The human vomeronasal organ. Part VI: A nonchemosensory vestige in the context of major variations of the mammalian vomeronasal organ

A vomeronasal organ (VNO) is found in most extant amphibians, reptiles, and mammals, but is absent in extant archosaurs (birds and crocodilians). In amniotes, the VNO differs greatly from its basal form, a simple neuroepithelial patch, as it still exists in most lissamphibians, and in some taxa (e.g., primates and bats) it presents extreme variations in epithelial structure. The history of the VNO literature since Ruysch [1703] prompts the question: what is a mammalian vomeronasal organ? Situated bilaterally, in the anteroventral nasal septum, the VNO is a part of a composite epithelial tube. Like any other sense organ, it includes a patch of sensory neuroepithelium (the vomeronasal neuroepithelium, VNNE). In certain species (e.g., man, chimpanzee), a low columnar ciliated, microvillar tube is generally present which also doubles as a septal glandular duct. The ancillary vomeronasal (VN) structures are the VN nerves (axons of the neurosensory VN receptors with the interspersed paravomeronasal ganglia), the accessory olfactory bulb and projections thereof, the chondro-osseous capsule, and glands all collectively called the vomeronasal organ complex. In order to standardize the terminology, our proposed definition of the primitive condition of the mammalian vomeronasal organ is: an epithelial patch or tube of microvillar chemosensory neuroepithelium. This neuroepithelium is generally continuous with a patch of ciliated “receptor-free epithelium”(RFE), or a bare nondescript epithelium that completes the tube around its lumen. Two broad categories of the mammalian VNO exist: chemosensory VNO or non-chemosensory vestige of the VNO.