Larval organogenesis in common dentex Dentex dentex L. (Sparidae): histological and histochemical aspects

The present study describes histological and histochemical aspects of Dentex dentex larval development up to day 36 after hatching and provides a tabular overview on the ontogeny of structures involved in general metabolic processes, mechanisms of defence against disease and, particularly, nutrition. Observations are related to main developmental stages defined on the basis of external morphological features. During stage 1, the mouth and the anus of the larvae are still closed. Stage 2 starts when the mouth opens and stages 3 and 4 are characterized by the appearance of opercular spines and notochord flexion, respectively. During the first three stages, dentex larvae undergo intense organogenesis, this being particularly intense during stages 2 and 3. Stage 4 is not characterized by the appearance of new structures but by the increase in size and complexity of pre-existing ones. During stage 1, the larva acquires morphological and enzymatic equipments (hepatocytes with glycogen stores, pancreatic cells with first zymogen granules, enterocytes with microvilli, non-specific esterase and alkaline and acid phosphatase activities) which will ensure absorptive and digestive processes before the mouth opening. Pronephric excretory structures and a tubular heart are present at hatching and endocrine elements (corpuscles of Stannius, endocrine pancreas and thyroid gland) appear. During stages 2 and 3, food capture as well as digestive and absorptive processes become fully guaranteed (pharyngeal as well as mandibular teeth and taste buds appear and then proliferate, the gut progressively differentiates, the mucosa folds, ATPase and aminopeptidase M activities appear, mucus secretion establishes, particularly in the oesophagus, and at the end of stage 3, gastric glands differentiate). Before this last event occurs, acidophilic supranuclear vesicles are seen in the hindgut enterocytes and disappear at the moment the stomach develops. Gill slits are evident at initial stage 2; filaments start differentiation by the end of this stage and lamellae appear by the second half of stage 3. By this time compartmentalisation of the heart is completed, trabeculae being evident in the ventricle, and mesonephric excretory structures appear. During stage 4, gastric glands proliferate, rodlet cells appear in the stomach and the oesophagus and rodlet and goblet cells proliferate in the intestine; gill filaments and lamellae increase in number and length and there is a notable development of the mesonephros. The lymphoid organs, which appear in the sequence pronephric kidney (present at stage 1), spleen (observed at stage 2) and thymus (first seen at stage 3) remarkably increase in size during this stage.