THE ALLOCATION OF EARLY BLASTOMERES TO THE ECTODERM AND ENDODERM IS VARIABLE IN THE SEA-URCHIN EMBRYO

During sea urchin development, a tier-to-tier progression of cell sign aling events is thought to segregate the early blastomeres to five dif ferent cell lineages by the 60-cell stage (E. H. Davidson, 1989, Devel opment 105, 421-445), For example, the sixth equatorial cleavage produ ces two tiers of sister cells called 'veg1' and 'veg2,' which were pro jected by early studies to be allocated to the ectoderm and endoderm, respectively, Recent in vitro studies have proposed that the segregati on of veg1 and veg2 cells to distinct fates involves signaling between the veg1 and veg2 tiers (O. Khaner and F. Wilt, 1991, Development 112 , 881-890), However, fate-mapping studies on 60-cell stage embryos hav e not been performed with modern lineage tracers, and cell interaction s between veg1 and veg2 cells have not been shown in vivo, Therefore, as an initial step towards examining how archenteron precursors are sp ecified, a clonal analysis of veg1 and vega cells was performed using the lipophilic dye, DiI(C16), in the seaurchin species, Lytechinus var iegatus, Both veg1 and veg2 descendants form archenteron tissues, reve aling that the ectoderm and endoderm are not segregated at the sixth c leavage, Also, this division does not demarcate cell type boundaries w ithin the endoderm, because both veg1 and veg2 descendants make an ove rlapping range of endodermal cell types, The allocation of veg1 cells to ectoderm and endoderm during cleavage is variable, as revealed by b oth the failure of veg1 descendants labeled at the eighth equatorial d ivision to segregate predictably to either tissue and the large differ ences in the numbers of veg1 descendants that contribute to the ectode rm, Furthermore, DiI-labeled mesomeres of 32-cell stage embryos also c ontribute to the endoderm at a low frequency, These results show that the prospective archenteron is produced by a larger population of clea vage-stage blastomeres than believed previously, The segregation of ve g1 cells to the ectoderm and endoderm occurs relatively late during de velopment and is unpredictable, indicating that later cell position is more important than the early cleavage pattern in determining ectoder mal and archenteron cell fates.