23.3: Embryonic Stage - Biology

The Most Important Time in Your Life?

In many cultures, marriage — along with birth and death — is considered the most pivotal life event. For pioneering developmental biologist Lewis Wolpert, however, these life events are overrated. According to Wolpert, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Gastrulation is a major biological event that occurs early in the embryonic stage of human development.

Defining the Embryonic Stage

After a blastocyst implants in the uterus around the end of the first week after fertilization, its internal cell mass, which was called the embryoblast, is now known as the embryo. The embryonic stage lasts through the eighth week following fertilization, after which the embryo is called a fetus. The embryonic stage is short, lasting only about seven weeks in total, but developments that occur during this stage bring about enormous changes in the embryo. During the embryonic stage, the embryo becomes not only bigger but also much more complex. Figure (PageIndex{2}) shows an eight to nine week old embryo. The embryo's finger, toes, head, eyes, and other structures are visible. It is no exaggeration to say that the embryonic stage lays the necessary groundwork for all of the remaining stages of life.

Embryonic Development

Starting in the second week after fertilization, the embryo starts to develop distinct cell layers, form the nervous system, make blood cells, and form many organs. By the end of the embryonic stage, most organs have started to form, although they will continue to develop and grow in the next stage (that of the fetus). As the embryo undergoes all of these changes, its cells continuously undergo mitosis, allowing the embryo to grow in size, as well as complexity.


Late in the second week after fertilization, gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. A gastrula has 3 germ layers--the ectoderm, the mesoderm, and the endoderm. Some of the ectoderm cells from the blastula collapse inward and form the endoderm.

The final phase of gastrulation is the formation of the primitive gut that will eventually develop into the gastrointestinal tract. A tiny hole, called a blastopore, develops in one side of the embryo. The blastopore deepens and becomes the anus. The blastopore continues to tunnel through the embryo to the other side, where it forms an opening that will become the mouth. Whether this blastospore develops into a mouth or an anus determines whether the organism is a protostome or a deuterostome. With a functioning digestive tube, gastrulation is now complete.

Each of the three germ layers of the embryo will eventually give rise to different cells, tissues, and organs that make up the entire organism, which is illustrated in Figure (PageIndex{4}). For example, the inner layer (the endoderm) will eventually form cells of many internal glands and organs, including the lungs, intestines, thyroid, pancreas, and bladder. The middle layer (the mesoderm) will form cells of the heart, blood, bones, muscles, and kidneys. The outer layer (the ectoderm) will form cells of the epidermis, nervous system, eyes, inner ears, and many connective tissues.

Table (PageIndex{1}): The germ layers and what they give rise to
Germ LayerGives rise to
EctodermThe epidermis, glands of the skin, some cranial bones, pituitary and adrenal medulla, the nervous system, the mouth between cheek and gums, the anus
MesodermConnective tissues, bone, cartilage, blood endothelium of blood vessels, muscles, synovial membranes, serous membranes, kidneys, the lining of gonads
EndodermThe lining of the airways and digestive system, except the moth and distal part of the digestive system. Digestive, endocrine, and adrenal cortex glands.


Following gastrulation, the next major development in the embryo is neurulation, which occurs during weeks three and four after fertilization. This is a process in which the embryo develops structures that will eventually become the nervous system. Neurulation is illustrated in Figure (PageIndex{4}). It begins when a structure of differentiated cells called a neural plate forms from the ectoderm. The neural plate then starts to fold inward until its borders converge. The convergence of the neural plate borders also results in the formation of a neural tube. Most of the neural tube will eventually become the spinal cord. The neural tube also develops a bulge at one end, which will later become the brain.


In addition to neurulation, gastrulation is followed by organogenesis, when organs develop within the newly formed germ layers. Most organs start to develop during the third to eighth weeks following fertilization. They will continue to develop and grow during the following fetal period.

The heart is the first functional organ to develop in the embryo. The primitive blood vessels start to develop in the mesoderm during the third week after fertilization. A couple of days later, the heart starts to form in the mesoderm when two endocardial tubes grow. The tubes migrate toward each other and fuse to form a single primitive heart tube. By about day 21 or 22, the tubular heart starts to beat and pump blood, even as it continues to develop. By day 23, the primitive heart has formed five distinct regions. These regions will develop into the chambers of the heart and the septa (walls) that separate them by the end of the eighth week after fertilization.

Other Developments in the Embryo

Several other major developments that occur during the embryonic stage are summarized chronologically below, starting with the fifth week after fertilization.

Week Five

By week five after fertilization, the embryo measures about 4 mm (0.16 in.) in length and has begun to curve into a C shape. During this week, the following developments take place:

  • Grooves called pharyngeal arches form. These will develop into the face and neck.
  • The inner ears begin to form.
  • Arm buds are visible.
  • The liver, pancreas, spleen, and gallbladder start to form.

Week Six

By week six after fertilization, the embryo measures about 8 mm (0.31 in.) in length. During the sixth week, some of the developments that occur include:

  • The eyes and nose start to develop.
  • Leg buds form and the hands form as flat paddles at the ends of the arms.
  • The precursors of the kidneys begin to form.
  • The stomach starts to develop.

Week Seven

By week seven, the embryo measures about 13 mm (0.51 in.) in length. During this week, some of the developments that take place include:

  • The lungs begin to form.
  • The arms and legs have lengthened, and the hands and feet have started to develop digits.
  • The lymphatic system starts to develop.
  • The primary prenatal development of the sex organs begins.

Week Eight

By week eight — which is the final week of the embryonic stage — the embryo measures about 20 mm (0.79 in.) in length. During this week, some of the developments that occur include:

  • Nipples and hair follicles begin to develop.
  • External ears start to form.
  • The face takes on a human appearance.
  • Fetal heartbeat and limb movements can be detected by ultrasound.
  • All essential organs have at least started to form.

Genetic and Environmental Risks to Embryonic Development

The embryonic stage is a critical period of development. Events that occur in the embryo lay the foundation for virtually all of the body’s different cells, tissues, organs, and organ systems. Genetic defects or harmful environmental exposures during this stage are likely to have devastating effects on the developing organism. They may cause the embryo to die and be spontaneously aborted (also called a miscarriage). If the embryo survives and goes on to develop and grow as a fetus, it is likely to have birth defects.

Environmental exposures are known to have adverse effects on the embryo include:

  • Alcohol consumption: Exposure of the embryo to alcohol from the mother’s blood can cause fetal alcohol spectrum disorder. Children born with this disorder may have cognitive deficits, developmental delays, behavioral issues, and distinctive facial features.
  • Infection by rubella virus: In adults, rubella (German measles) is a relatively mild disease, but if the virus passes from an infected mother to her embryo, it may have severe consequences. The virus may cause fetal death, or result in a diversity of birth defects, such as heart defects, microcephaly (abnormally small head), vision and hearing problems, cognitive deficits, growth problems, and liver and spleen damage.
  • Radiation from diagnostic X-rays or radiation therapy in the mother: Radiation may damage DNA and cause mutations in embryonic germ cells. When mutations occur at such an early stage of development, they are passed on to daughter cells in many tissues and organs, which is likely to have severe impacts on the offspring.
  • Nutritional deficiencies: A maternal diet lacking certain nutrients may cause birth defects. The birth defect called spina bifida is caused by a lack of folate when the nervous system is first forming, which happens early in the embryonic stage. In this disorder, the neural tube does not close completely and may lead to paralysis below the affected region of the spinal cord.

