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Gastrulation

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JITUMONI DEKA

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Elaborate the different steps of gastrula formation?

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  1. Ace The Race Geek

    Gastrulation
    Fusion of two gametes i.e. sperm and egg result in fertilization from where a new life of individual begins. Stages in the development of the new individual involve:

    After fertilization, cleavage is the next step of development which is a series of rapid mitotic divisions in which zygote cytoplasm divides into numerous smaller cells and results in blastula. After some time mitotic division slows down and blastomers (cells of blastula) undergo dramatic movements and change their relative position. This series of cell rearrangements is called as Gastrulation.

    Gastrulation result in three germ layers, the ectoderm, endoderm and mesoderm. The process of gastrulation is different in different species.

    Gastrulation in Frog (Xenopus)
    In case of frog, gastrulation begins at a point 180 degree opposite to the sperm entry with formation of a dimple, called blastopore. Gastrulation in frog embryos is initiated on the future dorsal side of the embryo, just below the equator in the region of the gray crescent. Here, the cells invaginate to form a slitlike blastopore. These cells change their shape dramatically. The main body of each cell is displaced toward the inside of the embryo while the cell maintains contact with the outside surface by way of a slender neck. These bottle cells line the archenteron as it forms. Thus, as in the gastrulating sea urchin, an invagination of cells initiates archenteron formation. However, unlike gastrulation in sea urchins, gastrulation in the frog begins not at the most vegetal region, but in the marginal zone: the zone surrounding the equator of the blastula, where the animal and vegetal hemispheres meet. Here the endodermal cells are not as large or as yolky as the most vegetal blastomeres.
    The next phase of gastrulation involves the involution of the marginal zone cells while the animal cells undergo epiboly and converge at the blastopore. When the migrating marginal cells reach the dorsal lip of the blastopore, they turn inward and travel along the inner surface of the outer animal hemisphere cells. Thus, the cells constituting the lip of the blastopore are constantly changing. The first cells to compose the dorsal blastopore lip are the bottle cells that invaginated to form the leading edge of the archenteron. These cells later become the pharyngeal cells of the fore gut. As these first cells pass into the interior of the embryo, the dorsal blastopore lip becomes composed of cells that involute into the embryo to become the prechordal plate (the precursor of the head mesoderm). The next cells involuting into the embryo through the dorsal blastopore lip are called the chordamesoderm cells. These cells will form the notochord, a transient mesodermal “backbone” that plays an important role in distinguishing and patterning the nervous system.

    Blastomeres migrate through the blastopore towards animal pole. These cells become dorsal mesoderm.

    • Now, blastospore expands to form a circle and the cells migrating through this circle became ventral and lateral mesoderm.
    • Other cells outside the circle become ectoderms and these cells expand vegetally to enclose the embryo entirely.
    • The cells present at the regetal end before the covering (by ectoderm layer) become endoderm.

    Therefore, at the end of gastrulation precess;
    • Ectoderm: present on the outside of the embryo and is the precursor of the epidermis and nerves.
    • Endoderm: present on the inner side of the embryo and is the precursor of the gut lining.
    • Mesoderm: present in between the endoderm and ectoderm layers and is the precursor for the connective tissue, blood, skeleton, gonards and kidneys.

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