Macro- and microscopic morphology of the rectal gland of the Brazilian guitarfish (Pseudobatos horkelii) from Southeastern Brazil.

Sharks and rays present an osmoregulatory mechanism essentially exercised by a rectal salt gland. Histological assessments of this gland, however, are notoriously lacking. In this sense, histological assessments of the rectal gland of the Brazilian guitarfish, Pseudobatos horkelii obtained from off the coast of Rio de Janeiro, Brazil, were carried out herein. Rectal gland samples were histologically processed with hematoxylin/eosin and special Masson's Trichrome stain. Three main regions were identified: the capsule, secretory parenchyma and central duct. Highly vascularized connective tissue was observed in the capsular region, surrounded by a superficial epithelium composed of a layer of cubic cells. Lymphoid tissue was present outside the capsule. The capsule presented connective tissue invaginations, forming interlobular septa. Each septum, surrounded by fibroelastic tissue, delimited the secretory lobes filled with secretory tubules, whose lumens exhibited a larger diameter and a greater number of secretory cells as they approached the central duct. The duct to which the organ's secretory tubules open, at the center of the rectal gland, presents a lumen lined with stratified epithelium, containing acidophilic intraepithelial and mucous cells. Most analyzed morphological characteristics are in accordance with morphological aspects reported in previous ray studies concerning other species presenting similar phylogeny, habitat and feeding characteristics as P. horkelii. These assessments are paramount in understanding species-specific osmoregulation and informing conservation strategies, particularly for threatened species like the Brazilian guitarfish.