Immunohistochemical study of the principal pancreatic islet of the toadfish, Halobatrachus didactylus (Pisces: Batrachoididae)

The endocrine pancreas of the toadfish, Halobatrachus didactylus, consists of one large circular principal islet (Brockman body) located in the dorsal side or neck region of the gallbladder, along with various accessory islets of variable sizes and shapes, embedded in the exocrine tissue located within the digestive organs connecting mesenteries. Islet cells showed variable shapes, angular or fusiform, with long cytoplasmic processes, granular cytoplasm, and a large eccentric nucleus. Cells were found scattered or as aggregates or cords. Four primary endocrine cell types immunoreactive for glucagon (α cells), insulin (β cells), somatostatin (δ cells), and pancreatic polypeptide (F cells) were identified within the toadfish principal islet. The α, δ, and F cells were located both at the periphery and in the central regions, while β cells, which were the predominant type, were present only in the central core. α and δ cells were found in moderate frequencies, while F cells were the least abundant. Macroscopically, the Brockman body of H. didactylus is visible as a milky white nodule separated from the exocrine tissue. Its size, location, and ease of extraction suggest that H. didactylus is suitable as experimental subject for biochemical, immunological, and physiological studies of the endocrine pancreas including in vitro investigations of hormone production, storage, and release.