Freeze-fracture analysis of synaptogenesis in glomeruli of mouse olfactory bulb

The relationship between intramembranous particle (IMP) aggregates appearing on the extracellular leaflet (E-face) of the postsynaptic membrane and postsynaptic densities was examined by electron microscopy during mouse olfactory bulb development. During prenatal development the IMP aggregates first increased in size and then decreased in size to the adult level, while the length of the postsynaptic densities tended to increase to a plateau. Concomitant with the size change, the shape of the IMP aggregates appeared to change during development from small, round clusters to large, anastomotic aggregations. Some of the IMP aggregates appeared to have a particle-free area in their centre. As development proceeded, the overall IMP density increased. The density of particles measuring 7-11 nm remained unchanged throughout prenatal life and decreased in the adult. These particles may be involved in stabilization of initial contacts and maintenance of mature synapses, rather than representing receptors or ion channels which would be expected to increase during development. The density of particles smaller than 7 nm increased prenatally, decreased at birth, then increased in the adult. These particles may represent two or more different macromolecules, one important in synaptogenesis, the other important in adult synapses.