Several structures form simultaneously with the embryo. These structures help the embryo grow and develop. These extraembryonic structures include the placenta, chorion, yolk sac, and amnion.


The placenta is a temporary organ that provides a connection between a developing embryo (and later the fetus) and the mother. It serves as a conduit from the maternal organism to the offspring for the transfer of nutrients, oxygen, antibodies, hormones, and other needed substances. It also passes waste products (such as urea and carbon dioxide) from the offspring to the mother’s blood for excretion from the body of the mother.

The placenta starts to develop after the blastocyst has implanted in the uterine lining. The placenta consists of both maternal and fetal tissues. The maternal portion of the placenta develops from the endometrial tissues lining the uterus. The fetal portion develops from the trophoblast, which forms a fetal membrane called the chorion (described below). Finger-like villi from the chorion penetrate the endometrium. The villi begin to branch and develop blood vessels from the embryo.

As shown in Figure (PageIndex{5}), maternal blood flows into the spaces between the chorionic villi, allowing the exchange of substances between the fetal blood and the maternal blood without the two sources of blood actually intermixing. The embryo is joined to the fetal portion of the placenta by a narrow connecting stalk. This stalk develops into the umbilical cord, which contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries, exchanges gases, and other substances with the mother’s blood, and travels back to the fetus through the umbilical vein.

Chorion, Yolk Sac, and Amnion

Besides the placenta, the chorion, yolk sac, and amnion also form around or near the developing embryo in the uterus. Their early development in the bilaminar embryonic disc is illustrated in Figure (PageIndex{5}).

  • Chorion: The chorion is a membrane formed by extraembryonic mesoderm and trophoblast. The chorion undergoes rapid proliferation and forms the chorionic villi. These villi invade the uterine lining and help form the fetal portion of the placenta.
  • Yolk Sac: The yolk sac (or sack) is a membranous sac attached to the embryo and formed by cells of the hypoblast. The yolk sac provides nourishment to the early embryo. After the tubular heart forms and starts pumping blood during the third week after fertilization, the blood circulates through the yolk sac, where it absorbs nutrients before returning to the embryo. By the end of the embryonic stage, the yolk sac will have been incorporated into the primitive gut, and the embryo will obtain its nutrients from the mother’s blood via the placenta.
  • Amnion: The amnion is a membrane that forms from extraembryonic mesoderm and ectoderm. It creates a sac, called the amniotic sac, around the embryo. By about the fourth or fifth week of embryonic development, amniotic fluid begins to accumulate within the amniotic sac. This fluid allows free movements of the fetus during the later stages of pregnancy and also helps cushion the fetus from potential injury.

Feature: My Human Body

Assume that you’ve been trying to conceive for many months and that you have just found out that you’re finally pregnant. You may be tempted to celebrate the good news with a champagne toast, but it’s not worth the risk. Alcohol can cross the placenta and enter the embryo’s (or fetus’s) blood. In essence, when a pregnant woman drinks alcohol, so does her unborn child. Alcohol in the embryo (or fetus) may cause many abnormalities in growth and development.

A child exposed to alcohol in utero may be born with a fetal alcohol spectrum disorder (FASD), the most severe of which is fetal alcohol syndrome (FAS). Signs and symptoms of FAS may include abnormal craniofacial appearance (Figure (PageIndex{6})), short height, low body weight, cognitive deficits, and behavioral problems, among others. The risk of FASDs and their severity if they occur depend on the amount and frequency of alcohol consumption, and also on the age of the embryo or fetus when the alcohol is consumed. Generally, greater consumption earlier in pregnancy is more detrimental. However, there is no known amount, frequency, or time at which drinking is known to be safe during pregnancy. The good news is that FASDs are completely preventable by abstaining from alcohol during pregnancy and while trying to conceive.


  1. When does the embryonic stage occur?
  2. Name a few of the major developments that occur during the embryonic stage.
  3. What is the embryonic disc? When and how does it form?
  4. Define gastrulation. When does it occur?
  5. Identify the three embryonic germ layers. Give examples of specific cell types that originate in each germ layer.
  6. What happens during neurulation? When does it occur?
  7. Define organogenesis. When does organogenesis take place in the embryo?
  8. What is the first functional organ to develop in the embryo? When does this organ start to function?
  9. Identify some of the developments that take place during weeks five through eight of the embryonic stage.
  10. List three environmental exposures that may cause birth defects during the embryonic stage.
  11. Identify extraembryonic structures that form at the same time as the embryo and help the embryo grow and develop. Give a function of each structure.
  12. Put the following events in order of when they occur, from earliest to latest:
    1. formation of the neural tube
    2. formation of the three germ layers
    3. formation of the primitive streak
    4. incorporation of the yolk sac into the embryo
  13. True or False: The nervous system develops from the same germ layer as skin cells do.
  14. True or False: Leg buds are formed during gastrulation.
  15. What are two tissues produced by the hypoblast?

Explore More

Learn more about spina bifida here:

Embryonic development

In developmental biology, embryonic development, also known as embryogenesis, is the development of an animal or plant embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). [1] Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo [2] after passing through an organizational checkpoint during mid-embryogenesis. [3] In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages. [2] [4]

Axolotl Embryo Staging Series

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

1. Fertilized egg in membranes

1. Animal pole

2. Two cells

3. Four cells

3. Eight cells

5. Sixteen cells, side view

5. Sixteen cells, animal pole

6. Thirty-two cells

7. Sixty-four cells

Blastula & Gastrula - Stages 8-12.5:

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

8. Early blastula

9. Late blastula

10. Early gastrula I,
vegetal hemisphere

10.5.Early gastrula II

10.75. Middle gastrula I

11. Middle gastrula II

11.5. Late gastrula I

12. Late gastrula II

12.5. Late gastrula III

Neurula - Stages 13-20:

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

13. Early neurula I,
vegetal hemisphere

13. Early neurula I,
side view

14. Early neurula II

15. Early neurula III

16. Middle neurula II

17. Late neurula I

18. Late neurula II

19. Late neurula III

20. Late neurula IV

Early Tailbud - Stages 21-25:

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

Middle Tailbud - Stages 26-30:

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

Late Tailbud - Stages 31-35:

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

Prehatched to Hatched - Stages 36-44:

Bordzilovskaya, N.P., T.A. Dettlaff, Susan T. Duhon, and George M. Malacinski. 1989. Developmental-stage series of axolotl embryos. In Developmental Biology of the Axolotl edited by J.B. Armstrong and G.M. Malacinski. Oxford University Press, New York, pp. 201-219.

The Bordzilovskaya and Dettlaff staging series is also available in Axolotl Newsletter #7 (Spring, 1979).

