MULTIPLE SIGNALING EVENTS SPECIFY ECTODERM AND PATTERN THE ORAL-ABORAL AXIS IN THE SEA-URCHIN EMBRYO

In the sea urchin embryo, the animal-vegetal axis is established durin g oogenesis and the oral-aboral axis is specified sometime after ferti lization. The mechanisms by which either of these axes are specified a nd patterned during embryogenesis are poorly understood. Here, we inve stigated the role of cellular interactions in the specification of the ectoderm territories and polarization of the ectoderm along the oral- aboral axis. Isolated animal halves (mesomeres), which are fated to gi ve rise to oral and aboral ectoderm, developed into polarized embryoid s that expressed an oral ectoderm-specific marker uniformly. These emb ryoids also produced neuron-like cells and serotonergic neurons, sugge sting that mesomeres are autonomously specified as oral ectoderm. Meso mere-derived embryoids did not express any aboral ectoderm-specific ma rkers, although we previously showed that aboral ectoderm-specific gen es can be induced by 25 mM lithium chloride, which also induced endode rm formation (Wikramanayake, A. H., Brandhorst, B. P. and Klein, W. R. (1995). Development 121, 1497-1505). To ascertain if endoderm formati on is a prerequisite for induction of aboral ectoderm by lithium and f or normal ectoderm patterning in animal halves, we modulated the lithi um treatment to ensure that no endoderm formed. Remarkably, treating a nimal halves with 10 mM LiCl at similar to 7 hours postfertilization r esulted in embryoids that displayed oral-aboral axis patterning in the absence of endoderm. Application of 25 mM LiCl to animal halves at si milar to 16 hours post-fertilization, which also did not induce endode rm, resulted in polarized expression of the aboral ectoderm-specific L pS1 protein, but global expression of the Ecto V antigen and no induct ion of the stomodeum or ciliary band. These results suggest that at le ast two signals, a positive inductive signal to specify the aboral ect oderm and a negative suppressive signal to inactivate oral ectoderm-sp ecific genes in the prospective aboral ectoderm territory, are needed for correct spatial expression of oral and aboral ectoderm-specific ge nes. Transmission of both these signals may be prerequisite for induct ion of secondary ectodermal structures such as the ciliary band and st omodeum. Thus, differentiation of ectoderm and polarization of the ora l-aboral axis in Lytechinus pictus depends on cellular interactions wi th vegetal blastomeres as well as interactions along the oral-aboral a xis.