The first two pictures show a 30-hour old gastrula of the sand dollar Dendraster
excentricus. During echinoid gastrulation the first cells to enter the
blastocoel (the spatial gel-filled cavity inside the embryo) are called primary
mesenchyme cells. Then, the archenteron (the primary gut) buckles in
(invaginates). Through a series of cell rearrangements called convergence and
extension, the archenteron (a large tube visible here inside the embryo)
elongates and extends towards the roof of the blastocoel. In the meantime the
primary mesenchyme cells form two groups and begin to secrete calcareous larval
spicules (two tri-radiate spicules are visible here on either side of the
archenteron). Approximately two thirds of the way up, a second group of cells
ingress into the blastocoel from the roof of the archenteron. This is where the filopodia come in.
This image is a close up view of the tip of the archenteron, showing a secondary mesenchyme cell with a long conical projection reaching
toward the roof of the blastocoel. These processes attach to the roof of the
blastocoel and aid in the final extension of the archenteron.
This picture shows a 62 hour old gastrula of
a sea star Pisaster
ochraceous. Asteroids lack the primary mesenchyme cells, so the
mesenchyme cells that ingress at the tip of the archenteron are the only
mesenchyme they have. Note the lack of primary mesenchyme cells in the
blastocoel and the numerous branched filopodia on the mesenchyme cells at the
tip of the archenteron.
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