BIOL 1911 Chapter Notes - Chapter 47.2: Gastrulation, Amnion, Allantois
29 Apr 2018
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47.2 – Morphogenesis in animals involves specific
changes in cell shape, position, and survival
• Morphogenesis – the cellular and tissue-based processes by which an animal body takes shape that occur over the last
two stages of embryonic development
Gastrulation
• A dramatic reorganization of the hollow blastula into a two-layered or three-layered embryo called a gastrula
• the embryos of all animals gastrulate
• During gastrulation:
o a set of cells at or near the surface of the blastula moves to an inferior location
o cell layers called embryonic germ layers are established
o a primitive digestive tube is formed
• germ layers
o ectoderm forms the outer layer and endoderm forms the lining of the digestive tract in the late gastrula
o only the endoderm and ectoderm form in a few radially symmetrical animals called diploblasts
o vertebrates and other bilaterally symmetric animals are triploblasts because the mesoderm, the third germ
layers, forms between the ectoderm and the endoderm
Gastrulation in Frogs
• each embryonic germ layer contributes to a distinct set of structures in the adult animal
o ectoderm forms the nervous system and outer body layer
o mesoderm gives rise to muscles and skeleton
o endoderm lines many organs and ducts
• there are exceptions to embryonic germ layers being reflected in the adult
• details of gastrulation
o gastrulation begins when cells on dorsal side invaginate to from an indented crease (BLASTOPORE)
o dorsal lip = part above the crease
o sheet of cells begins to spread out of the animal hemisphere as the blastopore is forming, and it rolls over the
dorsal lip (involution) and moves into the interior
o these cells will form endoderm and mesoderm in the interior, with the endodermal layer on the inside
o cells at the animal pole change shape and begin spreading over the outer surface
o blastopore extends around both sides of the embryo as more cells invaginate
o then ends of the blastopore meet = blastopore forms a circle that becomes smaller as ectoderm spreads
downward over the surface
o continued involution on the internal level expands the endoderm and mesoderm
o archenteron forms and grows as the blastocoel shrinks and eventually disappears
o the cells remaining on the surface make up the ectoderm
o endoderm is the innermost layer
o mesoderm lies between the ectoderm and the endoderm
o circular blastopore surrounds a plug of yolk-filled cells
• cell movements that begin gastrulation occur on the dorsal side, opposite where the sperm entered the egg
• frog’s anus develops from the blastopore
• mouth eventually breaks through at the opposite end of the archenteron
Gastrulation in Chicks
• at the onset of gastrulation, an upper and lower layer of cells – the epiblast and hypoblast – lie atop a yolk mass
• all the cells that will form the embryo come from the epiblast
o during gastrulation, some epiblast cells move toward the midline, detach, and move inward toward the yolk
o
primitive streak
= a visible thickening produced by the pileup of cells moving inward at the midline
▪ some move down and form endoderm, pushing aside the hypoblast cells,
▪ others migrate laterally and form mesoderm
▪ cells left behind on the surface at the end of gastrulation will become ectoderm
• hypoblast cells later segregate from the endoderm and eventually form part of the sac that surrounds the yolk and also
part of the stalk that connects the yolk mass to the embryo
• the rearrangements and movements of cells in gastrulation of different species exhibit a number of fundamental
similarities
Gastrulation in Humans
• humans eggs are small and store little in food reserves
• fertilization takes place in the oviduct
• development begins while the embryo completes its journey down the oviduct to the uterous
• detailed process
1. At the end of cleavage (6 days) the embryo has more than 100 cells arranged around a central cavity and has
reached the uterus
a. Embryo at this stage = blastocyst, the mammalian version of a blastula
b. Inner cell mass = a group of cells clustered at one end of the blastocyst cavity which will develop
into the embryo proper (source of embryonic stem cell lines)
2. Implantation of the embryo is initiated by the trophoblast, the outer epithelium of the blastocyst
a. Enzymes secreted by the trophoblast during implantation break down molecules of the endometrium
(the lining of the uterus), allowing invasion by the blastocyst
b. Trophoblast also extends finger-like projections that cause capillaries in the endometrium to spill out
blood that can be captured by trophoblast tissues
c. Inner cell mass of the blastocysts forms a flat disk with an inner layer of cells, the
epiblast
, and an
outer layer, the
hypoblast
d. Human embryo develops almost entirely from epiblast cells
3. Trophoblast continues to expand into the endometrium following implantation and four new membranes
appear
a. Extraembryonic membranes arise from the embryo and enclose specialized structures located inside
the embryo
