The present study on the main olfactory system (MOS) and the accessory olfactory system (AOS) documents the functional morphology of the rodent olfactory region and that of the vomeronasal organ (VNO) using light and electron microscopical techniques. Special attention is given to the cytoarchitecture of the sensory epithelia, i. e. the olfactory epithelium (OE) of the regio olfactoria and the neuroepithelium of the VNO (VNO-NE). Both sensory epithelia consist of a pseudostratified columnar epithelium composed of three types of cells, i. e. receptor cells, supporting cells and progenitor cells. Even at the light microscopical level, however, distinctive morphological features can be distinguished which illustrate important differences between the two sensory epithelia. For example, the height of the respective epithelia differs considerably, the VNO-NE is approximately 170 mum tall and the OE is only about 90 mum. The receptors of the VNO-NE lack olfactory knobs which are typically found in the sensory cells of the OE. The perikarya of the receptor cells of the VNO-NE are very large when compared to those of the sensory cells of the OE. In contrast to the OE, blood vessels are found within the neuroepithelial layer of the VNO. The progenitor 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 level. The olfactory knobs of the sensory cell dendrites of the OE reach the nasal cavity with numerous cilia. These olfactory hairs, on average 11 per knob, consist of a short proximal segment and a long and thin distal segment. This distal segment runs parallel to the epithelial surface and is embedded in the neuroepithelial mucosal layer. The dendrites of the receptor cells of the VNO-NE reach the lumen of the VNO with numerous branched microvilli which are also embedded in the mucous layer. Horizontal ultrathin sections through the apical portion of the OE reveal that each supporting cell completely envelopes several dendrites. This glia-like relationship is not found in the corresponding layer of the VNO-NE. The sensory cell perikarya of the OE contain only a few endoplasmatic reticulum (ER) profiles while the receptor cells of the VNO are characterized by an extensive smooth endoplasmatic reticulum (SER). In contrast to the fila olfactoria, numerous axons within 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 present study add to the body of morphological knowledge of these organs and aid in the understanding of the specific functioning of the respective systems.
