cell fate determination cell in sea urchin cell fate determinination

1. Presented by: Rabia shehzadi
Presented to: Sir Majid
Presentation on : Early development in sea urchin
Course name : Developmental biology

2. cell fate determination cell in sea urchin
In sea urchin eggs, a protein binds to the growing end of
microfilament and to mRNA encoding a cytoplasmic
determinant.
As microfilaments grows towards one end of the cell, it carries
the mRNA along with it.
The asymmetrical distribution of the mRNA leads to the
similar distribution of the proteins it encodes.

3. How were these paths are determined??
• Bicoid mutants produce larvae with no head and thorax.
• Cytoplasm with the anterior end of wild-type eggs will
produce normal larvae.
• Cytoplasm from the anterior end of wild-type eggs, injected
into posterior end of another egg, will produce anterior
structures there.

4. How is cell fate determined
• If the sea urchin 8-cell embryo is cut vertically, it develops
into two small larvae.
• If it is cut horizontally, the bottom develops into larva, the top
remains embryonic.
• This indicates that the top and bottom halves have already
developed distinct fates.

5. • The cytoskeleton contributes to asymmetric distribution of cytoplasmic
determinants:
• Microtubules and microfilaments have polarity.
• Cytoskeleton elements can binds motor proteins that transport the
cytoplasmic determinants.

7. Gastrulation
• Sea urchin gastrulation begins with the ingression of primary, or
skeletogenic, mesenchyme 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 tip of
the archenteron, and at about this time the archenteron begins to
elongate across the blastocoel.

8. • Eventually the tip of the archenteron makes contact with the ectoderm
near the animal pole. Meanwhile, the PMCs formed patterned arrays,
and they ultimately secrete calcium carbonate-containing skeletal rods
called spicules.
• The ventral (oral) ectoderm flattens, giving rise to what is known as
the prism stage. Finally, the gut, ectoderm, and the skeleton undergo
further obvious differentiation to make the pluteus larva

10. Ingression of primary mesenchyme
• Function of primary mesenchyme cells
• Shortly after the blastula hatches from its fertilization envelope, the
vegetal side of the spherical blastula begins to thicken and flatten (9
hours). At the center of this flat vegetal plate, a cluster of small cells
begins to change. These cells begin extending and contracting long,
thin processes called filo podia from their inner surfaces.

11. • The cells then dissociate from the epithelial monolayer and ingress
into the blastocoel 9–10 hours). These cells, derived from the
micromeres, are called the primary mesenchyme.

13. • They will form the larval skeleton, so they are sometimes called the
skeletogenic mesenchyme. At first the cells appear to move randomly
along the inner blastocoel surface, actively making and breaking
filopodial connections to the wall of the blastocoel. Eventually,
however, they become localized within the prospective ventrolateral
region of the blastocoel. Here they fuse into syncytial cables, which
will form the axis of the calcium carbonate spicules of the larval
skeleton.

15. 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 the gaps caused by the
ingression of the primary mesenchyme.

16. • Moreover, the vegetal plate bends inward and invaginates about one-
fourth to one-half the way into the blastocoel (10.5–11.5 hours). 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

18. Second and third stages of archenteron invagination
• 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 (12 hours; Figure 8.23). To
accomplish this extension, the cells of the archenteron rearrange
themselves by migrating over one another and by flattening
themselves (Ettensohn 1985; Hardin and Cheng 1986).

19. • 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.

20. Third stage of archenteron
• In at least some species of sea urchins, a third stage of archenteron
elongation occurs. This last phase is initiated by the tension provided
by secondary mesenchyme cells, which form at the tip of the
archenteron and remain there (13 hours;).
• Filo podia are extended from these cells through the blastocoel fluid
to contact the inner surface of the blastocoel wall (Dan and Okazaki
1956; Schroeder 1981).

21. • The filo podia attach to the wall at the junctions between the
blastoderm cells and then shorten, pulling up the archenteron. Hardin
(1988) ablated the secondary mesenchyme cells with a laser, with the
result that the archenteron could elongate to only about two-thirds of
the normal length.
• If a few secondary mesenchyme cells were left, elongation continued,
although at a slower rate. The secondary mesenchyme cells, then, play
an essential role in pulling the archenteron up to the blastocoel wall
during the last phase of invagination.

23. • 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 (see Figure
8.17, 13.5 hours). 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.