CELLS ARE ADDED TO THE ARCHENTERON DURING AND FOLLOWING SECONDARY INVAGINATION IN THE SEA-URCHIN LYTECHINUS-VARIEGATUS

In the present investigation, nuclei of endodermal cells, primary and secondary mesenchyme cells (PMCs and SMCs), and small micromere descen dants (SMDs) of the sea urchin Lytechinus variegatus were counted and mapped at five developmental stages, ranging from primary invagination to pluteus larva. The archenteron and its derivatives were measured t hree dimensionally with STERECON analytical software. For the first ti me SMC production is included in the kinetic analysis of archenteron f ormation. While the archenteron lumen doubled in length during seconda ry invagination, the number of archenteron cells increased by at least 38% (over 50% when SMCs that emigrated from the tip of the archentero n were included). The volume of the archenteron epithelial wall plus t he volume of 17 new SMCs increased by 40% over the equivalent volumes at the end of primary invagination. Because secondary invagination inv olves the addition of archenteron cells and an increase in volume of t he archenteron epithelium, we conclude that secondary invagination is not accomplished simply by the rearrangement and reshaping of the prim ary archenteron cells. Both archenteron cell number and wall volume co ntinued to increase at the same rates from the end of secondary invagi nation until the 27-h prism stage, although the lumen lengthened more slowly. SMCs were also produced at a constant rate from primary invagi nation until the prism stage. Because the production of both endoderma l and mesodermal cells continues until the late prism stage, we conclu de that gastrulation (defined as the establishment of the germ layers) also extends into the late prism stage. (C) 1998 Academic Press.