Embryogenesis

Embryogenesis

Formation of embryo from zygote includes following stages :

• Cleavage
• Blastulation
• Gastrulation
• Organogenesis

Cleavage (or Segmentation)

It is the mitotic cell division in zygote with following special features :

• Cell division without growth phase hence the cell number increases but size of embryo remains the same as that of zygote.
• The nucleo-cytoplasmic ratio of the resultant cell after every division increases.
• Early few division in all blastomeres are synchronous (except in human).
• Rapid division is complemented by very fast DNA synthesis, but in human cleavage divisions are slowest in the animal kingdom.
• O₂ consumption is very high for high rate of metabolism.
• The rate of division depends upon the species (of animal) and the temperature.

Process of Cleavage :

The first cleavage furrow appears at animal-vegetal axis (meridional)

2^nd division is also meridional but at right angle to the first one forming four blastomeres.

3^rd division is equatorial resulting into eight blastomeres.

4^th furrow is again meridional forming 16 cell stage called morula.

5^th furrow is equatorial though this may differ in different animals.

The synchrony of division is maintained upto 64 cell stage.

Types of Cleavage :

This primarily depends upon the type of egg

Holoblastic

The cleavage furrow divides the zygote (or blastomere) completely.

• Holoblastic equal : The blastomeres are divided equally in size, occurs in isolecithal (or microlecithal) eggs.
• Holoblastic unequal : On equatorial plane blastomeres are divided unequally due to higher amount of yolk on vegetal side. Hence, the blastomeres on this side are bigger (macromeres) and on animal pole side are smaller (micromeres).

 Meroblastic

The cleavage furrow in macrolecithal egg does not divide the zygote completely and is restricted to only animal pole. It is of two types :

• Meroblastic discoidal : The division is restricted to animal pole side resulting in only plate like structure e.g., In polylecithal eggs of reptiles and birds.
• Meroblastic superficial : This cleavage results in a superficial layer of blastomeres around the central yolk e.g.g., centrolecithal eggs of insects.

Fig. Types of Cleavage and the resulting blastula and gastrula

Pattern of Cleavage

• Radial : The resultant blastomeres get arranged in such a manner that the blastula acquires radial, symmetry e.g. sponges, coelenterates and some echinoderms.
• Biradial: The blastomeres get arranged on the planes of right and left sides, it is also basically of radial type e.g. animals of bilateral symmetry. These two are indeterminate cleavage as the fate of cells are not determined in this stage.

Spiral

Resultant blastomeres get arranged at different position and every new generation of cell is thus placed at the spiral plane in relation to their mother cell.

Since the fate of blastomeres are determined from the beginning, it is a determinate cleavage. e.g. some flatworms, annelids, most of molluscs and arthropods.

Blastulation

The early part of blastulation gives rise to a bunch of cells called morula (from 16 cell to 64 cell stage).

Cells of morula are similar undergoes a phase of compaction and generate two types of cells : peripheral cells and inner cell mass, the ball may be solid or it may acquire a cavity within.

Later it assumes a shape and distinction of area on it with or without cavity (= blastocoel) and this stage is called blastula.

Its types vary according to the egg type.

• Stereoblastula (solid blastula) : The blastomeres get arranged in a way that no or very small cavity is left. e.g. in sponges and Crepidula (mollusca).
• Coeloblastula :A prominent blastocoel cavity is formed within e.g. in all isolecithal (microlecithal) and mesolecithal eggs.
• Discoblastula : Blastula is always formed as a disc over the underlying yolk. e.g. Polylecithal egg.  
• Superficial blastula : Blastula develops as peripheral structure around the central yolk. e.g. in centrolecithal egg of insects.

Gastrulation

It is the morphogenetic movement (shifting) of blastomeres to form three germinal (Primary) layers ectoderm, mesoderm and endoderm with following features :

Differentiation of cells for future morphogenesis.

Characteristic and the pattern of movement vary in different species.

Formation of blastopore marks the beginning.

Formation of archenteron and closure of blastopore mark the end of gastrulation.

Cellular movement are primarily of two types :

(i) epiboly and (ii) emboly

• Epiboly : The outward movement of blastomeres which cover the embryo and form presumptive ectoderm.
• Emboly :  The inward movement of cells giving rise to neural fold, mesoderm and endoderm. It is further of various types:
• (a) Invagination : In Pushing of the entire layer of cells forming cup like structure. e.g. gastrulation of frog and Amphioxus.
• (b) Ingression : The inward movement of discrete individual cells as a result of rapid division, like formation of mesoderm in the chick embryo.
• (c) Involution : Inside out change of the entire layer. e.g. Gastrula formation of sponges.
• (d) Delamination : The separation or detachment of one layer of cells from another layer. e.g. separation of hypoblast from epiblast in the gastrulation of chick embryo.

Convergence, divergence and elongation are the specialized movement found in chick. Blastomeres of upper layer divide rapidly and converge at primitive streak. After entering by ingression they diverge to spread on both sides of primitive streak. At the same time the elongation also takes place.

From sixth division onwards there is no defined plane and regularity of divisions. Micromeres divide at faster rate while the macromeres at slower rate.

Blastulation

Coeloblastula begins to increase in size and acquire more fluid within it.

In fully formed blastula it becomes distinct in different regions from outside, so the fate map of these cells can be drawn.

The cells of respective areas are destined to form only those particular parts of the developing embryo.

Fig. Different developmental stages of frog

Gastrulation

Begins with the appearance of cut mark (blastopore) below grey crescent (future posterior side).

By epibolic movement of micromeres, the entire embryo is ultimately covered around.

The cells of grey crescent divide and start entering through blastopore by involution and ingression. These cells are presumptive (future) neural ectoderm and notochordal mesoderm.

The blastopore cut begins to increase on both lateral sides and finally making a round pore in the late gastrula with a yolk plug (of macromeres) in the midst.

Some of macromeres migrate inwards through this pore to form endoderm.

The inner layer of macromeres invaginate and form the archenteron as endodermal layer.

Blastocoel starts diminishing.

Features of late gastrula

Fully formed circular blastopore with yolk-plug.

Blastocoel disappears.

Archenteron is fully formed.

Embryo elongated at antero-posterior axis with blastopore at the posterior end.

End of gastrulation is marked by the withdrawal (disappearance) of yolk plug and closure of blastopore.

Fate of three germinal layers

As in all other chordates formation of different parts of the body from germinal layers takes place as follows:

• From ectoderm : Epidermis of skin, nervous tissue (nerves, brain), stomodaeum (mouth and pharynx) and proctodeum (cloaca), pituitary, eye lens, cornea, retina, medullary part of adrenal gland, internal ear, lateral line system and nasal epithelium.
• From mesoderm : Dermis, connective tissue, (bones, cartilage, muscles), kidney, gonads, reproductive and urinary ducts, cortical part of adrenal gland, blood vessels, heart and blood.
• From endoderm : Alimentary canal, lungs, thyroid, germinal epithelium, liver, pancreas.