Histochemistry of goblet cells and micro-computed tomography to study the digestive system in the long-snouted seahorse Hippocampus guttulatus

Abstract

Sygnathids are agastric teleosts (no stomach), relying on a digestive tract using different mechanisms to process and absorb nutrients. This results in a low digestion efficiency at early stages, forming a mayor bottleneck in the rearing of these fish. In agastric species, the numerous goblet cells present in the oesophagus could be considered as a morphological adaptation that replaces a functional stomach, although the specialization and number of these cells is species-specific and vary throughout the development and intestinal region. The present study aim to characterize the biochemical composition of goblet cells and investigate the morphology of the digestive system during the ontogeny of Hippocampus guttulatus seahorses (from 0 to 60 days post-partum, dpp) in order to understand the mechanisms of nutrient digestion in a species that lacks gastric glands. Goblet cells (GC) appear in the digestive tract of Hippocampus guttulatus from birth on, secreting a large amount of acid (carboxylated) and neutral glycoconjugates (GCs) released in the buccopharyngeal cavity and oesophagus, while sulphated and neutral GCs are detected throughout the complete digestive tract of the seahorses. The midgut mainly secretes neutral and acid (carboxylated) GCs at the early stages (<15 dpp) and acid (sulphated) GCs at later stages, while the hindgut is characterized by the presence of carboxylated and highly sulphated GCs throughout development. Based on the development of the digestive tract observed by 3D reconstructed μCT data, an increase in the intestinal absorption surface can be assumed. This increase is in correspondence with a change in the type of acid GCs observed in the midgut from 15 dpp. The present study demonstrates that the digestive system of H. guttulatus is functional at first feeding, although not fully developed. The different GCs secreted by the goblet cells in the digestive tract of H. guttulatus revealed that the glycosylation patterns vary according to the digestive region and stage of development. Neutral GCs could be involved in the digestion of simple substances (e.g. short chain fatty acids) in early stages of development, while in more advanced stages the change to acid GCs would promote the absorption of proteins. A change in the secretion of goblet cells from 15 dpp, in coincidence with the observation of a progress increase in the intestinal absorption surface, would explain the better digestive efficiency observed from that age.