Click on the buttons (72 dpi GIF images) below to view the larger images (150 dpi JPEG) of each stage.

16 Main Embryological Stages of Embryo | Biology

1. It is a multicellular, four-cornered structure, surrounded by a layer of epidermis.

2. In each comer develops one or more archesporial initials.

3. These initials divide by a periclinal wall into outer primary parietal cell and inner primary sporogenous cell.

4. Primary parietal cell divides periclinally as well as anticlinally and form 3 to 5 concentric layers of cells.

5. Innermost wall layer is called tapetum which is nutritive in function.

6. From the sporogenous tissue develop the pollen grains.

7. Some cells form the procambial strand in the centre of die anther.

Stage # 2. T. S. Anther Showing Four Mature Pollen Sacs:

1. It is a four-cornered structure containing a pollen sac. (Fig. 56).

2. Anther is surrounded by a layer of epidermis throughout.

3. Each pollen sac is surrounded by epidermis, an endothecial layer, one to three middle layers or wall layers and innermost layer of tapetum.

4. In each pollen sac or chamber are present many pollen tetrads which on separation form microspores.

5. A joint in the form of connective is present in the centre.

Stage # 3. T. S. Mature Anther Showing Dehiscence:

1. Four-cornered, four-chambered, multicellular body surrounded by a layer of epidermis.

2. Partition wall between the two pollen sacs is dissolved (Fig. 57).

3. Many pollen grains or microspores are present in the pollen sacs in the form of fine, powdery or granular mass.

4. Endothecium, middle layers and tapetal layers are present below the epidermis.

5. Along the line of dehiscence of each lobe, thin walled cells of endothecium form the stomium.

6. A connective is very clear.

Stage # 4. Pollen Tetrads:

(A) Isobilateral Tetrad:

All the four spores are formed in one plane because the spindles of first and second meiotic division remain at right angle to one another, e.g., Zea mays.

Out of the two lower spores, only one is visible. Both the upper ones are clear e.g., Magnolia.

In meiosis II upper cell divides to form two cells present side by side and the lower cell forms two cells lying one above the other e.g., Arislolochia.

All the four spores are present one above the other in a linear fashion e.g., Halophila.

(E) Compound Pollen Grain:

Sometimes microspore tetrads adhere to each other and form the compound pollen grain e.g., Typha, Cryptostegia.

Pollen grains of a pollen sac sometimes remain together to form a single mass called pollinium. Each pollinium consists of carpusculum, caudicle and pollinia e.g., Asclepiadaceae.

Stage # 5. Pollen Grain:

1. It is a unicellular, uninucleate structure (Fig. 59). But pollen grains are always 2- or 3 nucleate when shed.

2. It is surrounded by a double-layered wall, i.e., outer exine and inner intine.

3. Exine is thick, cutinized, pigmented, sculptured and perforated by germ pores.

4. Intine is thin, colorless, smooth and consists of cellulose.

5. In the cytoplasm are present water, protein, fats, carbohydrates, etc.

Stage # 6. Various Types of Ovules (Fig. 60):

(A) Orthotropous (Ortho, straight tropous, turned):

When micropyle, chalaza and funicle lie in one straight line e.g., Polygonaceae, Urticaceae.

(B) Anatropous (Ana, backwards tropous, turned):

Here, the body of the ovule turns backwards by an angle of 180° and so the micropyle becomes close to the hylum and placenta Sympetalae.

(C) Hemitropous (Hemi, half tropous, turned):

Here the body of the ovule is placed transversely or somewhat at right angle to funicle. Chalaza and micropyle are present here in one straight line e.g. Ranunculus.

(D) Campylotropous. (Kampylos, curved):

Here the body of the ovule is curved so that the chalaza and the micropyle do not lie in the same straight line e.g., Leguminosae.

Here the curvature of ovule is more pronounced and embryo sac becomes horse-shoe shaped e.g., Butomaceae.

Here the funicle is very long and the ovule rotates by an angle of 360° in such a fashion that it is completely circled around by the funicle. Micropyle faces upward e.g., Cactaceae.

Stage # 7. L. S. Anatropous Ovule (Fig. 61):

1. It is attached to the placenta with a stalk called funicle.

2. The point of attachment of funicle with the body of the ovule is known as hilum which extends above in the form of a ridge i.e., raphe.

3. Nucellus consists of parenchymatous cells.

4. Nucellus remains covered by one or two coverings called integuments.

5. Integuments remain disconnected at one point forming a passage called micropyle.

6. Embryo sac consists of three antipodals, two synergids, one egg cell and one secondary nucleus.

7. Antipodals are located near the chalaza end, and the egg cell and synergids towards the micropylar end.

Stage # 8. Archesporial Initial (Fig. 62):

1. It is hypodermal in origin.

2. Archesporial initial is bigger than that of its surrounding cells.

3. A conspicuous nucleus and dense cytoplasm is present in it.

4. In its later stages, it divides into two cells forming an outer parietal cell which form the parietal tissue and inner megaspore mother cell.

Stage # 9. Two-celled Stage of Megaspore Mother Cell:

1. Two cell are present one above the other.

2. These are formed after reduction division and so each cell contains haploid set of chromosomes.

3. From these two cells, tetrad is formed.

Stage # 10. Linear Tetrad of Megaspores:

1. Four megaspores are arranged in linear fashion.

2. These are haploid in nature.

3. Out of the four, only one remains functional which is near the chalazal end. Remaining three degenerate (Fig. 64).

4. Functional megaspore is the First cell of the female gametophyte and develops into the embryo sac.

Stage # 11. Ovule with Binucleate Embryo-Sac:

1. Two nuclei are present in the embryo sac.

2. These two nuclei are formed by the division of the nucleus of the functional megaspore.

3. After some time two nuclei are separated by a large vacuole and they reach at the corners.

Stage # 12. Ovule with 4-Nucleate Embryo-Sac:

1. Four nuclei are present in the embryo sac (Fig. 66).

2. Out of the four, two nuclei are present near the chalazal end and the rest two near the micropylar end.

3. In the centre is present a large central vacuole.

4. Traces of degenerated megaspores are also seen at the micropylar end.

Stage # 13. OvuIe with 8 – Nucleate, Polygonum type Embryo-sac:

1. Near the micropylar end is present the egg apparatus.

2. Egg apparatus consists of an egg and two synergids.

3. Near the chalazal end are present three antipodals (Fig. 67).

4. In the centre are present two polar nuclei which ultimately fuse and form a secondary nucleus.

5. Many small vacuoles are present throughout.

Stage # 14. Endosperm:

1. Endosperm is formed because of the fusion of two polar nuclei and one of the male gametes.

2. It has triploid number of chromosomes.

3. It is of following three different types (Fig. 68).

Different kinds of endosperm:

Endosperm nucleus divides many times thus forming many free nuclei which in the later stages may be separated by walls.

In this type all the nuclear divisions are accompanied by wall formation.

In this type, first the nuclear divisions are accompanied by wall formation but later on there is no wall formation and nuclei remain free. So it is an intermediate stage between nuclear and cellular.

