MORPHOLOGICAL-STUDIES ON THE RODENT MAIN AND ACCESSORY OLFACTORY SYSTEMS - THE REGIO OLFACTORIA AND VOMERONASAL ORGAN

The present study on the main olfactory system (MOS) and the accessory olfactory system (AOS) documents the functional morphology of the rod ent olfactory region and that of the vomeronasal organ (VNO) using lig ht and electron microscopical techniques. Special attention is given t o the cytoarchitecture of the sensory epithelia, i. e. the olfactory e pithelium (OE) of the regio olfactoria and the neuroepithelium of the VNO (VNO-NE). Both sensory epithelia consist of a pseudostratified col umnar epithelium composed of three types of cells, i. e. receptor cell s, supporting cells and progenitor cells. Even at the light microscopi cal level, however, distinctive morphological features can be distingu ished which illustrate important differences between the two sensory e pithelia. For example, the height of the respective epithelia differs considerably, the VNO-NE is approximately 170 mum tall and the OE is o nly about 90 mum. The receptors of the VNO-NE lack olfactory knobs whi ch are typically found in the sensory cells of the OE. The perikarya o f the receptor cells of the VNO-NE are very large when compared to tho se of the sensory cells of the OE. In contrast to the OE, blood vessel s are found within the neuroepithelial layer of the VNO. The progenito r cells of the OE are located in a clearly distinguishable cell layer which is lacking in the rodent VNO-NE. The differences between the two epithelial layers become more obvious at the electron microscopical l evel. The olfactory knobs of the sensory cell dendrites of the OE reac h the nasal cavity with numerous cilia. These olfactory hairs, on aver age 11 per knob, consist of a short proximal segment and a long and th in distal segment. This distal segment runs parallel to the epithelial surface and is embedded in the neuroepithelial mucosal layer. The den drites of the receptor cells of the VNO-NE reach the lumen of the VNO with numerous branched microvilli which are also embedded in the mucou s layer. Horizontal ultrathin sections through the apical portion of t he OE reveal that each supporting cell completely envelopes several de ndrites. This glia-like relationship is not found in the corresponding layer of the VNO-NE. The sensory cell perikarya of the OE contain onl y a few endoplasmatic reticulum (ER) profiles while the receptor cells of the VNO are characterized by an extensive smooth endoplasmatic ret iculum (SER). In contrast to the fila olfactoria, numerous axons withi n the vomeronasal nerve show ellipsoidal varicosities without synaptic vesicles which may indicate the existence of at least two vomeronasal nerve fibers. The cytoarchitectonic differences between the OE of the MOS and the VNO-NE, the sensory field of the AOS, described in the pr esent study add to the body of morphological knowledge of these organs and aid in the understanding of the specific functioning of the respe ctive systems.