Vomeronasal and olfactory nerves of adult and larval bullfrogs: II. Axon terminations and synaptic contacts in the accessory olfactory bulb

The ultrastructure of the accessory olfactory bulb (AOB) of the bullfrog tadpole and adult was examined, and the main difference between tadpole and adult is that the latter is more compact and shows more synapses. Except for vomeronasal (VMN) glomeruli, the AOB is not highly organized, with mitral cell neurons scattered throughout the neuropil. VMN axon terminals form asymmetric synapses with mitral cell dendrites in glomeruli; in VMN axon terminals, dense-cored vesicles are seen along with the more abundant lucent vesicles 40-50 nm in diameter. Counts indicated that more than 90% of the dendro-dendritic synapses between mitral cells and presumed granule cells are of the asymmetrical type, and reciprocal asymmetrical-symmetrical synapses are not common. Lucent vesicles with round or slightly ellipsoidal profiles and less abundant dense-cored vesicles 60-90 nm in diameter are found in pre- and postsynaptic dendrites; sometimes the dense-cored vesicles lie against or near the presynaptic membrane. Microtubules were often seen to be closely associated with pre- and postsynaptic elements of dendro-dendritic synapses. The most characteristic feature of mitral cell bodies, apart from their large size, is an extensive Golgi system that may extend well into their major dendritic extensions. Dense-cored vesicles are associated with Golgi membranes, from which they probably originate. Centrioles are associated with the Golgi system, and some become basal bodies and give rise to cilia in some mitral cells.