The Harderian gland of two species of skink (Tiliqua rugosa and Hemiergis decresiensis): a discussion of the significance of lymphatic tissue in the squamate Harderian gland

The Harderian gland is an orbital structure ubiquitous in reptiles. Numerous functions have been ascribed to this gland, including osmoregulation, chemoreception, and immunity. The anatomical, histological, histochemical, and ultrastructural characteristics of the Harderian gland of two species of skink, Tiliqua rugosa (Gray, 1825) and Hemiergis decresiensis (Cuvier, 1829), were examined. Mucous and serous secretory cells were observed in both species. In T. rugosa, mucous cells are located in a small anterior region, while serous cells occupy most of the gland. In contrast, the Harderian gland of H. decresiensis contains mainly serous cells, with a few anteriorly located mucous cells. In both species, the serous granules exhibit internal compartmentalization. There is no evidence of either lipid secretions or salt secreting cells. However, there were either a few plasma cells (H. decresiensis) or several lymphatic aggregations (T. rugosa) in the serous portion of the gland. The presence of such lymphatic tissue may suggest a role in either the head-associated lymphatic tissue (HALT) system or the eye-associated lymphatic tissue (EALT). The difference between these two is based upon terminology, the consolidation of which would allow meaningful comparative analyses. The presence of lymphatic tissue implies that the Harderian gland could play a role in ocular immunity.