The Questions of Developmental Biology

Ingression of primary mesenchyme
Function of primary mesenchyme cells
Shortly after the blastula hatches from
its fertilization envelope, the vegetal side of the
spherical blastula begins to thicken and flatten
(Figure 8.17, 9 hours). At the center of this flat
vegetal plate, a cluster of small cells begins to
change. These cells begin extending and
contracting long, thin (30 × 5 μm) processes
called filopodia from their inner surfaces. The
cells then dissociate from the epithelial
monolayer and ingress into the blastocoel
(Figure 8.17, 9 10 hours). These cells, derived
from the micromeres, are called the primary
mesenchyme. They will form the larval
skeleton, so they are sometimes called the
skeletogenic mesenchyme.
At first the cells appear to move randomly along
the inner blastocoel surface, actively making and
breaking filopodial connections to the wall of
the blastocoel. Eventually, however, they
become localized within the prospective
ventrolateral region of the blastocoel. Here they
fuse into syncytial cables, which will form the
axis of the calcium carbonate spicules of the
larval skeleton (Figure 8.18).
Importance of extracellular lamina inside the blastocoel
The ingression of the micromere descendants into the blastocoel
is caused by these cells losing their affinity for their neighbors and for
the hyaline membrane and acquiring a strong affinity for a group of
proteins that line the blastocoel. This model was first proposed by
Gustafson and Wolpert (1967) and was confirmed in 1985, when Rachel
Fink and David McClay measured the strengths of sea urchin blastomere
adhesion to the hyaline layer, to the basal lamina lining the blastocoel,
and to other blastomeres. Originally, all the cells of the blastula are
connected on their outer surface to the hyaline layer and on their inner
surface to a basal lamina secreted by the cells.