Gastrulation is the process of highly coordinated cell and tissue movements whereby the cells of blastula are dramatically rearranged.

1. Developmental Biology
MSc Zoology I
Lecturer: Miss Bushra Jajja

2. Gastrulation in Sea Urchin
‟It is not birth, marriage, or death, but gastrulation, which is
truly the most important time in your life; Lewis Wolpert
(1986).
Gastrulation is the process of
highly coordinated cell and
tissue movements whereby
the cells of blastula are
dramatically rearranged.

3. Main events in Gastrulation:
➢ Cell Migration/ Movement
➢ 3 Germ layers are Formed
➢ Formation of Archenteron/ Primitive Gut
Ectoderm
● Nervous system
● Epidermal glands
● Inner ear
● Eye lens
Endoderm
● Lungs
● Inner lining of digestive
tract and associated
organs
● Liver
● Pancreas
Mesoderm
● Notochord
● Skeleton
● Gonads
● Kidneys
● Heart
● Muscles

4. 2 Phylogenetic Divisions
1. Deuterostomes 2. Protostomes

5. Basic type of Cell Movement during
Gastrulation
★ Involution
★ Invagination

6. Basic type of Cell Movement during
Gastrulation
★ Ingression
★ Delamination

7. Basic type of Cell Movement during
Gastrulation
★ Epiboly

8. Gastrulation in Sea Urchin
Why we choose Sea Urchin as an Experimental Model?
Sea Urchin has been used as an experimental specimen since
1800 after the invention of microscope.
❏ Artificial Spawning
❏ Fertilization & Rearing
❏ Embryo’s optical Transparency
❏ Simple Organization

9. Gastrulation in Sea Urchin
Sea Urchin blastula consists of a singular layer of about 1000 cells
Ingression of Primary Mesenchyme:
Vegetal side begins to thicken.
A cluster of small cells begin extending and contracting long, thin
processes called filopodia .
Dissociate and ingress into blastocoel.
Primary (skeletogenic) mesenchyme Larval skeleton

10. Gastrulation in Sea Urchin
All the cells of blastula are connected
● Mesomeres: Precursors of Future Ectoderm
● Macromeres: Precursors of Future Endoderm
● Micromeres: Precursors of Primary Mesenchymal Cells
Hyaline Layer
Basal Lamina
Outer surface
Inner Surface
Strongly bind to
each other and
to Hyaline Layer
Loosely bind to
Basal lamina
Loose affinity with Hyaline Layer and
100 folds increased affinity with Basal
lamina & extracellular matrix Fibronectin

11. Gastrulation in Sea Urchin

12. Gastrulation in Sea Urchin
Micromeres separate from Hyaline layer and drawn in by basal
lamina
Migrate into blastocoel
Migrate along the extracellular matrix of
Blastocoel wall, extend their filopodia in
front of them.
Primary mesenchyme cells
arrange themselves in a ring at a
specific position along the
animal-vegetal axis
Near the future ventral
side PMC fuse together
and initiate calcium
carbonate spicule
formation
Dorsal- Ventral &
Animal- Vegetal
patterning cues
for ectoderm are
picked by
exploring filopodia

13. Gastrulation in Sea Urchin
First stage of archenteron Invagination
➔ Non mesenchymal cells move to fill the gap created by Primary mesenchymal
cells.
➔ Vegetal pole bends inwards and invagiate one forth to one half the way into
blastocoel.
➔ Invagination ceases and the invaginated region is called blastopore.
➔ Hyaline layer consists of outer and inner lamina. Microvilli from the vegetal plate cells extend
through the hyaline layer and their cytoplasm contains secretory vesicles that store a chondroitin
sulfate proteoglycan (CSPG). The storage granules secrete CSPG into the inner lamina of the hyaline
layer. The CSPG absorbs water and swells the inner lamina, while the outer lamina, to which it is
attached, does not swell. This causes the bending of the hyaline layer and its attached epithelium
inward.

14. Gastrulation in Sea Urchin

15. Gastrulation in Sea Urchin
Second phase of Archenteron Formation
➔ After a brief pause 2nd phase begins
➔ This is the process of Extension: the wide, short gut rudiment is transferred into
long, thin tube.
➔ Convergent extension→cells intercalate to narrow the tissue, and at the same time move it forward
➔ Cell Division +
More endodermal and secondary mesenchymal cells are produced.

16. Gastrulation in Sea Urchin
Second phase of Archenteron Formation

17. Gastrulation in Sea Urchin
Third Stage of Archenteron Elongation
➔ Where the archenteron contacts the wall, a mouth is eventually formed.
➔ The mouth fuses with the archenteron to create a continuous digestive
tube. Thus, the blastopore marks the position of the anus.
➔ At the stage when the skeletogenic mesenchyme cells begin ingressing
into the blastocoel, the fates of the vegetal plate cells have already been
specified.
➔ The endodermal cells adjacent to the micromere-derived mesenchyme
become foregut, migrating the farthest distance into the blastocoel.
➔ The next layer of endodermal cells becomes midgut, the last
circumferential row to invaginate forms the hindgut and anus.