Stage # 15. Monocot Embryo:

1. Only one cotyledon is present (Fig. 69).

2. Plumule forms the stem and radicle forms the root.

3. Hypocotyle and a small suspensor are also present.

Stage # 16. Dicot Embryo:

1. Two large cotyledones are present

2. Both the cotyledones covet a small stem apex.

4. Near the suspensor is present the root cap.

5. Central region forms the procambium which is present in between root cap and stem apex (Fig. 70).

The Difference Between Embryonic and Adult Stem Cells

There are also other types of stem cells, not to be confused with an embryonic stem cell. Embryonic stem cells are derived from embryos. There are also adult stem cells, umbilical cord stem cells, and fetal stem cells. Not only are these stem cells sometimes more ethically challenging, they are only multipotent, meaning they can only become a small range of cell types.

One example is umbilical cord blood stem cells, which have been used in medical treatments to treat various blood diseases and suppressed immune systems. The stem cells in the blood of the umbilical cord can differentiate into almost any type of blood or immune cell, making them multipotent. However, this limits their use in other areas of medicine.

There are also adult stem cells, which survive in various organs throughout the body. These cells are also multipotent, and can only differentiate into the kinds of tissue in which they are found. A common use of adult stem cells is the bone marrow transplant. In this procedure, a healthy donor must have their marrow extracted from their bones. The marrow is a blood-like substance on the inside of large bones which creates blood cells and immune cells.

Cancer patients, having undergone radiation and chemotherapy, lose most of their immune cells and become immunocompromised. Often a bone marrow transplant is needed to replace these tissues. The new stem cells begin producing new immune cells, which help the patient recover and fight off infection and disease.


Meckel, Serres, Geoffroy Edit

The idea of recapitulation was first formulated in biology from the 1790s onwards by the German natural philosophers Johann Friedrich Meckel and Carl Friedrich Kielmeyer, and by Étienne Serres [5] after which, Marcel Danesi states, it soon gained the status of a supposed biogenetic law. [6]

The embryological theory was formalised by Serres in 1824–26, based on Meckel's work, in what became known as the "Meckel-Serres Law". This attempted to link comparative embryology with a "pattern of unification" in the organic world. It was supported by Étienne Geoffroy Saint-Hilaire, and became a prominent part of his ideas. It suggested that past transformations of life could have been through environmental causes working on the embryo, rather than on the adult as in Lamarckism. These naturalistic ideas led to disagreements with Georges Cuvier. The theory was widely supported in the Edinburgh and London schools of higher anatomy around 1830, notably by Robert Edmond Grant, but was opposed by Karl Ernst von Baer's ideas of divergence, and attacked by Richard Owen in the 1830s. [7]

Haeckel Edit

Ernst Haeckel (1834–1919) attempted to synthesize the ideas of Lamarckism and Goethe's Naturphilosophie with Charles Darwin's concepts. While often seen as rejecting Darwin's theory of branching evolution for a more linear Lamarckian view of progressive evolution, this is not accurate: Haeckel used the Lamarckian picture to describe the ontogenetic and phylogenetic history of individual species, but agreed with Darwin about the branching of all species from one, or a few, original ancestors. [9] Since early in the twentieth century, Haeckel's "biogenetic law" has been refuted on many fronts. [10]

Haeckel formulated his theory as "Ontogeny recapitulates phylogeny". The notion later became simply known as the recapitulation theory. Ontogeny is the growth (size change) and development (structure change) of an individual organism phylogeny is the evolutionary history of a species. Haeckel claimed that the development of advanced species passes through stages represented by adult organisms of more primitive species. [10] Otherwise put, each successive stage in the development of an individual represents one of the adult forms that appeared in its evolutionary history.

For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo not only roughly resembled gill slits of fish, but directly represented an adult "fishlike" developmental stage, signifying a fishlike ancestor. Embryonic pharyngeal slits, which form in many animals when the thin branchial plates separating pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes.

Haeckel produced several embryo drawings that often overemphasized similarities between embryos of related species. Modern biology rejects the literal and universal form of Haeckel's theory, such as its possible application to behavioural ontogeny, i.e. the psychomotor development of young animals and human children. [11]

Contemporary criticism Edit

Haeckel's drawings misrepresented observed human embryonic development to such an extent that he attracted the opposition of several members of the scientific community, including the anatomist Wilhelm His, who had developed a rival "causal-mechanical theory" of human embryonic development. [12] [13] His's work specifically criticised Haeckel's methodology, arguing that the shapes of embryos were caused most immediately by mechanical pressures resulting from local differences in growth. These differences were, in turn, caused by "heredity". His compared the shapes of embryonic structures to those of rubber tubes that could be slit and bent, illustrating these comparisons with accurate drawings. Stephen Jay Gould noted in his 1977 book Ontogeny and Phylogeny that His's attack on Haeckel's recapitulation theory was far more fundamental than that of any empirical critic, as it effectively stated that Haeckel's "biogenetic law" was irrelevant. [14] [15]

Darwin proposed that embryos resembled each other since they shared a common ancestor, which presumably had a similar embryo, but that development did not necessarily recapitulate phylogeny: he saw no reason to suppose that an embryo at any stage resembled an adult of any ancestor. Darwin supposed further that embryos were subject to less intense selection pressure than adults, and had therefore changed less. [16]

Modern status Edit

Modern evolutionary developmental biology (evo-devo) follows von Baer, rather than Darwin, in pointing to active evolution of embryonic development as a significant means of changing the morphology of adult bodies. Two of the key principles of evo-devo, namely that changes in the timing (heterochrony) and positioning (heterotopy) within the body of aspects of embryonic development would change the shape of a descendant's body compared to an ancestor's, were however first formulated by Haeckel in the 1870s. These elements of his thinking about development have thus survived, whereas his theory of recapitulation has not. [17]

The Haeckelian form of recapitulation theory is considered defunct. [18] Embryos do undergo a period or phylotypic stage where their morphology is strongly shaped by their phylogenetic position, [19] rather than selective pressures, but that means only that they resemble other embryos at that stage, not ancestral adults as Haeckel had claimed. [20] The modern view is summarised by the University of California Museum of Paleontology:

Embryos do reflect the course of evolution, but that course is far more intricate and quirky than Haeckel claimed. Different parts of the same embryo can even evolve in different directions. As a result, the Biogenetic Law was abandoned, and its fall freed scientists to appreciate the full range of embryonic changes that evolution can produce—an appreciation that has yielded spectacular results in recent years as scientists have discovered some of the specific genes that control development. [21]

The idea that ontogeny recapitulates phylogeny has been applied to some other areas.

