Gastrulation in sea urchins

Gastrulation Process

1. Gastrulation in Sea Urchins by Tariq Yasin Dept. of Zoology (University of Education, Lahore)

2. Introduction of Sea Urchins: Sea urchins  are typically spiny, globular animals, echinoderms in the class Echinoidea . Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals.

3. Gastrulation: Gastrulation is the process during embryonic development that changes the  e mbryo from a blastula with a single layer of cells to a gastrula containing multiple layers of cells. Gastrulation typically involves the blastula folding in upon itself or dividing, which creates two layers of cells. Organisms that do not form a third layer are

4. known as diploblastic organisms, while the organisms performed the third layer are known as triploblastic organisms. Gastrulation in Sea Urchins: Gastrulation in the sea urchin embryo involves two separable morphogenetic processes: ingression of primary mesenchyme cells and invagination of the archenteron, or the primitive gut.

5. Ingression of primary mesenchyme cells Primary mesenchyme cells in sea urchin embryos emerge from the epithelium of the blastula wall and move into the blastocoel in a process referred to

6. as ingression.  Ingressing cells are bottle-shaped with their basal end protruding into the blastocoel and their apical end narrowed into a thin strand.

7. Invagination of the archenteron Invagination of the archenteron is further divided in two stages: First stage of Archenteron Invagination As the ring of primary mesenchyme cells leaves the vegetal region of the blastocoel, important changes are occurring in the cells that remain at the vegetal plate. These cells remain bound to one another and to the hyaline layer of the egg, and they move to fill

8. the gaps caused by the ingression of the primary mesenchyme. Moreover, the vegetal plate bends inward and invaginates about one-fourth to one-half the way into the blastocoel. Then invagination suddenly ceases. The invaginated region is called the archenteron (primitive gut), and the opening of the archenteron at the vegetal region is called the blastopore.

9. Second and third stages of Archenteron Invagination The invagination of the vegetal cells occurs in three discrete stages. After a brief pause, the second phase of archenteron formation begins. During this time, the archenteron extends dramatically, sometimes tripling its length. In this process of extension, the wide, short gut rudiment is transformed into a long, thin tube. To accomplish this extension, the cells of the archenteron rearrange themselves by migrating over one another and by flattening themselves.

10. This phenomenon, wherein cells intercalate to narrow the tissue and at the same time move it forward, is called  convergent extension . Moreover, cell division continues, producing more endodermal and secondary mesenchyme cells as the archenteron extends. Filopodia are extended from these cells through the blastocoel fluid to contact the inner surface

11. of the blastocoel wall. The filopodia attach to the wall at the junctions between the blastoderm cells and then shorten, pulling up the archenteron. There is a specific “target” site for the filopodia that differs from other regions of the animal hemisphere. The filopodia extend, touch the blastocoel wall at random sites, and then retract. However, when the filopodia contact a particular region of the wall, they remain attached

12. there, flatten out against this region, and pull the archenteron toward it. When Hardin and McClay poked in the other side of the blastocoel wall so that the contacts were made most readily with that region, the filopodia continued to extend and retract after touching it. Only when the filopodia found their “target” did they cease these movements. If the gastrula was constricted so that filopodia never reached the target area, the secondary mesenchyme cells

13. continued to explore until they eventually moved off the archenteron and found the target tissue as freely migrating cells. As the top of the archenteron meets the blastocoel wall in the target region, the secondary mesenchyme cells disperse into the blastocoel, where they proliferate to form the mesodermal organs. Where the archenteron contacts the wall, a mouth is eventually formed. The mouth fuses with the archenteron to create a continuous digestive tube. Thus, as is characteristic of deuterostomes, the blastopore marks the position of the anus.

14. General Over-view of Gastrulation: Sea urchin gastrulation begins with the ingression of primary mesnenchyme cells from the flattened epithelium at the vegetal pole of the embryos known as the vegetal plate. Following ingression of PMCs, there is a pause, and then the vegetal plate invaginates to form a stout cylinder called the archenteron. After another pause, secondary mesenchyme cells appear at the

15. tip of the archenteron, and at about this time the archenteron begins to elongate across the blastocoel. Eventually the tip of the archenteron makes contact with the ectoderm near the animal pole. Meanwhile, the PMCs formed patterned arrays, like anus is developed from the blastopore and mouth is formed from the archenteron.