Target Recognition by Mesenchyme Cells During Sea Urchin Gastrulation

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Abstract
SYNOPSIS. During sea urchin gastrulation, mesenchyme cells must migrate to the correct locations and attach to specific sites within the embryo. Primary mesenchyme cells (PMCs) aggregate into two major clusters in the ventrolateral region of the embryo and other PMCs form an elaborate pattern that presages the skeletal pattern of the larva. Numerous experiments, including alteration of dorsoventral differentiation of the ectoderm with NiCl2, indicate that the ectoderm provides much of the pattern information to which these cells respond. Secondary mesenchyme cells (SMCs) attach to the ectoderm at a specific site near the animal pole at the end of gastrulation, undergoing a dramatic change in protrusive behavior as they do so. Experiments indicate that the target region to which they respond is restricted to a small domain that will form the larval mouth, and that this region is unique in its ability to elicit these changes in behavior. A third population of patterned mesenchyme has recently been identified by their expression of the sea urchin homologue of the Drosophila transcription factor, snail. Snail expressing cells form two bilateral clusters near the sites where the larval arms will form as gastrulation ends. Finally, we have identified two interactions between cells in the early embryo that seem to be important for the establishment of pattern information for mesenchyme: (1) interactions between macromere descendants and neighboring cells can establish new bilateral patterning sites for PMCs, and (2) interactions between the animal pole and cells in a more vegetal location appear to result in induction of the oral field and associated pattern information for both PMCs and SMCs.

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