DEVELOPMENT OF OLFACTORY GLOMERULI - TEMPORAL AND SPATIAL INTERACTIONS BETWEEN OLFACTORY RECEPTOR AXONS AND MITRAL CELLS IN OPOSSUMS AND RATS

Mitral cells are the primary output neurons of the vertebrate olfactor y bulb and are major recipients of sensory input from the periphery. T he morphogenesis of mitral cell dendrites was followed to elucidate th eir early spatial and temporal interactions with olfactory receptor ne urons and glia during the construction of olfactory glomeruli. Monodel phis domestica, a marsupial born at an extremely immature stage, and r ats were examined. Mitral cells were retrogradely labeled by applicati on of the lipophilic dye 1'dihexadecyl-3,3,3'3'-tetramethylindocarbocy anine perchlorate (DiI) to the lateral olfactory tract. In double-labe ling experiments, olfactory receptor neurons were stained with 3,3'dih exadecyloxacarbocyanine perchlorate (DiO), or olfactory nerve Schwann cells were visualized using S-100 protein immunohistochemistry. Tissue was examined with a confocal laser scanning microscope. Some preparat ions were subsequently investigated with an electron microscope. In Mo nodelphis, differentiation of mitral cells starts with an outgrowth of numerous, uniform, and widespread dendrites. As soon as terminals of olfactory receptor axons coalesce into glomerular knots within the pre sumptive glomerular layer, dendrites of individual mitral cells innerv ate several adjacent glomeruli where they receive sensory synaptic inp ut. With maturation, supernumerary mitral cell dendrites retract, leav ing one primary dendrite bearing a terminal glomerular tuft. Simultane ously, secondary dendrites begin to arise. The formation of glomeruli begins earlier and progresses faster in the rat compared to Monodelphi s. Nevertheless, mitral cell differentiation in both species follows a common sequence: overproduction of dendrites, selection of usually on e primary apical dendrite, and elimination of supernumerary processes. Since olfactory receptor neurons form synaptic contacts with the wide spread mitral cell dendrites, considerable synaptic rearrangement must occur within the olfactory glomeruli during maturation. (C) 1996 Wile y-Liss, Inc.