The vomeronasal organ of the South American armadillo Chaetophractus villosus (Xenarthra, Mammalia): anatomy, histology and ultrastructure

The vomeronasal organ (VNO) is a chemoreceptive structure that has not been extensively studied in the Xenarthran order. Tissue samples from the VNO o f the armadillo Chaetophractus villosus were prepared for light and electro n microscopy. The VNO is located in the anterior part of the base of the na sal septum. It is tubular in shape, similar to 18 mm in length and opens in the rostral region of the nasal cavity and with a blind caudal end. Its lu men is lined by sensory (SE) and nonsensory (NSE) epithelium. The SE shows sensory, supporting and basal cells whereas the NSE contains ciliated and n onciliated secretory cells and basal cells. At the ultrastructural level, t he sensory cells appear as bipolar neurons with conspicuous microvilli on t heir free surface. The supporting cells of the SE contain numerous membrane -bound vesicles in their apical regions. A peculiar feature not found in ot her mammals, is the presence of concentric whorls of RER cisterns frequentl y observed in their basal expansions. Infiltrating plasma cells can be dete cted in the SE basal region close to the dorsal junctional area. This regio n also exhibits an unusual type of basal cell, probably responsible for the generation of new vomeronasal receptor neurons. The ciliated NSE cells exh ibit numerous ovoids or irregularly shaped membranous protrusions projectin g from the plasma membrane of the cilia. As far as we know, this is the fir st study reporting the presence of this feature in ciliated NSE cells. The nonciliated cells are characterised by scarce large secretory granules and apical microvilli. The vomeronasal glands are compound-branched tubuloacina r glands with serous acinar cells. Four types of secretory granules are pre sent. The ducts of these glands reach the lumen in the dorsolateral region between the NSE and SE. Hypolemmal nerve terminals were observed contacting secretory cells. Fenestrated and nonfenestrated capillaries constitute the vascular supply to these glands. Plasma cells, intimately associated with acinar cells, were frequently observed.