Architecture Tradeoff Analysis Method(ATAM)

But these guidance

when to stop their movement and primary mesenchyme cells

animal-vegetal axis. At two primary mesenchyme cells formation. If a labeled m a gastrulating sea urchin e formation of the embry

provided by the prospective ectode mesenchyme cells (and not

these patterning cues. The existence skeleton-forming mesench

locomotion; rather, they responsible for picking u ectoderm .

Fibropellins are stored in secretory granules within the oocyte, and are secreted from those granules after cortical granule exocytosis releases the hyalin protein.

By the blastula stage, the fibropellins have formed a meshlike network over the embryo surface. At the time of invagination, the vegetal plate cells (and only those cells) secrete a chondroitin sulfate proteoglycan into the inner lamina of the hyaline layer directly beneath them. This hygroscopic (water-absorbing) molecule swells the inner lamina, but not the outer lamina. This causes the vegetal region of the hyaline layer to buckle (Figure 8.21C). Slightly later, a second force arising from the movements of epithelial cells adjacent to the vegetal plate may facilitate this invagination by drawing the buckled layer inward

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, and the last circumferential row to invaginate forms the hindgut and anus.

3.2.2 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 (see Figure 8.17, 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.

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

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. Filopodia are extended from these cells through the blastocoel fluid to contact the inner surface of the

blastocoel wall. The filopodia attach to the wall at the junctions between the blastoderm cells and then shorten, pulling up the archenteron. T he 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.

But can the secondary mesenchyme filopodia attach to any part of the blastocoel wall, or is there a specific target in the animal hemisphere that must be present for attachment to occur? Is there a region of the blastocoel wall that is already committed to becoming the ventral side of the larva? Studies show that 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 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 continued to explore until they eventually moved off the archenteron and found the target tissue as freely migrating cells. There appears, then, to be a target region on what is to become the ventral side of the larva that is recognized by the secondary mesenchyme cells, and which positions the archenteron in the region where the mouth will form.

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.