GASTRULATION

GASTRULATION

gastrula, from the Greek word gaster, ..meaning stomach The 2nd major phase of embryonic dvtThe embryo is transformed from hollow ball of cells (the blastula) into a 3-layered stage  Gastrula A dramatic re-arrangement of the cells of the blastula Cells are given new positions and new neighbours, and the multilayered body plan of

the organism is established At this stage,cells that will form the endodermal and mesodermal organs are brought inside the embryo, whilethe cells that will form the skin and nervous system are spread over its outside surface.3 germ layers

–outer ectoderm, inner endoderm, and interstitial mesoderm—are first produced during gastrulation The movements of gastrulation involve the entire embryo.

Cell migrations in one part of the gastrulating embryo must be intimately coordinated with other movements occurring simultaneously The patterns of gastrulation vary enormously throughout the animal kingdom…….but  there are basic types of cell movements The gastrulation of any particular

organism is a collection of several

of the following cell movements

Types of cell movements during gastrulation

Gastrulation usually involves some combination of these types of movements

Invagination: The infolding of a region of cells, much like the indenting of a soft rubber ball when it is poked. (cell movt) Involution: The inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells. (cell movt) Ingression: The migration of individual cells from the surface layer into the interior of the embryo. (cell movt)•Delamination: The splitting of one cellular sheet into two more or less parallel sheets. Epiboly:  The movement of epithelial sheets (usually of ectodermal cells) that spread as a unit, rather than individually, to enclose the deeper layers of the embryo. (cell movt) Gastrulation process will result into formation of a three-layered embryo with a primitive gut (the archenteron)The three cell layers will later develop into all the parts of the adult animal.………. How does a cell know what it is supposed to grow up? •Induction is the process during which individual cells are "told" what they are supposed to become. Spemann and his graduate student Hilde Mangold (1898–1924) demonstrated that specific cells of the blastopore are the only determining region.Further experimentation demonstrated that during gastrulation cells became committed to their developmental fates.

GASTRULATION in Sea Urchin

The late sea urchin blastula consists of a single layer of about 1000 cells that form a hollow ball, somewhat flattened at the vegetal end.The process of gastrulation in sea urchin develops to the pluteus larva stage

The process of gastrulation in sea urchin.

A. Ingression of primary mesenchyme ;Thickenning & flattlenning of the vegital pole to form a vegital plate (vp) Cells at the centre of the vp beginns to extend & contract long by the process called filopodia;The cells dissociate to form the epithelial monolayer These cells, derived from the micromeres, are called the primary mesenchyme (pm) The ingression of the micromere descendants into the blastocoel is caused by these cells losing their affinity for their neighbours and for the hyaline membrane and acquiring a strong affinity for a group of proteins that line the blastocoel

B. First stage of archenteron invagination ;Cells remain after the pm has left undergoes some important changes.They remain bound to one another and to the hyaline layer of the eggs They move to fill the gaps caused by the ingression of the pm.The vegetal plate bends inward and invaginates into the blastocoel to form the archenteron & the opening called blastopore

C. Second and third stages of archenteron invagination  The 2nd phase of archenteron formation begins after a brief pause from the 1st stage,The archenteron extends dramatically, sometimes tripling its length.The wide, short gut rudiment is transformed into a long, thin tube;Here the cells of the archenteron rearrange themselves by migrating over one another and by flattening themselves. This phenomena is known as convergent extension. (cell movt) Moreover, cell division continues, producing more endodermal and secondary mesenchyme cells as the archenteron extends

Cell rearrangement during the extension of the archenteron in sea urchin embryos

The early archenteron has 20 to 30 cells around its circumference

The archenteron has a circumference  6 to 8 cells.

2nd & 3rd stage……cont’d

The last phase is initiated by the tension provided by secondary mesenchyme cells, which form at the tip of the archenteron,Filopodia are extended from these cells through the blastocoel fluid to contact the inner surface of the blastocoel wall,The filopodia attach to the wall at the junctions between the blastoderm cells and then shorten, pulling up the archenteron.As the top of the archenteron meets the blastocoel wall in the target region, the secondary mesenchyme cells disperse into the blastocoel, where they proliferate to form the mesodermal organs

Where the archenteron contacts the wall, a mouth is eventually formed

GASTRULATION in Frogs 

At specific region below the equator the blastoderm cells assume an elongated bottle like shape.They move toward the interior of the blastula. As the cells move further inside, an invagination happens.A deepening of invagination results in a cavity called archenteron or gastrocoel.The opening of the archenteron on the surface of blastula is called blastopore.The region dorsal to the blastoporal opening is called dorsal lip and on the ventral edge is a ventral lip.The surface cells representing several prospective zones of the embryo begin to wander inside through the blastopore.These inwandering of cells is termed as involution.

While the exterior mesodermal cell involute inside, their place is taken up by the ectodermal cells.The expansion of the ectoderm is due to epiboly.The blastopore is gradually covered by certain endodermal cells. The closing cells of the blastopore constitute the yolky plug. Gradually the yolky plug withdraws to the interior and the blastopore gets reduced into a narrow slit.

GASTRULATION in Frogs

REFERENCES

Gilbert,S.F(2013).Develepmental Biology,Sanderland Mass,Sineaur Association Inc

Slack,J.M.W(2013).Essential Develepmental Biology OXFORD, Willey-Blackwell

Wolpert.l and Tickle.c (2011),Principles of Develepmental Oxford and New York University Press