Anatomy, histology, histochemistry and fine structure of the Harderian gland in the South American armadillo Chaetophractus villosus (Xenarthra, Mammalia)

The anatomical, histological, histochemical and ultrastructural characteristics of the Harderian gland of the armadillo Chaetophractus villosus were described. The gland is the largest structure in the bony orbit. It is situated in the anteroventral region of the orbit. Obvious structural differences are not observed between males and females. The gland is compound-branched tubulo-alveolar, being characterized by a single layer of columnar cells surrounded by myoepithelial cells. It possesses a single excretory duct opened into the inner canthus. All glandular cells show yellow-green autofluorescence and additionally some glandular lumen may contain dense autofluorescent solid accretions. There are two peculiar and outstanding cytoplasmic features. One is represented by the smooth endoplasmic reticulum (SER), forming a closely woven meshwork. The other one is represented by "membranous bodies" apparently derived from the SER, RER and cytoskeleton with a "Star of David" configuration situated in the supranuclear region. Three types of vesicles are detected in the cytoplasm. Histochemical staining methods reveal lipids, proteins, neutral and acidic containing glycoconjugates in secretory vesicles. The mechanism of secretion appears either merocrine or apocrine. The epithelium of the intra- and interlobular excretory ducts suggests secretory activity. Tubulo-acinar glands similar to those seen in the lacrimal gland and nictitans glands are found related to the intralobular and main excretory ducts. The capillary network is characterized by fenestrated endothelium. The stroma possesses unmyelinated axons and plasma cells. The normal secretion of the secretory endpieces, particularly lipids, proteins and glycoconjugates, is complemented by mucous and serous secretions released by ductal cells and glands associated to the ducts.