Histological and histochemical characterization of the ink gland from Aplysia argus (Mollusca: Heterobranchia: Aplysiida), Red Sea

This study aimed to investigate the histological and histochemical characterizations of the ink gland of the sea hare “Aplysia argus”.This gland is composed of two covering epithelium and an inner matrix. The covering epithelium involved the cuboidal facing the mantle cavity and the columnar facing the mantle shelf. Underneath the covering cuboidal epithelium, there are bundles of collagenous fibers permeated by two types of granulated cells. The columnar covering epithelium underlies an adipose connective tissue that includes dispersed cells between adipocytes. The inner matrix of the ink gland possesses three types of secretory vesicles; namely, dark-Red, granulated, and transparent. Histochemical investigation indicated that the granulated cell's content included neutral mucopolysaccharides and protein, while they were devoid of acid mucopolysaccharides. The dark-red vesicles possess a mixture of acid and neutral mucopolysaccharides; whereas, the granulated vesicles contain acid mucopolysaccharides only. The dark-red and granulated vesicles contain protein contents. The histological characteristics of the ink gland of Aplysia argus corroborated the general organization of aplysiids. To the best of our knowledge, new data were added in the current work (for the first time) about the two different types of granulated cells and adipose connective tissue, in addition to the investigation of the histochemical contents of granulated cells, secretory vesicles and dispersed cells.