Development of the digestive tract of gilthead sea bream (Sparus aurata L.).: Light and electron microscopic studies

Light and electron microscopic studies of the digestive tract of gilthead sea bream were carried out from hatching to 69 days. Five significant phases were established. Phases I and II comprise the lecitotrophic period. During phase I, the yolk sac was large and the uniform digestive tract showed a layer of squamous epithelial cells with numerous free ribosomes. Phase II was characterized by the opening of the anus and the differentiation of three digestive regions: the esophagus, with a stratified epithelium, the presumptive stomach, whose cuboid epithelial cells had some apical processes and clear vesicles, and the intestine with large intercellular spaces among prismatic epithelia] cells that had a PAS-positive striated border. Phase III, or lecitoexotrophic period, began with the opening of the mouth; absorption of the yolk sac started and the intestine became differentiated into two regions separated by a valve. Intestinal epithelia] cells showed basal lamellar structures and lipoprotein particles. Some columnar cells appeared inside the epithelium of the esophagus. Phases IV and V comprise the exotrophic period; phase IV began with the disappearance of the yolk sac; mucous cells containing sulphomucin-type acid mucosubstances appeared in the esophagus and goblet cells, with acid and neutral mucosubstances appeared in the intestine. The epithelia] cells of the first and posterior intestinal segments showed large lipid droplets and heavy pinocytosis with large supranuclear vesicles and numerous lysosomes, respectively. Phase V was marked by the appearance of neutral mucosubstances in the esophageal mucous cells and in the stomach epithelial cells, and the differentiation of pyloric caeca and gastric glands. The ultrastructural features of glandular cells indicated that they secrete both pepsinogen and hydrochloride acid. The epithelia] cells of the first intestinal segment showed large lipid droplets, often close to mitochondria, at the beginning of this phase. These lipid droplets decreased in size, while the rough and smooth endoplasmic reticulum and Golgi complex, related to lipoprotein synthesis, progressively developed during this phase. Pinocytotic and large supranuclear vesicles disappeared from epithelia] cells of the posterior intestinal segment. (C) 2004 Elsevier B.V. All rights reserved.