Embryonic PMCs and late larval cells resemble each other: both are rounded in shape (not deformed like secondary mesenchyme cells) and have several filopodia.
In the sea urchin embryo, another type of mesenchymal cell, secondary mesenchyme cells (SMCs), potentially have an ability to become skeletogenic cells.
Cells were treated with DHT rather than testosterone to control for potential treatment effects on the intracellular concentration of this high-affinity ligand for the androgen receptor, which is formed from testosterone in UGS mesenchyme cells in vivo, and also to avoid the intracellular metabolism of testosterone to [E.sub.2] by aromatase; DHT is not a substrate for aromatase (Kokontis and Liao 1999).
The UGS primary cell cultures that we examined were thus homogenous populations of mesenchyme cells that retained mesenchymal differentiation markers throughout the incubation period.
An invagination of the body wall occurred after the appearance of mesenchyme cells, and the coelomic pouches elongated toward the posterior (Fig.
The morphogenetic and organogenic processes appeared to be similar to those we observed in regenerating asteroid larvae, with aggregations of mesenchyme cells with pseudopodia evident at the site of bisection (Vickery, 2002).
During secondary invagination, a population of secondary mesenchyme cells (SMCs) connect the archenteron tip to the inner surface of the apical plate and exert the force to pull up the archenteron (Dan and Okazaki, 1956; Hardin, 1988).
Figure 7 shows the secondary mesenchyme cells observed at the archenteron tip of the midgastrulae.
This suggests that the ECM is required for differentiation of secondary mesenchyme cells
along the myogenic pathway.
The early gastrula is filled with yolky mesenchyme cells
[ILLUSTRATION FOR FIGURE 5D OMITTED] in each of which the nucleus is difficult to see because of the very abundant yolk [ILLUSTRATION FOR FIGURE 5E OMITTED].
Stereo views of sequential slices, generated from limited sets of serial optical sections, bring out further details of the cellular organization, including the regular pavement-like arrangement of the circumoral epithelium and the shape of the mesenchyme cells
that were masked by the overlapping complex structure in the whole embryo.