A FATE MAP OF THE VEGETAL PLATE OF THE SEA-URCHIN (LYTECHINUS-VARIEGATUS) MESENCHYME BLASTULA

Previous lineage tracing experiments have shown that the vegetal blast omeres of cleavage stage embryos give rise to all the mesoderm and end oderm of the sea urchin larva, In these studies, vegetal blastomeres w ere labeled no later than the sixth cleavage division (60-64 cell stag e). In an earlier study we showed that single cells in the vegetal pla te of the blastula stage Lytechinus variegatus embryo could be labeled in situ with the fluorescent, lipophilic dye, DiI(C-18), and that cel ls labeled in the central region of the vegetal plate of the mesenchym e blastula primarily gave rise to homogeneous clones consisting of a s ingle secondary mesenchyme cell (SMC) type (Ruffins and Ettensohn (199 3) Dev. Biol. 160, 285-288), Our clonal labeling showed that a detaile d fate map could be generated using the DiI(C-18) labeling technique, Such a fate map could provide information about the spatial relationsh ips between the precursors of specific mesodermal and endodermal cell types and information concerning the movements of these cells during g astrulation and later embryogenesis. We have used this method to const ruct the first detailed fate map of the vegetal plate of the sea urchi n embryo, Ours is a latitudinal map; mapping from the plate center, wh ere the mesodermal precursors reside, through the region which contain s the endodermal precursors and across the ectodermal boundary, We fou nd that the precursors of certain SMC types are segregated in the mese nchyme blastula stage vegetal plate and that prospective germ layers r eside within specific boundaries, To determine whether the vegetal pla te is radially symmetrical with respect to mesodermal cell fates, sing le blastomeres of four cell stage embryos were injected with lysyl-rho damine dextran (LRD). The resulting ectodermal labeling patterns were classified and correlated with the SMC types labeled, This analysis in dicates that the dorsal and ventral blastomeres do not contribute equa lly to SMC derivatives in L. variegatus.