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13.12: Embryological Development - Biology

13.12: Embryological Development - Biology



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Learning Objectives

  • Compare and contrast the embryonic development of diploblasts and triploblasts, and protostomes and deuterostomes

Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that they are predetermined to develop into the animal’s specialized tissues and organs. Animals develop either two or three embryonic germs layers (Figure 1). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called diploblasts. Diploblasts have a non-living layer between the endoderm and ectoderm. More complex animals (those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called triploblasts.

Practice Question

Which of the following statements about diploblasts and triploblasts is false?

  1. Animals that display radial symmetry are diploblasts.
  2. Animals that display bilateral symmetry are triploblasts.
  3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
  4. The mesoderm gives rise to the central nervous system.

[reveal-answer q=”815922″]Show Answer[/reveal-answer]
[hidden-answer a=”815922″]Statement d is false.[/hidden-answer]

Each of the three germ layers is programmed to give rise to particular body tissues and organs. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.

Presence or Absence of a Coelom

Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. It houses many organs such as the digestive system, kidneys, reproductive organs, and heart, and contains the circulatory system. In some animals, such as mammals, the part of the coelom called the pleural cavity provides space for the lungs to expand during breathing. The evolution of the coelom is associated with many functional advantages. Primarily, the coelom provides cushioning and shock absorption for the major organ systems. Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.

Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or coelomates) (Figure 2). A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms, and those animals are called pseudocoelomates. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on certain features of their early embryological development.

Embryonic Development of the Mouth

Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as chordates but also some simple animals such as echinoderms. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from the Greek word meaning “mouth first,” and deuterostome originates from the word meaning “mouth second” (in this case, the anus develops first). The mouth or anus develops from a structure called the blastopore (Figure 3).

The blastopore is the indentation formed during the initial stages of gastrulation. In later stages, a second opening forms, and these two openings will eventually give rise to the mouth and anus (Figure 3). It has long been believed that the blastopore develops into the mouth of protostomes, with the second opening developing into the anus; the opposite is true for deuterostomes. Recent evidence has challenged this view of the development of the blastopore of protostomes, however, and the theory remains under debate.

Another distinction between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. The coelom of most protostomes is formed through a process called schizocoely, meaning that during development, a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.

The earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo spiral cleavage, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of the cleavage. Deuterostomes undergo radial cleavage, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles.

There is a second distinction between the types of cleavage in protostomes and deuterostomes. In addition to spiral cleavage, protostomes also undergo determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A cell does not have the ability to develop into any cell type. In contrast, deuterostomes undergo indeterminate cleavage, in which cells are not yet pre-determined at this early stage to develop into specific cell types. These cells are referred to as undifferentiated cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.

Try It

One of the first steps in the classification of animals is to examine the animal’s body. Studying the body parts tells us not only the roles of the organs in question but also how the species may have evolved. One such structure that is used in classification of animals is the coelom. A coelom is a body cavity that forms during early embryonic development. The coelom allows for compartmentalization of the body parts, so that different organ systems can evolve and nutrient transport is possible. Additionally, because the coelom is a fluid-filled cavity, it protects the organs from shock and compression. Simple animals, such as worms and jellyfish, do not have a coelom. All vertebrates have a coelom that helped them evolve complex organ systems.

Animals that do not have a coelom are called acoelomates. Flatworms and tapeworms are examples of acoelomates. They rely on passive diffusion for nutrient transport across their body. Additionally, the internal organs of acoelomates are not protected from crushing.

Animals that have a true coelom are called eucoelomates; all vertebrates are eucoelomates. The coelom evolves from the mesoderm during embryogenesis. The abdominal cavity contains the stomach, liver, gall bladder, and other digestive organs. Another category of invertebrates animals based on body cavity is pseudocoelomates. These animals have a pseudo-cavity that is not completely lined by mesoderm. Examples include nematode parasites and small worms. These animals are thought to have evolved from coelomates and may have lost their ability to form a coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.


13.12: Embryological Development - Biology

Homologies: developmental biology

Studying the embryological development of living things provides clues to the evolution of present-day organisms. During some stages of development, organisms exhibit ancestral features in whole or incomplete form.

Snakes have legged ancestors.
Some species of living snakes have hind limb-buds as early embryos but rapidly lose the buds and develop into legless adults. The study of developmental stages of snakes, combined with fossil evidence of snakes with hind limbs, supports the hypothesis that snakes evolved from a limbed ancestor.

Above left, the Cretaceous snake Pachyrhachis problematicus clearly had small hindlimbs. The drawing at right shows a reconstruction of the pelvis and hindlimb of Pachyrhachis.

Baleen whales have toothed ancestors.
Toothed whales have full sets of teeth throughout their lives. Baleen whales, however, only possess teeth in the early fetal stage and lose them before birth. The possession of teeth in fetal baleen whales provides evidence of common ancestry with toothed whales and other mammals. In addition, fossil evidence indicates that the late Oligocene whale Aetiocetus (below), from Oregon, which is considered to be the earliest example of baleen whales, also bore a full set of teeth.


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).


Chicken stages - Hamburger & Hamilton staged the chicken embryo in 1951. The Hamburger Hamilton Stages are most commonly used series for chicken staging. The original paper had approx 25 citations between 1955 - 59, while in the year 1991 alone there were over 300 citations. Series of Embryonic Chicken Growth. J. Morphology, 88 49 - 92 (1951). Atlas recently republished by J.R. Sanes in Developmental Dynamics 195 229-275 (1993).

Original 1951 paper (and all data) was republished in 1992. <pubmed>1304821</pubmed> PDF

Note that there was also an earlier Witschi staging, and a 1900 staging series by Franz Keibel and Karl Abraham Ζ] , and an earlier (1883) series by Foster, Balfour, Sedgwick, and Heape. Η]


Watch the video: General Embryology Review in 20 minutes (August 2022).