Left: A L. variegatus early gastrula, viewed from the side (courtesy of Dr. Charles Ettensohn, Carnegie-Mellon Univ). Right: a L. pictus early gastrula (by Jeff Hardin, Univ. of Wisconsin).

The vegetal plate undergoes an initial invagination, termed primary invagination, to produce the archenteron. Since sea urchins are deuterstomes, the blastopore forms the anus of the larva later in development. The vegetal plate is an epithelial monolayer, and so the process of primary invagination seems like a deceptively simple case of invagination. However, appearances can be deceiving! The mechanisms of primary invagination are not well understood. However, several observations and experiments indicate at least some of the events that occur during primary invagination.

Models of primary invagination. Image by Jeff Hardin, Univ. of Wisconsin.

Several models of primary invagination have been proposed. These include:

(1) apical constriction - in this model, which has been proposed for many decades, the apical ends of some cells in the vegetal plate would actively constrict, forcing the vegetal plate to buckle.
(2) apical "tractoring" (Burke et al., 1991. Dev. Biol. 146,542-57) - in this model, cells lateral to the vegetal plate actively migrate towards the vegetal pole, forcing the vegetal plate to indent. In this model apical surfaces of the lateral cells would have to migrate actively.
(3) swelling of proteoglycan (Lane et al., 1993. Development 117, 1049-60) - in this model, local secretion of chondroitin sulfate proteoglycan would result in invagination if after its release the proteoglycan swells, and the swollen material is held in place by the hyaline layer.

Click on the thumbnails to view two movies of primary invagination. Left: The first third of gastrulation in L. variegatus. Movie courtesy of David McClay, Duke Univ. Right: Lytechinus pictus primary invagination: (4 hours elapsed). The embryo is trapped using calcium carbonate encrusted Nitex mesh, a classic technique for immobilizing sea urchin embryos. Courtesy of Lance Davidson, Univ. of Virginia.
Primary invagination in L. variegatus (8.1 Mb)
Primary invagination in L. pictus (1.3 Mb)