Sea Urchin Gastrulation

At the beginning of gastrulation, the vegetal plate starts to flatten. Note the blastocoel cavity is not empty. Many thin fibronectin fibers fill this cavity, but do not show up at this magnification under bright field illumination.

The primary mesenchyme cells (PMCs) lose their adhesions to their neighboring cells and to the hyaline layer. They then change their shape and start to ingress into the blastocoel cavity.

The PMCs migrate along the fibers in the blastocoel cavity and then settle down in two clusters near the vegetal plate. The endodermal cells in the vegetal plate start to invaginate.

The archenteron elongates by convergence and extension reaching up to the roof of the blastocoel cavity. The PMCs are starting to form spicules (skeletal rods).

(courtesy of Dr. J. Hardin, Univ. of Wisconsin)

	At the beginning of gastrulation, the vegetal plate starts to flatten. Note the blastocoel cavity is not empty. Many thin fibronectin fibers fill this cavity, but do not show up at this magnification under bright field illumination.
	The primary mesenchyme cells (PMCs) lose their adhesions to their neighboring cells and to the hyaline layer. They then change their shape and start to ingress into the blastocoel cavity.
	The PMCs migrate along the fibers in the blastocoel cavity and then settle down in two clusters near the vegetal plate. The endodermal cells in the vegetal plate start to invaginate.
	The archenteron elongates by convergence and extension reaching up to the roof of the blastocoel cavity. The PMCs are starting to form spicules (skeletal rods).