Digestive system formation during the metamorphosis and definitive organogenesis in Apostichopus japonicus

Abstract

We studied the ultrastructure of the digestive system in late doliolaria, pentactula, and 1-month-old juvenile of the holothurian Apostichopus japonicus. In late doliolaria and pentactula, the digestive system is divided into three segments: pharynx, gut, and cloaca. Pharynx and the posterior part of cloaca are lined by cuticular epithelium and apparently of ectodermal origin. The anterior part of cloaca and the entire gut is not differentiated histologically and is lined by a single type of digestive cells, vesicular enterocytes I. These cells are characterized by large secretory granules, containing an acidic substance, found in cytoplasm. As the anterior part of the cloaca is lined by cells typical of the endodermal segment of digestive tube (vesicular enterocytes I), we suggest this part to be of endodermal origin and probably formed from the larval stomach. In 1-month-old juveniles, the structure of the digestive system grows more complicated. In addition to vesicular enterocytes I, three more types of enterocytes appear in luminal epithelium. The specific distribution of the four types of digestive cells divides the intestine into three parts, each probably performing its own function. All enterocytes develop long microvilli, which indicate the intensification of the extracellular digestion processes and an increased absorption of dissolved nutrients.