Microscopic anatomy of the orbital Harderian gland in the common tree shrew (Tupaia glis)

The orbital Harderian gland of the common tree shrew (Tupaia glis) was investigated at the macroscopic and microscopic levels. In the glands of both sexes only one acinar cell type was found. The cell is characterized by the presence of numerous lipid vacuoles of variable size and by a small number of PAS-positive, electron-dense granules distributed throughout the cytoplasm, which are predominant at the basal portion of each acinar cell. The duct system is well developed within the gland. The content of lipid vacuoles within the acinar cells is secreted from the apical portions by exocytosis, indicating the exocrine function of the organ. Apart from the lipid vacuoles, both acinar and ductal luminal contents of the Harderian gland also contain accretion of electron-dense materials. The vascularization within the Harderian gland is unique in that two capillary types (small fenestrated and irregular sinusoidal capillaries) could be demonstrated. The presence of fenestrated capillaries together with other morphological features (such as accumulation of the small electron-dense granules at the basal pole and the presence of basolateral microvilli) near the basal portion of the acinar cells suggest that the Harderian gland in T. glis might also be involved in an endocrine function.