Development of the digestive tract of sea bass (Dicentrarchus labrax L). Light and electron microscopic studies

The developing gut of sea bass was studied by light and electron microscopy , four phases being established. Phase I, from hatching to the opening of t he mouth, was a lecitotrophic period, in which the gut appeared as a straig ht undifferentiated tube lined by a simple epithelium that became stratifie d in the most caudal region. The epithelial cells increased in length towar ds the caudal zone, as did the number and height of the apical microvilli a nd the magnitude of the lamellar structures in their basal region. Cilia we re more numerous in the caudal region than in the rest of the gut. Signs of lipid but not of protein absorption were found in the epithelial cells at this phase. Phase II, from the opening of the mouth to the complete resorpt ion of the yolk sac, was a lecitoexotrophic period in which an esophagus, a gastric region, an intestine and a rectum, the last two separated by a val ve, were present. During this phase the differentiation of the gut started at the esophagus and the rectum. In the esophagus, the epithelium became st ratified and goblet cells containing acid mucosubstances, including sulphom ucins, appeared. In the epithelial cells of the rectum, supranuclear vacuol es and an incipient endocytotic apparatus that seemed to be involved in the absorption and digestion of proteins were found. In both regions the mucos a was folded. Phase III, from the complete resorption of the yolk sac to th e appearance of the first gastric glands, initiated the exclusively exotrop hic period. During this phase the intestine formed the mucosa folds, while the first pyloric caeca and the epithelial cells acquired the ultrastructur al features of mature absorptive cells with many lipid inclusions. Goblet c ells containing neutral mucosubstances appeared and increased in number in both the intestine and the rectum. Neutral mucosubstances were also present in the cells lining the gastric region. During phase IV, from the appearan ce of the first gastric glands onwards, the intestinal absorptive surface i ncreased with the formation of new pyloric caeca and two intestinal loops. The stomach acquired its definitive anatomy and histology with the developm ent of the caecal and pyloric regions alongside differentiated gastric glan ds. The glandular cells had the ultrastructural features of the cells that secrete both pepsinogen and hydrochloride acid in the adult teleost stomach .