CLEAVAGE AND GASTRULATION IN AMPHIBIANS

Amphibian Cleavage

• Radially symmetrical, holoblastic • but unlike sea urchin, mesolecithal egg • Yolk is concentrated in vegetal pole• Cell divisions are slower in the vegetal hemisphere• First cleavage bisects the grey crescent– Second cleavage begins in animal pole, while

first cleavage is not yet complete in vegetal pole– As in sea urchins, there are no Gap phases in

the cell cycle to allow for rapid divisions

• First & Second cleavage – Meridional– At right angle to first one and also Meridional

• Third cleavage– Equatorial (but not actually at the equator)– Divides the embryo into 4 small– micromeres, 4 large micromeres

• As cleavage continues _ animal pole packed with many small cells

-vegetal pole has fewer large yolk-laden cells

• At 16-64 cells, embryo is called a morula – Solid ball of cells

• At 128 cell stage, embryo is a blastula– Open cavity called blastocoel has appeared in animal pole

• FUNCTION OF BLASTOCOEL1. Permits cell migration during gastrulation2. Prevents cells below from interacting with the cells above

prematurely.

Amphibian Gastrulation

• Different in different species• Goals–Bring endoderm cells to the inside of the

embryo–Allow ectoderm cells to coat the outside of

the embryo–Position mesoderm cells in between

Fate-maps Fate-mapping of blastula stage embryos has

p provided some insight– Using vital dyes to mark cells– Superficial layers of embryo form ectoderm and endoderm–Mesoderm lie mostly in the deeper layers of cells– Surface of animal hemisphere will become cells of ectoderm– Vegetal hemisphere will form cells of gut and associated organs –Mesodermal cell will form internal cytoplasm around equator

Cell Movements in Amphibian Gastrulation

• Gastrulation begins on future dorsal side–Below the equator, in region of grey

crescent–Cells invaginate to form a slit like blastopore –Dorsal lip of blastopore will become

important organizing region of embryo (Spemann organizer)–Cells become elongated as they contact the

inner surface (Bottle cells)

• Bottle cell line the archenteron as it forms• Invagination of cells initiate archenteron

formation• Gastrulation begins at marginal zone ,not at

vegetal zone as in sea urchin

Cell Movements in Amphibian Gastrulation

• Next steps:– Involution of the cells at the marginal zone

(outer sheet spreads over inner sheet)–Cells from Animal pole undergo epiboly • Converge at the blastopore• When reach blastopore, travel inward

–Bottle cells continue to migrate, form leading edge of archenteron (primitive gut)

Amphibian Gastrulation

• Cells from the dorsal lip (the first cells that migrated inward) become prechordal plate (will form head mesoderm)

• Next cells that involute form chordamesoderm (will become notochord)– Important for patterning the nervous system

• Next yolk plug formation

• YOLK PLUG : Yolk plug is the remaining patch

of endodermal cells that is created during the formation of the dorsal lip of the blastopore which remains exposed on the vegetal surface of the blastula that will eventually be internalized by epiboly.

Dorsal lip of the blastopore

yolk plug

ventral lip of the blastopore.

Amphibian Axis Formation and “The Organizer”

• Amphibian gastrulation and axis formation are an example of regulative development

• Inductive interactions occur between cells• This was demonstrated by Hans Spemann

and Hilde Mangold – Nobel Prize winners

Spemann and Mangold

• Performed many types of transplants at the early gastrula and late gastrula stages in the newt embryo

• These experiments showed that in most cases, the cells of the embryo are not committed until at least the late gastrula stage

• But - There is ONE tissue from the early gastrula that is already committed. . .

CONDITIONAL DEVELOPMENTEarly newt gastrula cells were not committed to a specific fate .such cells are said to exhibit conditional development

AUTONOMOUS DEVELOPMENTLate newt gastrula cells that were committed to specific fate such cells are said to exhibit autonomous development

• There is ONE tissue from the early gastrula that is already committed is dorsal lip of blastophore,the tissue derived from the gray crescent cytoplasm• When this tissue transplanted into presumptive belly skin region of another gastrula not only continued to form blastopore lip • but also initiated gastrulation and embryogenesis in the surrounding tissue• Two conjoined embryos were formed instead of one