Gastrulation in Frog and Chicks | Zoology Optional Notes for UPSC PDF Download

Meaning of Gastrulation

The transition from the blastula stage to the gastrula stage is facilitated by the process of gastrulation, a pivotal phase in the developmental process of an animal. Gastrulation holds immense significance as it lays the foundation for the future organizational structure of the organism.

This crucial and dynamic process involves the reorganization of major presumptive organ-forming areas within the blastula. Through gastrulation, these areas undergo a transformation that facilitates their conversion into the fundamental body plan characteristic of a species. Gastrulation is primarily characterized by the migration of cells within the embryo, accompanied by substantial nuclear differentiation.

In nearly all animals, gastrulation leads to:

(i) The establishment and differentiation of three primary germinal layers—ectoderm, mesoderm, and endoderm.
(ii) The initiation of nuclear differentiation.
(iii) The commencement of genetic factors exerting control over the developmental process.

Basic Mechanism in Gastrulation

Gastrulation comprises three key cellular activities—cell movement, cell contact, and cell division. These mechanisms are executed in a well-coordinated and integrated manner.

Morphogenetic Movement of Cells in Gastrulation

In the process of gastrulation, cells migrate within the embryo to assume their destined positions. Two terms, emboly and epiboly, which have contrasting meanings, are commonly used to describe this movement.

• Epiboly:
  • Definition: Extension along the anteroposterior axis and peripheral divergence characterize epiboly during gastrulation.
  • Explanation: Cells exhibit outward movement and extend along both the embryo's front-to-back axis and its periphery.
• Emboly:
  • Definition: The inward movement of cells during gastrulation, with various types based on the behavior of the migrating cells.
  • Explanation: This inward movement involves different behaviors of cells, leading to diverse types of embryonic cell migration.

These are:

• Invagination: Invagination refers to the folding inward of a cell layer to create a cavity surrounded by the folded cells. In the gastrulation of Amphioxus and frogs, the blastoderm's wall is pushed inside the blastocoel, forming a new cavity known as the archenteron. This cavity connects with the exterior through a blastopore. The inpushed layer continues this inward movement, ultimately forming the archenteron, which completely eliminates the blastocoel.
• Involution: Involution involves the inward movement of cells, as observed in the gastrulation of amphibian and avian eggs. Starting from one end near the blastoderm's edge, cells initiate inward movement to constitute the inner lining of the blastoderm.
• Convergence: Convergence denotes the movement of cells toward a specific region of the gastrula. In amphibian eggs, this is seen as the migration of cells to the external edge of the blastoporal lip, termed convergence.
• Divergence: Divergence is the opposite of convergence, where involuted cells diverge to assume their future positions inside the gastrula.
• Infiltration: Infiltration involves the movement of blastoderm cells near the bottom of the blastocoel to form a second layer.
• Delamination: Delamination is the separation of a group of cells from others, forming discrete cellular masses.
• Extension: Extension refers to the elongation of presumptive areas after their movement inside the embryo.
• Cell Proliferation: Cell proliferation is the increase in the number of cells during gastrulation.
• Concrescence: Similar to convergence, concrescence involves cells migrating anteriorly along one axis, but in concrescence, cells from two sides unite before moving anteriorly.

This crescent-shaped invagination, resembling a cleft, represents the dorsal lip of the blastopore. As gastrulation proceeds, this cleft undergoes expansion, transforming from a crescent shape to a semicircular appearance, eventually becoming horse-shoe-shaped and forming a ring. This ring marks the blastopore, serving as the central point for gastrulation activities.

The initiation of cell migration inside the gastrula occurs along the newly formed dorsal lip of the blastopore. The inward movement is instigated by the folding of endodermal cells, directed inward and forward toward the prospective anterior end of the embryo (see Fig. 5.19). The upper margin of the blastopore is identified as the dorsal lip, while the lower edge is termed the ventral lip.

Simultaneously, as invagination progresses within the blastocoel, cells from the upper part of the dorsal side, specifically the prechordal plate cells, move inward. This inward movement results in the creation of a new cavity called the archenteron, which communicates with the exterior through the blastopore. The continued advancement of invagination leads to the archenteron's expansion, eventually obliterating the blastocoel.

The cells moving inward form a new border beneath the outer cells. The roof of the archenteron comprises the involuted layer, encompassing both endoderm and mesoderm. Adjacent to this layer lies the ectodermal layer. The floor of the archenteron is constituted by a layer of endodermal cells, derived from the large yolk cells initially located in the vegetal hemisphere of the blastula.

As the cells undergo inward movement through the dorsal lip, a concurrent movement takes place on the outer side. The pigmented cells from the animal hemisphere initiate the enclosure of the macromeres from the vegetal hemisphere. This enclosure process continues until the outer cells reach the ventral lip.

During this process, a small mass of macromeres remains temporarily uncovered, serving as a plug for the blastopore, and it is referred to as the yolk plug. At this stage, the embryo is characterized by two distinct strata, each consisting of numerous layers of cells.

Differentiation of three primary germ-layers

The blastula of a frog is initially a mono-layered structure that undergoes gastrulation to transform into a triploblastic stage, comprising three cell layers known as the primary germ layers—embryonic ectoderm, embryonic mesoderm, and embryonic endoderm. All the organs of the developing embryo originate from these three primary germ layers.

• Ectoderm: The pigmented cells from the animal pole, which extend to envelop the macromeres of the vegetal hemisphere, differentiate into the ectoderm.
• Endoderm: The dorsal and lateral sheets of cells that constitute the roof of the archenteron serve as both endoderm and mesodermal material. Following gastrulation, a single layer of endodermal cells, derived from the thickened roof of the archenteron, lines the roof and sides of the archenteron.
• Mesoderm: Upon dorsal and lateral separation of the endodermal sheet from the involuted cells, the mesodermal sheet forms between the endoderm and ectoderm. Mesodermal differentiation initiates anteriorly and progresses posteriorly. The mesodermal sheet is divided into two halves by a narrow band of median cells that develop into the notochord. The lateral extensions of the mesodermal sheets grow downward, eventually uniting in the mid-ventral line to constitute a continuous mesodermal sheet. This distinctive feature in amphibian development involves the formation of mesoderm first, followed by the development of endoderm during gastrulation.