Can the filopodia of non-skeletogenic mesenchyme cells 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 by Hardin and McClay (1990) show that there is indeed a specific target site for 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 filopodia contact a particular region of the wall, they remain attached and flatten out against this region, pulling the archenteron toward it. When Hardin and McClay poked in the other side of the blastocoel wall so that 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 tissue did they cease these movements. If the gastrula was constricted so that the filopodia never reached the target area, the non-skeletogenic mesenchyme cells continued to explore until they eventually moved off the archenteron and found the target 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 non-skeletogenic mesenchyme cells, and which positions the archenteron near the region where the mouth will form. Thus, as is characteristic of deuterostomes, the blastopore marks the position of the anus.
Hardin, J. D. and D. R. McClay. 1990. Target recognition by the archenteron during sea urchin gastrulation.Dev. Biol.142: 86–102.
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