CONTENTS INTRODUCTION DIFFERENT TYPE OF CELL MOVEMENT CELL MOVEMENT DURING AMPHIBIAN GASTRULATION REFERENCES
The blastula consists of numerous cells, the positions of which were established during cleavage. During gastrulation, these cells are given new positions and new neighbors, and the multi-layered body plan of the organism is established. The cells that will form the endodermal and mesodermal organs are brought to the inside of the embryo, while the cells that will form the skin and nervous system are spread over its outside surface. Thus, the three germ layers—outer ectoderm, inner endoderm, and interstitial mesoderm— are first produced during gastrulation. In addition, the stage is set for the interactions of these newrly positioned tissue
Although patterns of gastrulation vary enormously throughout the animal kingdom, there are only a few basic types of cell movements: • Invagination: The infolding of a region of cells, much like the indenting of a soft rubber ball when it is poked. • Involution: The in turning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.
• Ingression: The migration of individual cells from the surface layer into the interior of the embryo. The cells become mesenchymal (i.e., they separate from one another) and migrate independently. • Delamination (Intercalation). The splitting of one cellular sheet into two more or less parallel sheets. While on a cellular basis it resembles ingression, the result is the formation of a new sheet of cells. • 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. Epiboly can occur by the cells dividing, by the cells changing their shape, or by several layers of cells intercalating into fewer layers. Often, all three mechanisms are used.
The first precondition for gastrulation is the activation of the genome. In Xenopus, the nuclear genes are not transcribed until late in the twelfth cell cycle. At that time, different genes begin to be transcribed in different cells, and the blastomeres acquire the capacity to become motile.
The vegetal cells are critical in determining the location of the blastopore, as is the point of sperm entry. The microtubules of the sperm direct cytoplasmic movements that empower the vegetal cells opposite the point of sperm entry to induce the blastopore in the mesoderm above them. This region of cells opposite the point of sperm entry will form the blastopore and become the dorsal portion of the body.
Amphibian gastrulation is first visible when a group of marginal endoderm cells on the dorsal surface of the blastula sinks into the embryo. The outer (apical) surfaces of these cells contract dramatically, while their inner (basal) ends expand. The apical-basal length of these cells greatly increases to yield the characteristic "bottle" shape. In salamanders, these bottle cells appear to have an active role in the early movements of gastrulation
Involution begins dorsally, led by the pharyngeal end mesoderm and the prechordal plate. These tissues will migrate most anteriorly beneath the surface ectoderm. The next tissues to enter the dorsal blastopore lip contain notochord and somite precursors. Meanwhile, as the lip of the blastopore expands to have dorsolateral, lateral, and ventral sides, the prospective heart mesoderm, kidney mesoderm, and ventral mesoderm enter into the embryo.
As mesodermal movement progresses, convergent extension continues to narrow and lengthen the involuting marginal zone. The IMZ contains the prospective endodermal roof of the archenteron in its superficial layer (IMZS) and the prospective mesodermal cells, including those of the notochord, in its deep region (IMZD). During the middle third of gastrulation, the expanding sheet of mesoderm converges toward the midline of the embryo.
The major mechanism of epiboly in Xenopus gastrulation appears to be an increase in cell number (through division) coupled with a concurrent integration of several deep layers into one.
REFERENCES: Gilbert, Scott. F. Developmental Biology, 7th edition, Sinauer Associates, Inc., Publisher. P-(221-258). Calson, M. Bruce. Foundations of Embryology, 6th edition, Tata McGraw-Hill Publishing Company Ltd. P- (189-226). Slack, J.M.W. From Egg to Embryo, 2nd edition, Cambridge University Press. P-(149-153).