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A L. variegatus late gastrula (ventral view). The archenteron has made an attachment to the ventral (oral) ectoderm near the animal pole. Primary mesenchyme cells have begun to produce skeletal spicules. Image by Jeff Hardin, Univ. of Wisconsin.


Both primary and secondary mesenchyme cells localize to specific regions of the embryo, a striking example of pattern formation. The sea urchin embryo is an excellent system for examining how cues to which mesenchymal cells respond are presented within the context of an intact embryo. In particular, experimental embryology has been used very successfully to identify cell-cell interactions in the sea urchin embryo that contribute to mesenchymal patterning.

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A L. variegatus late gastrula, immunostained to show the bilateral pattern of primary (skeletogenic) mesenchyme (green) and a subset of secondary mesenchyme that expresses the SNAIL protein (orange). Image by Jeff Hardin, Univ. of Wisconsin.

As an aid to such studies, the two types of mesenchyme express different genes, such as cell surface molecules and transcription factors, as the image below shows.

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A schematic showing the localization of two types of mesenchyme in the sea urchin embryo. Pink: primary (skeletogenic) mesenchyme; green; secondary mesenchyme. Image by Jeff Hardin, Univ. of Wisconsin.

Both primary and secondary mesenchyme cells have striking localization patterns.