Cognitive development Edit

English philosopher Herbert Spencer was one of the most energetic proponents of evolutionary ideas to explain many phenomena. In 1861, five years before Haeckel first published on the subject, Spencer proposed a possible basis for a cultural recapitulation theory of education with the following claim: [22]

If there be an order in which the human race has mastered its various kinds of knowledge, there will arise in every child an aptitude to acquire these kinds of knowledge in the same order. Education is a repetition of civilization in little. [23]

G. Stanley Hall used Haeckel's theories as the basis for his theories of child development. His most influential work, "Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education" in 1904 [24] suggested that each individual's life course recapitulated humanity's evolution from "savagery" to "civilization". Though he has influenced later childhood development theories, Hall's conception is now generally considered racist. [25] Developmental psychologist Jean Piaget favored a weaker version of the formula, according to which ontogeny parallels phylogeny because the two are subject to similar external constraints. [26]

The Austrian pioneer of psychoanalysis, Sigmund Freud, also favored Haeckel's doctrine. He was trained as a biologist under the influence of recapitulation theory during its heyday, and retained a Lamarckian outlook with justification from the recapitulation theory. [27] Freud also distinguished between physical and mental recapitulation, in which the differences would become an essential argument for his theory of neuroses. [27]

In the late 20th century, studies of symbolism and learning in the field of cultural anthropology suggested that "both biological evolution and the stages in the child's cognitive development follow much the same progression of evolutionary stages as that suggested in the archaeological record". [28]

Music criticism Edit

The musicologist Richard Taruskin in 2005 applied the phrase "ontogeny becomes phylogeny" to the process of creating and recasting music history, often to assert a perspective or argument. For example, the peculiar development of the works by modernist composer Arnold Schoenberg (here an "ontogeny") is generalized in many histories into a "phylogeny" – a historical development ("evolution") of Western music toward atonal styles of which Schoenberg is a representative. Such historiographies of the "collapse of traditional tonality" are faulted by music historians as asserting a rhetorical rather than historical point about tonality's "collapse". [29]

Taruskin also developed a variation of the motto into the pun "ontogeny recapitulates ontology" to refute the concept of "absolute music" advancing the socio-artistic theories of the musicologist Carl Dahlhaus. Ontology is the investigation of what exactly something is, and Taruskin asserts that an art object becomes that which society and succeeding generations made of it. For example, Johann Sebastian Bach's St. John Passion, composed in the 1720s, was appropriated by the Nazi regime in the 1930s for propaganda. Taruskin claims the historical development of the St John Passion (its ontogeny) as a work with an anti-Semitic message does, in fact, inform the work's identity (its ontology), even though that was an unlikely concern of the composer. Music or even an abstract visual artwork can not be truly autonomous ("absolute") because it is defined by its historical and social reception. [29]

Stages of Mammalian Embryonic Development

Although there are some inherent differences between species, the embryos of most vertebrate species involve the same processes during embryogenesis. Most of the notable differences tend to become more apparent during the later stages of development. In mammals, embryogenesis proceeds in the following distinct stages:


Following fertilization, the zygote begins to divide by mitosis in a manner in which there is a lack of growth, and the resulting cluster of cells remains the same size as the initial fertilized cell (shown below). After four rounds of cleavage, the 16-celled cluster is termed the morula. The cells comprising the morula eventually form an outer layer called the trophoblast and an inner cluster of cells, termed the inner cell mass, which will form the embryo. Fluid will then fill the space between the trophoblast and the inner cells, with the two cell formations connecting at one pole, termed the embryonic pole.

Blastula Stage

After seven rounds of cleavage, the cell cluster comprised of 128 cells is known as the blastula. The blastula is characterized by a circular layer of cells termed the blastoderm surrounding an inner cell mass termed the blastocyst (shown below). The fluid-filled cavity residing between the two groups of cells is termed the blastocoel. During this stage, the trophoblast as described above is divided into an outer layer called the syncytiotrophoblast and an inner layer termed the cytotrophoblast. These layers do not form the embryo, but will eventually help form the placenta. The inner cluster of cells, termed the inner cell mass, also begins to undergo organization during this stage. At the center of the inner cell mass is a layer of flat, differentiated cells termed the endoderm. The endoderm forms the yolk sac which will supply the growing embryo with nutrients and a source of blood supply until the formation of the placenta is complete. Between the remaining cells, the amniotic cavity is formed, the bottom of which is composed of prismatic cells called the ectoderm, and forms a structure called the embryonic disk. The embryonic disk then begins to change conformation and forms a pore with the yolk sac. The cells of the ectoderm gradually descend to meet the endoderm. A third layer of cells is also formed and is situated laterally between the endoderm and ectoderm. These layers are termed the germ layers and will eventually form the various tissues of the organism. It is also during this stage that implantation of the embryo into the uterine wall occurs.

Gastrula Stage

Once the three germ layers have been formed and move towards the center of the blastula, the embryo is called a gastrula (shown below). Although the differentiation of the various cell types occurs during the blastula stage, the organization of the cell into three distinct layers is known as gastrulation. Gastrulation typically occurs during the third week of pregnancy, and the process begins with the formation of a thick structure along the midline of the embryonic disk, termed the primitive streak. The primitive streak defines the major axes of the embryo (left, right, cranial, and caudal sides). At the cranial end of the embryonic disk, the primitive streak expands to form a primitive node and begins to extend along the midline to the caudal end and to form a primitive groove. At this point, the outer layer of cells begins to fold inward and detach along the primitive streak via a process termed invagination. The first cells which move inward displace the outer layer of cells and are replaced by a new cell layer termed the definitive endoderm. Inside the embryo, the cells that were internalized join and form the definitive ectoderm. The group of cells residing between the definitive ectoderm and endoderm form the definitive mesoderm.

Organogenesis Stage

Study the Stages of the Human Embryonic Development

Embryonic growth depends directly on mitosis. Through this type of cell division, the zygote divides, producing a series of cells that also compose differentiated tissues and organs via mitosis until the formation of a complete individual.

More Bite-Sized Q&As Below

2. What is the function of the vitellus in vertebrate eggs? How are these eggs classified according to the amount of vitellus within them?

Vitellus (yolk) is the nutritive material that accumulates in the cytoplasm of the egg (zygote), and has the function of nourishing the embryo. Depending on the amount of vitellus in them, vertebrate eggs are classified as oligolecithal (little yolk), centrolecithal, or heterolecithal (more yolk diffusely distributed) and telolecithal (more yolk concentrated at one end of the egg).

3. What are the animal pole and the vegetal pole of vertebrate eggs?

The animal pole of a telolecithal egg is t he portion of the egg wi th little vitellus. It is opposite to the vegetal pole, which is the region where the yolk is concentrated.

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The Stages of Embryonic Development

4. What are the four initial stages of embryonic development?

The four initial stages of embryonic development are the morula stage, the blastula stage, the gastrula stage and the neurula stage.

5. What is cell division during the first stage of embryonic development called? How can this stage be described?

Cell division during the first stage of embryonic developments is called cleavage, or segmentation. During this stage, several mitoses occur within the zygote to form the new embryo.

6. What are the cells produced during the first stage of embryonic development called?

The cells that are produced during cleavage (the first stage of embryonic development) are called blastomeres. In this stage the embryo is called the morula (similar to a “morus”, or mulberry).

7. After the morula stage, what is the next stage? What is the morphological feature that defines this stage?

After passing the morula stage in which the embryo is a compact mass of cells, the next stage is the blastula stage. In the blastula stage, the compactness is lost and an internal cavity filled with fluid appears inside it, called the blastocele.

