The harderian gland of desert rodents: a histological and ultrastructural study.

This study describes the structure of the harderian gland in desert rodents: 3 Gerbillidae species (Gerbillus gerbillus, Meriones crassus, Psammomys obesus) and 1 Ctenodactylidae species (Ctenodactylus vali). In all these species the gland consists of tubules lined by a single layer of epithelial cells and possesses myoepithelial cells within their basal laminae. The gland contains porphyrin which is stored as solid intraluminal deposits. The glandular epithelium presents a single cell type (type I) in Psammomys obesus, 2 cell types (I and II) in Ctenodactylus vali and 3 (I, II and III) in Gerbillus gerbillus and Meriones crassus. The type I and II cells are columnar. They are characterised by many lipid vacuoles and a well developed vesicle-like structure of smooth endoplasmic reticulum. In Gerbillus gerbillus and Meriones crassus the type I cells can be distinguished from type II cells by cytoplasmic clefts approximately 1 micron in length. In Ctenodactylus vali type I cells are characterised by cytoplasmic rod-shaped crystalloid structures approximately 0.5 microns in length which are frequently observed in the mitochondrial matrix. These structures are also present in the sole cell type of Psammomys obesus. Most of the secretory lipid vacuoles of the type I cell contain an electron-dense material, possibly porphyrin, which presents different appearances according to species: it is lamellar in Gerbillus gerbillus, trilamellar in Meriones crassus, and amorphous in Psammomys obesus and Ctenodactylus vali. Secretory lipid vacuoles are released primarily by exocytosis, but holocrine and apocrine secretion is also observed. The type III cells are pyramidal. This cell type is characterised by the presence of an extraordinarily well developed granular endoplasmic reticulum, organised in concentric lamellae in Gerbillus gerbillus, and very numerous mitochondria. Epithelial cells are frequently binucleate. The single excretory duct contains both mucous and serous cells. Mast cells, plasma cells, macrophages, fenestrated capillaries and unmyelinated nerve endings with clear or dense-cored vesicles are present in the connective tissue. Melanocytes are very numerous in the interstices of the Gerbillidae harderian gland. The gland is surrounded by a collagenous capsule and an outer layer of endothelial cells derived from the orbital venous sinus.