The human vomeronasal organ. Part II: prenatal development

During the 20th century, the human vomeronasal organ (VNO) has been controv ersial regarding its structure, function, and even identity. Despite report s that provide evidence for its presence throughout prenatal and postnatal ontogeny, some studies and numerous textbooks declare its absence in late f etal and postnatal humans. To that end, the present study was designed to e stablish firmly whether the human VNO is homologous with that or other mamm als and whether it degenerates (partially or completely) or persists throug hout prenatal development. Fifty human embryos and fetuses (33 d to 32 wk f ertilisation age) and 2 neonates were examined by light microscopy. Four em bryonic primates (mouse lemurs) were examined for a comparison of VNO embry ogenesis. The presence or absence and structural characteristics of the VNO and supporting tissues are described. The first appearance of the VNO was in the form of bilateral epithelial thickenings of the nasal septum, the vo meronasal primordium. The primordia invaginated between 37 and 43 d of age and formed the tubular VNO. The tubular VNO was located dorsally at a varia ble distance from, but was always spatially separated from the paraseptal c artilages. The mouse lemurs examined in this study and other reports from t he literature indicate that the human VNO resembles that of primates having functional VNOs until just after a tubular VNO is formed. Examination of t he VNO and adjacent tissues suggested that the VNO may lose receptor cells and corresponding vomeronasal nerves and become a ciliated, pseudostratifie d epithelium between similar to 12 and 14 wk of age. Our findings indicate the prenatal human VNO goes through 3 successive stages: early morphogenesi s, transformation (of the epithelium), and growth. These observations indic ated that (1) all embryonic humans develop a vomeronasal organ which is hom ologous with the VNOs of other mammals, but which has become displaced and highly variable in relative location during embryogenesis; (2) the human vo meronasal organ does not degenerate prenatally, but very likely loses the f unctional components of the vomeronasal complex of other mammals; and (3) t he remnant of the human VNO persists until birth and beyond.