Importance of Electron Microscopy to reveal species-specific characteristics of gland secretion

Secretory granules of Squamata Harderian glands (HG) show a different ultrastructure as compared to those described for Chelonian HG. The “special secretory granules” described for the HG of the lizard Podarcis s. sicula show a composite structure. A similar structure is present in Coluber viridiflavus, a species of the same order. For the gecko Tarentola mauritanica, the osmiophilic secretory granules show a crescent-like cup of higher density. The secretory granules of Testudo graeca HG are formed from a moderately dense homogeneous material. Two types of secretory granules are found in the Pseudemys scripta HG, i.e., one that is electron-dense with an electron-lucent core and another that is moderately osmiophilic with an electron-dense core. The variety of secretory granules observed at the ultrastructural level leads to the hypothesis that they serve other possible functions besides the obvious lubrication of the eyeball. Furthermore, the presence of plasma cells among the glandular cells in the HG of T. graeca strongly supports the possible immunoresponses related role of the gland, as suggested in mammals and birds. The electron microscopy also indicated that the HG of Chelonians differs from that of Squamata in terms of the presence of “salt cells” among the glandular cells. These characteristic ion-transporting cells are more numerous in the terrapin. The presence of salt cells in Chelonian HGs leads us to hypothesize that this gland may play a role in osmoregulation, which would be unique among vertebrates.