8. After the blastula stage, what is the following stage of embryonic development? What is the passage from the blastula to the next stage called?

The blastula turns into the gastrula through a process known as gastrulation.

9. What is gastrulation? How are the first two germ layers formed during gastrulation? What are these germ layers?

Gastrulation is the process through which a portion of the blastula wall invaginates inside the blastocele, forming a tube called the archenteron (a primitive intestine). The cells of the inner side of the tube form the endoderm (a germ layer) and the cells of the outer side form the ectoderm (another germ layer). This is the beginning of tissue differentiation in embryonic development.

10. What are the archenteron and the blastopore? During what stage of embryonic development are these structures formed? What happens to the archenteron and the blastopore?

The archenteron is the tube formed during gastrulation by means of the invagination of the blastula wall inside the਋lastocoel. It turns into the gastrointestinal tract. The blastopore is archenteron's exterior opening . The blastopore produces one of the extremities of the digestive tract: the mouth in protostome organisms, or the anus in deuterostome organisms.

11. How is the mesoderm (third germ layer) of triploblastic animals formed?

The mesoderm is formed through the differentiation of the endodermal cells that cover the dorsal region of the archenteron.

Germ Layers, Diploblastic and Triploblastic Animals

12. What are the three types of germ layers that form tissues and organs in animals?

The three germ layers are the ectoderm, the mesoderm and the endoderm.

13. How are animals classified according to the germ layers present during their embryonic development?

Cnidarians are diploblastic, meaning that they only have an endoderm and ectoderm. With the exception of poriferans, all remaining animals are triploblastic. Poriferans do not have differentiated tissue organization and, as a result, have no classification regarding germ layers (although sometimes they are considered diploblastic).

The Neural Tube and Notochord

14. How does the embryo turn into the neurula from gastrula? How is the neural tube formed? What is the embryonic origin of the nervous system in vertebrates?

The neurula stage is characterized by the appearance of the neural tube along the dorsal region of the embryo. The growth of the mesoderm in that region causes the differentiation of the ectodermal cells just above it. These cells then differentiate to form the neural tube. Therefore, the origin of the nervous system is the ectoderm (the same germ layer that produces the skin).

15. What is the notochord? How is this structure formed?

The notochord is a rodlike structure that forms the supporting axis of the embryo and which produces the spine in vertebrates. It is formed through the differentiation of mesodermal cells.

The Coelom

16. What is the coelom? What structures are produced by the coelom? Are all animals coelomate?

Coeloms are cavities delimited by a mesoderm. Coeloms turn into cavities where the internal organs of the body are located, such as the pericardial cavity, the peritoneal cavity and the pleural cavity.

In addition to coelomate animals, there are also acoelomate animals, such as platyhelminthes, and pseudocoelomate animals, such as nematodes.

17. From which germ layer are coeloms produced?

Coeloms are produced from the mesoderm.

18. What are the pleura, the pericardium and the peritoneum?

The pleura is the membrane that covers the lungs and the inner wall of the chest the pericardium is the membrane that covers the heart and the peritoneum is the membrane that covers most organs of the gastrointestinal tract and part of the abdominal cavity. All these membranes surround coeloms (internal cavities).


19. After the neurula stage, how can the morphology of the embryo be described, starting at its ventral portion and ending at its dorsal portion?

In a schematic longitudinal section of an embryo after the neurula stage, the outermost layer of cells is the ectoderm. In the ventral region, the archenteron tube is formed of endodermal cells. In both sides of the embryo, coeloms covered by a mesoderm are present. In the central region above the archenteron and in the middle of the coeloms, the notochord is located. In the dorsal region just above the notochord, the neural tube is located.

20. What are somites?

Somites are differentiated portions of mesodermal tissue which are longitudinally distributed along the embryo. Somites turn into muscle tissue and portions of connective tissues.

Histogenesis and Organogenesis

21. What are histogenesis and organogenesis?

Histogenesis is the process of tissue formation during the embryonic development. Organogenesis is the process of organ formation. Before histogenesis and organogenesis, primitive embryonic structures have been already formed: germ layers, the neural tube, the notochord, coeloms, and somites.

22. From which germ layer are the epidermis and the nervous system produced? What other organs and tissues are made from that germ layer?

The epidermis and the nervous system have the same embryonic origin: the ectoderm. Epidermal appendages (such as nails, hair, sweat glands and sebaceous glands), the mammary glands, the adenohypophysis, the cornea, the crystalline lens and the retina are also produced from the ectoderm.

23. From which germ layer are blood cells produced? What other organs and tissues are made from that germ layer?

Blood cells have a mesodermal e mbryonic origin. Other organs made from the mesoderm are: serous membrane coverings such as the pericardium, the peritoneum and the pleura, muscles, cartilage, the dermis, adipose tissue, the kidneys, the ureters, the bladder, the urethra, the gonads, blood and lymph vessels, and bones.

24. From which germ layer are the liver and the pancreas produced? What other organs and tissues are made from that germ layer?

The liver and the pancreas are produced from the endoderm. Also of endodermal origin are the epithelium of the airway, the epithelium of the bladder, the epithelium of the urethra and the epithelium of the GI tract (except for the mouth and anus), the alveolar cells of the lungs and the thyroid and parathyroid glands.

Twins and Polyembryony

25. What are twins? Genetically, what are the two types of twins that can occur?

Twins are simultaneously generated (within the mother’s uterus) offspring. Twins are classified according to zygosity as monozygotic or dizygotic twins.

Monozygotic twins, also known as identical twins, are those that originate from one single fertilized ovum (therefore from one single zygote) monozygotic twins are genetically identical, meaning that they have identical genotypes and are necessarily of the same sex. Dizygotic twins, also known as fraternal twins, are those generated from two different ova fertilized by two different sperm cells therefore, they are not genetically identical and they are not necessarily of the same sex.

26. What is polyembryony?

Polyembryony is the phenomenon in which a single embryo in its initial embryonic stage divides itself to form many new individuals of the same sex and who are genetically identical. This is the way, for example, in which reproduction takes place in armadillos of the genus Dasypus. Polyembryony is an example of natural “cloning”.

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Throughout the process of fetal development, the growing fetus goes through three distinct stages, each characterized by specific events.

2 ¼ to 4 months

As the embryo enters the fetal stage of development, the placenta becomes functional. The fetus typically measures 30 mm from the crown to the rump and weighs approximately 8 g (shown below). By the end of this stage, the fetus is approximately 15 cm. During this time, several organs can be observed, including the hands, feet, heart, and brain. The pancreas and liver begin to secrete fluids. In addition, the genitals begin form and the head is prominent, comprising almost half of the fetal body. The fetus also exhibits unregulated movements required for the lung, muscle, and neurological development that is occurring.

4 ¼ to 6 ¼ months

During this period of fetal development, the mother begins to feel the movements of the growing fetus, which grows from approximately 15 cm to 38 cm and weighs about 500 g by the end of this stage (shown below). During this stage, the eyebrows and eyelashes form, muscle development increases and the fetus becomes more active, the lung continues to develop with the formation of the alveoli. In addition, the nervous system rapidly develops, exhibited by the development of the inner ear, control over the opening and closing of the eyelids, as well as other bodily processes. The genitals are fully formed and the sex can be reliably discerned.

