This document discusses cell movement during amphibian gastrulation. It begins by introducing gastrulation and how it establishes the three germ layers and body plan. There are a few basic types of cell movement during gastrulation: invagination, involution, ingression, delamination, and epiboly. The document then focuses on amphibian gastrulation, noting that bottle cells first sink in and the blastopore forms opposite the sperm entry point. Involution begins dorsally and the mesoderm enters through the expanding blastopore lip through the end of gastrulation.

1. LS-403(Z)

2. CONTENTS
 INTRODUCTION
 DIFFERENT TYPE OF CELL MOVEMENT
 CELL MOVEMENT DURING AMPHIBIAN GASTRULATION
 REFERENCES

3.  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

4.  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.

5.  • 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.

7. 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.

8. 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.

9.  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

10.  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.

11.  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.

12. 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.

13. 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).