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The opossum Monodelphis domestica possesses a well-developed vomeronasal system. Uptake of chemical stimuli into the vomeronasal organ (VNO) involves a stereotypical "nuzzling" behavior. In the present study, ten animals were examined by light and electron microscopy. The peripheral oro-nasal structures that apparently enhance access of solutes into the VNO include: (1) two lateral grooves of the ventral rhinarium and a network of channels leading into them, (2) dental gap adjacent to the grooves, (3) butterfly-shaped incisive papilla, and (4) unique bristle/cup-shaped filiform papillae on the tongue. The longitudinal axis of the vomeronasal complex is composed of the VNO proper at its rostral end, and an extensive compound serous gland at its caudal end with a distinct transition zone in between. The transition zone is characterized by the following features: merging of the main excretory duct of the large vomeronasal gland with the VNO lumen and drainage of auxiliary glandular clusters into the lumen, irregularities in the sensory epithelium ("rosette" appearance), and the ending of the cartilaginous support surrounding the VNO. Multiple elongated bundles of smooth muscle are positioned between the sensory epithelium and the cartilaginous capsule and more caudally are intertwined with the glandular parenchyma. These bundles become more numerous at the transition zone. Contraction and extension of these muscles may function to enhance the flow of solutes and glandular secretion within the lumen. Two extensive venous sinuses are associated with the opossum VNO complex: the internal vein bordering the sensory epithelium at its rostral end, and the external vein alongside the nonsensory epithelium. This arrangement suggests a unique dual pumping mechanism. |
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