Early Amphibian Development
Cleavage
to Gastrulation
Cleavage
•
Holoblastic and radially
symmetrical
•
Yoke is concentrated in
the vegetal hemisphere - impedes cleavage
•
First cleavage begins in
the animal pole - moves slowly to the vegetal pole
•
Second cleavage is at a
right angle to the first
Early
Cleavage
Cleavage
•
Third cleavage is
equatorial - but not a the equator because of the dense yolk
–
Results in 4 small
blastomeres (micromeres) & 4 large blastomeres (macromeres)
•
Two major regions
develop
–
Animal region - rapid
divisions & small cells
–
Vegetal region - fewer
yolk laden macromeres
Overview
of Cleavage
Morula to
Blastula
•
16 to 64 cell stages -
morula
•
At 128 cell stage -
blastula
•
Blastocoel is apparent
–
Actually the blastocoel
can be traced back to the first cleavage - expands with each successive
division
–
Functions
•
Permits cell migration
•
Prevents cells beneath it
from prematurely interacting with
the cells above
–
Numerous cell adhesion
molecules hold blastula together - EP-cadherin is a major player
Formation
of the Blastocoel
With and
Without Cadherin mRNA
Amphibian
Gastrulation
•
Starts on the future
dorsal side of the embryo
•
Cells invaginate - form
a slit-like blastopore
•
Bottle cells (called
this because of their shape) line the archenteron as it forms
•
This beginning region is
called the marginal zone - where the animal and vegetal hemispheres meet
Early
Gastrulation
Amphibian
Gastrulation
•
Next is involution of
the marginal zone cells - at the same time animal cells undergo epiboly and
convergence at the blastopore
•
When these cells reach
the dorsal lip of the blastopore - turn inward & travel along the inner
surface of the outer animal hemisphere
–
Thus lip cells
constantly change
Amphibian
Gastrulation
•
The bottle cells are the
first cells of the dorsal lip of the blastopore - form the leading edge of the
archenteron
–
Become foregut cells
•
Next dorsal lip becomes
composed of cells that will be mesoderm & eventually the cells that will be
the notochord
•
The blastopore will
eventually form a ventral lip & will be a ring around endodermal cells -
the yoke plug
Overview
Gastrulation
Invagination
and Involution
•
Gastrulation begins -
when marginal endodermal cells on the dorsal surface of the blastula sink into
the embryo
•
The outer or apical
surface of these cells contract while the inner (basal) surface expands - this
forms a “bottle shape” to the cells
•
Major factor in
the movement of cells into the embryo - involution of subsurface marginal
cells - not superficial ones
Convergent
Extension of Dorsal Mesoderm
•
Involution begins
dorsally, led by the pharyngeal endomesoderm and the prechordal plate (will be
head mesoderm)
•
Next mesoderm that forms
the heart, kidney and general ventral mesoderm enters the dorsal blastopore lip
•
Convergent extension
of mesoderm - several streams
of mesoderm fuse - now a long narrow band moving toward the animal pole
Epiboly
of Ectoderm
•
While involution is
occurring at the blastopore lips - ectodermal precursors are expanding over the
entire embryo
•
Mechanism of epiboly
appears to be simply an increase in cell number - while several deep layers are
integrated into one
Early
Movements - Gastrulation
Review of
Gastrulation Movements