Ultrastructural and Immunohistochemical Characterisation of the Pancreas in the American Bullfrog (Lithobates catesbeianus), with Special Emphasis on Telocytes.

The pancreas is a crucial digestive and endocrine organ, yet the histology of the animal pancreas remains incompletely elucidated, particularly in amphibians. This study aimed to elucidate the histology of the American bullfrog (Lithobates catesbeianus) pancreas using histological, immunohistochemical, transmission electron microscopic methods. Histologically, the pancreas displayed typical exocrine acini and scattered endocrine islets. Two types of endocrine cells were distinguished based on their secretory granules: Type I, found diffusely among acini and containing rounded granules (0.28 ± 0.07 μm), and Type II, forming islets and containing rod-shaped granules (major axis 0.37 μm; minor axis 0.12 μm). Importantly, telocytes (TCs) were observed within the pancreatic stroma, characterised by small cell bodies and long telopodes (Tps) with podoms (0.51-2.71 μm) and podomers (0.08-0.14 μm). TCs established close contacts with acinar cells, islets, capillaries and nerve fibres. Most notably, ribosome-associated vesicles (RAV) (mean diameter: 0.27 ± 0.07 μm) were found adjacent to Tp podoms. Immunohistochemistry revealed CD34+, c-Kit+ and vimentin+ interstitial cells with morphological features consistent with TCs, forming a stromal network surrounding acini and islets. This study presents the first ultrastructural and immunohistochemical evidence of TCs and their associated RAV in the pancreas of L. catesbeianus. These findings demonstrate that TCs are structurally integrated interstitial cells in the amphibian pancreas; moreover, RAVs were observed in the vicinity of Tps.