6 ½ to 9 ½ months

At this stage, the fetus begins to gain weight as the body fat increases. The lungs continue to mature and become capable of gas exchange. In addition, hair begins to thicken on the head and breast buds appear. The fetus is considered to be full-term at approximately 38 weeks (between 36 and 40 weeks) of pregnancy (shown below). While fetuses born prior to 36 weeks can survive outside the uterus, medical intervention is required to promote survival, particularly due to the underdevelopment of the lungs in premature infants.

1. Which of the following statements is TRUE:
A. The lungs are one of the first organs to fully mature in a developing fetus.
B. The heart is one of the first organs to form in the developing fetus.
C. The sex of the fetus can be observed as early as 13 weeks.
D. A fetus born at 37 weeks has a low probability of survival.

Somatic Embryogenesis: Process and Applications | Plants

In this article we will discuss about: 1. Process of Somatic Embryogenesis 2. Embryo Maturation and Synchronisation 3. Cultural Conditions 4. Recurrent Embryogenesis and Mass Production 5. Applications.

After fertilization, zygote is transformed into adult status through a series of embryogenic processes. Despite the same genetic constituents, somatic cells on the other hand, do not reorient towards embryo production. However, isolated somatic cells under in vitro conditions have the potential to develop into embryo under the influence of growth factors.

Somatic cells are able to develop into whole plant through the stages of embryogenesis without gametic fusion. Therefore, somatic embyros are non-zygotic embryos originated from sporophytic cells. Somatic embryo production is either direct or indirect in vitro. Somatic embryos may be direct when embryonic cells develop directly from the explants’ cells or indirect when developed through the callus.

Plant cells undergoing somatic embryogenesis are either pro-embryonic determined cells (PEDC) or induced embryogenic determined cells (IEDC). There have been reports on the induction of somatic embryos frequently from various tissues like seedlings, shoot meristem, young inflorescence and zygotic embryos. In addition, other tissues such as root, nucellus has also yielded somatic embryos.

The favorable responses of a few of the above tissues actually contain proembryogenic determinant cells (PEDC) or these cells may require minor reprogramming to enter embryogenic state. The first report on the production of somatic embryos in carrot suspension cells was published by Steward and co-workers in 1958. Thereafter reports were flooded on the production of somatic embryos in plants.

Process of Somatic Embryogenesis:

Significance of Auxin:

Reprogramming of somatic cells and its entry into the embryogenic status requires ex­tensive proliferation through unorganized callus cycle and exposure to high doses of synthetic auxin such as 2, 4-D or picloram. Somatic embryo induction can also be accomplished by plasmolysis of explant cells.

The significance of auxin for embryo induction status from vegetative cells and tissue was recognized as the prime controlling factor. This is based on a critical assessment in species like Daucus carota, Atropa belladona and Ranunculus sceleratus. Transformation of embyrogenic cells into the callus system due to the differentiation of single cell is followed by the appearance of dense cytoplasm, prominent nucleus and high profiles of organelles.

These groups of small densely packed cytoplasmic cells arise by internal division. These groups of cells constitute pro-embyros which can develop into globular embryos. The formation of a mature embyro and plantlet via heart and torpedo shaped stages may proceed undisturbed even when exogenous auxin remains present at lowest concentration in the later development.

The later process in the prevalent media condition however, may disturb further establishment of embryogenesis unless auxin is completely omitted. It was even evidenced that embryogenic process may be completely arrested during transition of embryo to plantlet. Therefore auxin is reduced or entirely withdrawn once such anomaly appears during culture.

In date palm tissue culture, liquid media enriched with low amount of plant growth regulator resulted in the differentiation of large number of somatic embryos. High concentration of auxin may not encourage embryo formation. Therefore, two distinct conclusions can be drawn by the role of auxin in entire embryogenic episode.

First, induction of cells with reprogrammed embryogenic competence under the influence of auxin. Second one is directing embryogenic cells to undergo complete development by withdrawing auxin from the media. Low level of endogenous auxin can equally determine embryo induction.

Auxin deprivation acts as a development switch from nonpolar embryogenic units to induce somatic embryogenesis in maize. This developmental switch is accompanied by cytoskeletal rearrangements in embryogenic cells. Whole somatic embryogenetic process may derail the establishment of polarity if exogenous auxin is supplied.

One of the negative factors implicated in somatic embryogenesis is the pro­duction of ethylene in presence of auxin for a considerable period of time in the culture media. Production of ethylene in turn elevates the activity of enzymes, probably, cellulase and pectinase which degrade pectin compounds and consequently disturb establishment of polarity by reduc­ing cell to cell interaction and contact due to separation.

Role of 2, 4-D in particular, for the induction of somatic embryogenesis is exemplary. Literature survey has shown that this synthetic auxin is very often suitable in inducing somatic embryogenesis in most of the species. Another synthetic auxin NAA has been found to be suit­able for somatic embryo induction.

However, the role of phytohormones in somatic embryo induction is highly a complex process and varies depending on plant species as well as its en­dogenous concentration. Under no circumstances, gibberellic acid is useful for somatic embryo induction. But its role has been implicated in the maturation of somatic embryos.

Role of Reduced Nitrogen:

The embryogenic competent cells seem to have preference for high salt strength and specific nitrogen source. This was considered to be a second pre-requisite for somatic embryo induction after auxin. The reduced form of nitrogen, ammonia, provides triggering factors for embryogenesis.

Similarly, nitrogen in the form of casein hydrolysate can equally contribute in the stimulation of somatic embryos and has been critically assessed in carrot as a model plant. Presence of proline and serine, capable of stimulating somatic embryo induction was reported in carrot plant.

Addition of reduced nitrogen, ammonium ion (NH4 + salt) or amino acids into the media is conductive for embryogenesis after shifting callus from auxin to auxin free media. It is however, concentration of auxin and nitrogen rather than critical concentration of reduced ni­trogen which is crucial in empowering embryogenesis.

High frequency of somatic embryogenesis was achieved in cucumber plant. Addition of diazuron and sucrose treatment (3-6%) exerted positive effect on the relative position of somatic embryo induction. Addition of copper sulphate in the media induces high frequency somatic embryo induction.

Similarly, thiadiazuron when supplemented in the medium induced shoot organogenesis at low concentration and somatic embryogenesis at high concentration. Enhanced somatic embryo production and maturation into normal plants in cotton was achieved when calli cultured on half strength MS media.

A thorough examination of the role of reduced nitrogen ammonia shows that embryo formation is promoted when as little as 0.1 mM ammonium chloride is supplied to nitrate me­dia. Embryogenesis is promoted by 40 mM potassium nitrate and 30 mM ammonium chloride as optimum concentration.

Glutamine and alanine can serve as sole nitrogen source for the growth and embryo formation. Although nitrate is required for embryogenesis on several in­stances, ammonium alone can produce embryo in carrot suspension culture, provided pH of the medium containing 10mM ammonium chloride and 20 mM potassium chloride was controlled at pH 5.4.

