1. Gastrulation

2. Cell movement and reorganization within the embryo (morphogenetic movement) to the interior of the embryo forming 3 primary germ layers.

3. Gastrulation 1 In sea star and amphioxus
cells from blastula wall invaginate
eventually meet opposite wall, forming archenteron

4. Blastocoel Archenteron Blastopore 50 µm 50 µm 50 µm (f) Early gastrula
Figure 50.3: LMs showing sea star development.
(a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star.
Archenteron
Blastopore
50 µm
50 µm
50 µm
(f) Early gastrula
(d)
16-cell stage
(e) Blastula
Fig (d-f), p. 1084

5. Archenteron Mouth Anus Stomach Blastopore 1 mm 50 µm 50 µm (h)
Figure 50.3: LMs showing sea star development.
(a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star.
1 mm
50 µm
50 µm
(h)
Sea star larva
(i)
Young sea star
(g)
Middle gastrula
Fig (g-i), p. 1084

6. Archenteron Ectoderm Endoderm Blastopore Fig. 50-4 (h-j), p. 1085
Figure 50.4: Cleavage and gastrulation in amphioxus.
As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed.
Endoderm
Blastopore
Fig (h-j), p. 1085

8. Gastrulation 2 In the amphibian
invagination at vegetal pole obstructed by large, yolk-laden cells
cells from animal pole move down over yolk-rich cells and invaginate, forming dorsal lip of the blastopore

9. The Gray Crescent Of an amphibian zygote
determines body axis of embryo

11. Gastrulation 3 In the bird invagination occurs at the primitive streak
no archenteron forms

12. Cleavage in Birds What kind of cleavage occurs in birds?
discoidal meroblastic

13. Cleavage in Birds
What is the area pellucida, area opaca and subgerminal cavity?
pellucida – one-cell thick layer in center of blastoderm
opaca – peripheral ring of blastoderm cells that contain deep cells
subgerminal cavity – fluid-filled space between blastodisc and yolk

14. Cross sections through Primitive streak
AO = area opaca, AP = area pellucida
P = region of thick blastoderm that marks the future posterior region of the embryo; area where gastrulation begins
Arrows = movement of rapidly dividing cells of epiblast layer
AO = area opaca, AP = area pellucida, P= posterior
The primitive streak is the region where epiblast cells converge (solid arrows) and move through a groove into the blastocoel (dashed arrows). The primitive steak consists of a lightly staining groove bordered on each side by darker staining ridges. Hensen's node marks the anterior of the streak and P marks the posterior.
Cross sections through Primitive streak
Key to Upper Cross section: AO = area opaca, . AP = area pellucida, ^^ = remnants of yolk granules, arrows = direction of ingression of embryonic cells
Lower Cross Section shows the three germ layers formed by the process of gastrulation

15. Gastrulation in Birds How is the two-layered blastoderm formed?
ingression of epiblast cells in to subgerminal cavity and migration of marginal zone cells form hypoblast
embryo comes from epiblast; hypoblast forms some parts of extraembryonic membranes (yolk sac)

16. Gastrulation in Birds What is the primitive streak?
seen in birds reptiles and mammals
establishes axes of embryo
cells ingress through primitive streak into blastocoel

17. Gastrulation in Birds What is important about the primitive groove?
the primitive knot or Henson’ node?
How does the primitive streak define the embryonic axes?
depression in streak; groove – migrating cells enter here; node – similar to dorsal lip and embryonic shield
extends posterior to anterior; cells move from dorsal to ventral side and is dividing line for left-right

18. Gastrulation in Birds
What is the fate of cells migrating through the primitive streak?
first cells through Henson’s node migrate anteriorly – pharyngeal endoderm – displace hypoblast cells
hypoblast becomes confined to anteriormost part of area pellucida (germinal crescent)
next cells move anteriorly but not as ventrally – head mesenchyme, prechordal plate mesoderm – will form head process
then cells form chordamesoderm
cells passing through more posterior parts of streak –endoderm & extra-embryonic membranes and mesoderm

19. Gastrulation in Birds What happens to the primitive streak?
regresses; Henson’s node move posteriorly- formation of notocord
anterior end of embryo develops faster than posterior

20. Gastrulation in Birds
What is needed to complete the process of gastrulation?
epiboly of ectoderm – yolk becomes enclosed by ectoderm

21. links
Human cleavage
Sea urchin
Frog