Animal-vegetal axis patterning mechanisms in the early sea urchin embryo

We discuss recent progress in understanding how cell fates are specified al ong the animal-vegetal axis of the sea urchin embryo. This process is initi ated by cell-autonomous, maternally directed, mechanisms that establish thr ee unique gene-regulatory domains. These domains are defined by distinct se ts of vegetalizing (beta-catenin) and animalizing transcription factor (ATF ) activities and their region of overlap in the macromeres, which specifies these cells as early mesendoderm. Subsequent signaling among cleavage-stag e blastomeres further subdivides fates of macromere progeny to yield major embryonic tissues. Zygotically produced Wnt8 reinforces maternally regulate d levels of nuclear beta-catenin in vegetal derivatives to down regulate AT F activity and further promote mesendoderm fates. Signaling through the Not ch receptor from the vegetal micromere lineages diverts adjacent mesendoder m to secondary mesenchyme fates. Continued Wnt signaling expands the vegeta l domain of beta-catenin's transcriptional regulatory activity and competes with animal signaling factors, including BMP2/4, to specify the endoderm-e ctoderm border within veg(1) progeny. This model places new emphasis on the importance of the ratio of maternally regulated vegetal and animal transcr iption factor activities in initial specification events along the animal-v egetal axis. (C) 2000 Academic Press.