Level of dissolved oxygen has some role to play in somatic embryogenesis at least in carrot plant where embryogenesis takes place only below critical level of dissolved oxygen (i.e., above 1.5 ppm). Higher level favors rhizogenesis. Addition of activated charcoal into the culture media can promote embryo induction by adsorbing inhibiting substances produced by tissue.

Embryo Maturation and Synchronisation:

Studies on embryo germination process shows that embryo development completes without any anomalies in the absence of auxin in the media. However, any abnormalities due to endogenous hormones can be avoided by supplementing balanced concentrations of abscisic acid (ABA), zeatin, and GA3. Addition of charcoal may increase the maturation of somatic embryo.

Presence of charcoal in the media reduces the level of auxin like IAA due to its binding effect. Somatic embryo maturation can be enhanced by subjecting to osmotic desiccation. Sucrose is generally used at different concentrations to achieve embryonic growth and maturation. This is achieved by providing sucrose concentration between 4 and 6%.

In certain species, progressive increase in sucrose concentration upto 4% is required for maturation, which consequently produces vigorous plantlets. Similarly, imposition of temporary desiccation before embryo germination facilitates conversion to plantlets. Imposition of desiccation can be progressed by placing somatic embryos in empty petridish and incubated at desiccated condition for 2-3 weeks and some plants upto several weeks.

Somatic embryos, when shriveled to 50% of their original volume rapidly imbibe water when rehydrated by transfer to media. The whole exercise of desiccation in embryo is to influence metabolic process for germination. Somatic embryos when subjected to show desiccation, it stimulates the production of high frequency of shoot regeneration.

Imposition of desiccation improves conversion to plantlets several times the frequency of non-desiccated embryos. In Alfalfa culture, somatic embryos have been trained to withstand desiccation by treating them with ABA at the torpedo stage. ABA treatment can promote the development of cotyledons and block the production of embryo clusters.

Cultural Conditions of Somatic Embryogenesis:

High light intensity can influence the process of somatic embryogenesis. However, cul­tures were incubated under both light and dark periods. Early maturation takes place more predominantly under complete dark conditions.

Reports on the influence of temperature on somatic embryogenesis are scarce. In citrus nucellus culture, embryogenic potential drops when the temperature was reduced from 27°C to 12°C. Similarly, conditioning of somatic embryos by cold treatment can escape dormancy and facilitate development.

Recurrent Embryogenesis and Mass Production of Somatic Embryogenesis:

The primary somatic embryo when fails to undergo maturation may enter continuous successive cycles of embryos. Certain specific superficial cells of the hypocotyl or cotyledon exhibit this tendency in provoking successive cycles of embryos or in other words continuous production of supernumerary embryos from somatic embryos itself.

This phenomenon is also known as secondary embryogenesis, recurrent embryogenesis, repetitive or accessory embryogenesis (Fig. 8.1). Recurrent embryogenic cycle can be maintained in culture by the removal of growth regulators and cycles can be spontaneous as this was evidenced in Alfalfa (Medicago sativa).

Recurrent embryogenesis cycle can be made spontaneous by locking the development of so­matic embryos particularly at proembryogenic status, beyond which they cannot proceed to develop. This can be accomplished by initial exposure to very high concentration of 2, 4-D upto 40 mg/L for brief period followed by exposure to a lowest concentration (3-5 mg/L).

This high concentration of auxin treatment may be involved in reprogramming of cells and reinduce embryogenic competence. Repetitive embryogenesis may be a serious problem during spontane­ous cycles of somatic embryo production when germination and further development is required.

Gene Expression Programme in Somatic Embryogenesis:

One of the most striking features of somatic embryogenesis is the successful crackdown on RNA expression in embryogenic and nonembryogenic tissues. Several striking similarities were cited in the gene products expressed in embryogenic and nonembryogenic cultures. The tissue culture conditions are typically defined as nonembryogenic. The pattern of gene expres­sion between embryogenic and nonembryogenic systems exhibit least diversity about RNA ex­pression profiles.

Limited number of changes has been recorded in protein expression pattern during somatic embryogenesis. Similarly, changes in mRNA populations take place during tran­sition from nonembryogenic to various embryo stages. Removal of auxin from the media during embryo induction triggers new profiles of gene expression that are eventually coupled to ob­serve morphogenetic events.

Applications of Somatic Embryogenesis:

Micro-Propagation Industries:

One of the most promising applications of somatic embryogenesis is large scale propagation of somatic embryos, which shows several advantages such as innumerable number of embryo production (60,000-70,000 embryos per litre of media), presence of both root and shoots meristems, easy to scale up and convert them into seedlings efficiently as far as commercial significance is considered. Somatic embryos are genetically well programmed to make a complete plant. Thus, unlike other micro-propagation systems, somatic embryogenesis avoids certain stages of micro-propagation particularly, the rooting stage.

Synthetic Seed Production:

Synthetic seeds or artificial seeds are the somatic embryos encapsulated by gel entrap­ment solution. Artificial seeds are generally produced in plant species which exhibit seed steril­ity and difficulty or slow phase of vegetative propagation.

This can be prepared by placing somatic embryos in alginate slurry (2%) as gel entrapment matrix and subsequently trans­ferred to calcium chloride (100 mM) solution to form beads in which embryos get entrapped. Artificial seeds can be stored at 4°C for a considerable period of time and used as efficient system for germplasm conservation. For regeneration, seeds can be placed in culture media or in sterile soil to facilitate germination and seedling development (Fig. 8.2).

Repetitive embryogenesis often provides innumerable number of somatic embryos, which in turn is useful in the mass production of plant propagules. Several embryo specific metabolites like seed storage proteins and lipids of industrial value can be recovered. Lack of seed tissue surrounding somatic embryos proves significant advantage for certain lipids such as α-linolenic acid present at high level in Borage seeds.

This lipid is of high commercial significance in the treatment of atopic eczema. Surprisingly, somatic embryos as an analogue of zygotic embryo also synthesize the same amount of α-linolenic acid. Similarly, jojoba plant contains high qual­ity industrial lubricant in their seeds.

Somatic embryos obtained from zygotic embryos as the explant possesses waxes identical to that of zygotic embryos. In addition, novel metabolites can be produced in somatic embryos throughout the season.

Somatic Embryos in Gene Transfer:

Somatic embryos are an ideal system for gene transfer process. This particular approach can avoid protoplast mediated regeneration of transformed plants which generally requires additional care. Moreover, protoplast mediated regenerated plants can exhibit genetic variation. Since somatic embryos maintain genetic stability, regenerated plants are not susceptible for somaclonal variation.

Somatic embryos can be transformed by incubating them in Agrobacterium solution or subjected for particle bombardment. Embryo cloning by recurrent approach is well suited for direct gene transfer to the mass of somatic embryos. The stably transformed somatic embryos can be farther subjected for recurrent embryogenic cycle to procure millions of transgenic plants.

Watch the video: Development of Embryo. Reproduction in Animals. Dont Memorise (January 2022).