The Sea Urchin Blastula: Extent of Cellular Determination

Abstract

The extent of cellular determination at the blastula stage of sea urchins has been investigated by using as parameters the morphological differentiation and cellular interactions of dissociated blastula cells under conditions that either promote or discourage their reaggregation and reconstitution into embryos. The dissociated blastula cells have the capacity to differentiate into at least three different specific cell types that occur in the normally developing sea urchin embryo. Of these, ciliated epithelium is known to originate in the animal half, and spindle-shaped cells and polyfilamentous cells are of mesenchymal origin and are derived from the vegetal half. The formation of ciliated epithelium and blastulation requires calcium and hyalin; the cellular differentiation and interactions of the mesenchymal cells do not. This basic difference in calcium requirement is explored in terms of differential hyalin synthesis or secretion by the cells at either equatorial half of the embryo. The animalizing effect of zinc and the vegetalizing effect of lithium can be explained in part by their enhancement and reduction, respectively, of hyalin gelation by calcium. Literature relating to other basic differences along the animal-vegetal axis of the embryo is appraised. These differences include the use and the assembly of microtubule protein and quantitative RNA synthesis during both the cleavage and blastula stages.