Ciliary band formation in the doliolaria larva of Florometra: I. The development of normal epithelial pattern

The development of ciliary band pattern in the doliolaria larva of Florometra serratissima is described based on scanning and transmission electron microscopy. The uniformly ciliated epithelium of the post-hatching larva develops four regularly spaced bands over a period of approx. 20 h generating an epithelial pattern that is, essentially, a series of stripes. The first visible events of pattern formation progress over the larval surface in a posterior-to-anterior and dorsal-to-ventral sequence, but the initial pattern is not, in fact, striped. It instead consists of a close-packed array of oval interband domains separated and surrounded by belts of band cells. Secondarily the interband domains expand laterally and coalesce to form continuous, broad stripes, while the bands remain as narrow stripes between them. Two possible explanations for this unusual sequence of events are discussed: (1) that it can be understood in evolutionary terms with reference to band pattern in other echinoderm larvae, and (2) that it is a morphogenetic necessity because limitations inherent in the patterning mechanism prevent the direct formation of regular stripes.