Vomeronasal and olfactory nerves of adult and larval bullfrogs: I. Axons and the distribution of their glomeruli.

The distribution of primary olfactory and vomeronasal (VMN) glomeruli is demonstrated in the forebrain of the larval (stage III tadpole) and adult bullfrog. Olfactory glomeruli are massed at the anterior end of the olfactory bulb; farther posterior, they are located at the periphery and then become distributed in a superficial layer all along the ventral surface of the bulb. Glomeruli of the accessory olfactory bulb (AOB) of the tadpole are less compact and show larger dendritic profiles and fewer synaptic junctions per unit area than those of the adult AOB. Axons of both tadpole and adult VMN nerve are ultrastructurally indistinguishable from those of the primary olfactory nerve; in comparing VMN axons of tadpole and adult, however, differences were observed in the number of neurofilament profiles in axons at the proximal and distal ends of the VMN nerve. Counts of neurofilament profiles in the two ends of the nerve suggest that neurofilament segments accumulate near terminations of their axons. The VMN nerve of tadpole and adult frog are roughly the same size, and calculations indicate that they contain comparable numbers of axons. The primary olfactory nerve of the tadpole is estimated to contain about 660,000 axons; the adherent VMN nerve of the tadpole contains about 270,000 axons (a ratio of about 2.5:1). In the adult, the ratio of primary to VMN axons is about 64:1, reflecting the quantitative growth of the primary olfactory nerve with development from an aquatic tadpole to an air-breathing adult.