Development of the olfactory epithelium and vomeronasal organ in the Japanese reddish frog, Rana japonica

Histological and ultrastructural development of the olfactory epithelium (OE) and vomeronasal organ (VNO) was investigated in the Japanese reddish frog, Rana japonica. Tadpoles, from hatching to the end of metamorphosis, and adult frogs were examined. In the adult, olfactory cells of the OE were equipped with olfactory vesicles with long cilia, but supporting cells with microvilli. The supporting cells of the OE contained secretory granules, PAS-positive by light microscopy, in their apical cytoplasm. On the contrary, sensory cells of the VNO were equipped with microvilli, and supporting cells of the VNO were equipped with cilia, but without secretory granules. Embryologically, the olfactory cells were indistinguishable from the supporting cells in the olfactory placode (primitive OE) lining the nasal pit, at hatch. The VNO appeared as a concave of the ventral part of the OE at 4 days after hatch. At this time, the olfactory and supporting cells of the OE became distinguishable from each other. Secretory granules were formed in the supporting cells of the OE at 36 days after hatch, and the OE was similar in fine structure to that in the adult. While, the VNO remained immature at 24-36 days after hatch, and did not complete its ultrastructural development at 60 days after hatch, the end of metamorphosis. These findings suggest that the OE may take part in the olfaction earlier than the VNO in ontogeny.