Gastrulation means all processes and events that transform a blastula into a gastrula.
Besides the formation of the archenteron, this process also involves the formation of 3 primordial germ layers which are ectoderm, endoderm, and mesoderm.
According to Balinsky(1970), “The process of gastrulation involves displacement of parts of the early embryo, in which the single layer of cells called blastoderm gives rise to three germinal layers, the ectoderm, endoderm, and mesoderm“.
Process of Gastrulation
It is essentially a process of movement of the parts of an embryo. As a result of these movements, new structural elements such as the archenteron, the neural tube, and the notochord are formed in place of the blastoderm.
All this happens through the process of morphogenesis. Gastrulation involves the following morphogenetic movements:
- Epiboly– It involves the gradual growth of the blastopolar lip over the yolk or the yolk-filled vegetal cells.
- Emboly-This morphogenetic movement involves the growth of endoderm under the ectoderm forming regions. This occurs differently by the following methods:
- Involution or inflection,
- Extension or elongation,
Gastrulation is a dynamic process.
The blastomeres move actively from one place to another to take definite places in the embryo.
The moving cells arrange themselves to form definite cell layers. These cell layers are called germ layers.
Thus, gastrulation is the transformation of the single-layered blastula into a two-layered or three-layered gastrula.
Gastrulation is brought about the morphogenetic movements. These movements are of two types. They are epiboly and emboly.
Epiboly is the movement of blastomeres of an animal pole (micromeres) and covers the megameres. It includes convergence, extension, etc.
Emboly is the movement of megameres into the interior of the embryo. It includes invagination, involution, divergence, etc.
Several changes or process taking place simultaneously during gastrulation are as described below:
A fold of rapidly dividing pigmented cells of micromeres of animal pole gradually grows over the yolky cells or megameres of the vegetal or vegetative pole. This process of overgrowth is called epiboly.
(b) Formation of Blastopore
At the beginning of epiboly, a small crescentic groove appears posterodorsally on blastula, a little behind the edge of a grey crescent in the presumptive endoderm.
Its anterior pigmented margin is called the dorsal lip of the blastopore. Its backwardly projecting lateral margins are called the lateral tips.
As epiboly progresses, the lateral lips finally meet below forming the ventral lip.
Thus, the crescentic, groove becomes a complete circle, or blastopore, through which is visible a tiny white spot of yolky endodermal cells, called the yolk plug. It is the place where megameres have not been covered by micromeres.
(c) Invagination of Endoderm
As the prospective ectoderm cells or micromeres advance, the future endoderm cells or megameres gradually migrate towards the blastopore and gradually sink inside.
With the completion of the blastopore, the whole of yolky megameres or future endoderm becomes invaginated inside the embryo.
(d) Foramtion of archenteron
With the inturning of megameres the earlier crescentic groove gradually grows inward into a new cavity, the archenteron or primitive gut, which opens to outside through blastopore.
As a result of inwardly rotating endodermal cells, the blastocoel becomes gradually reduced and finally obliterated.
The advancing future notochord cells or chorda cells become inflected or turned inside round over the dorsal lip of the blastopore.
They extend beneath the neural plate cells which however remain on the surface. With the formation of lateral lips of the blastopore, the future mesoderm cells also roll inside over the lateral lips.
Internally they occupy positions on either side of chorda cells between the surfaces of the epidermis and endoderm.
In a completed gastrula, the blastocoel is obliterated. Archenteron is well developed and opens to the outside through blastopore later reduced to a slit (proctodaeum).
The roof of the archenteron is made of notochord cells and sides of mesoderm cells.
The floor of the archenteron contains a mass of yolky endoderm cells visible through the blastopore as a yolk plug.
The outer layer of pigmented cells consists of ectoderm and neural plate. The formation of the gastrula is now complete.
Development After Gastrulation or Post Gastrular Development
During this process, a neural tube is formed and the embryo in the stage is called a neurula.
The cells of the presumptive neural plate in the gastrula now enlarge and get distinguished from the cells of the remaining epidermis.
Towards the end of gastrulation, the presumptive neural plate area, running forwards from the blastopore, becomes thickened forming a mid-dorsal pear-shaped medullary or neural plate and which forms the nervous system.
A fold is formed at either end of this plate and soon the plate deepens to form a groove. These folds are called neural folds and the groove is known as the neural groove.
Gradually, the neural folds grow up and meet each other as a result of which the neural groove takes the form of a tube called a neural tube that extends from one end of the embryo to the other.
The neural tube gradually forms the brain and spinal cord. Its anterior part enlarges and then narrows at three places to form primary cerebral vesicles which develop into the fore, mid, and hindbrain, respectively.
The rest of the neural tube forms the spinal cord. A neural crest is present on the lateral sides of the neural tube.
This crest is formed by the ectodermal cells and later on GROOVE forms dorsal nerve roots and autonomic ganglia.
The neural tube is attached to the archenteron by a narrow canal called the neurenteric canal which opens to the outside by a pore known as a neuropore. This stage of the embryo is called neurula.
Some cells of mesendoderm, called Chordata mesoderm, separate from it as a group to form the notochord. These cells are large in size and possess vacuoles.
After some time two strong fibrous layers are formed around them. The outer layer is elastic and the inner is fibrous.
These two layers form the covering of the notochord called a notochordal sheath.
Thus, the notochord is formed as a solid, rod-like structure extending from one end of the embryo to the other differentiated into three fairly distinct zones, (a) Dermatome, (b) Myotome, and (c) Sclerotome.