| Abstract: | The total number of vomeronasal nerve synapses was estimated in a series of mice from 1 to 12 months of age. The volume of the glomerular layer of the accessory olfactory bulb was derived from area measurements through a series of semithin sections. The specialized appositions constituting the vomeronasal nerve synapses were identified in electron micrographs of the glomerular layer by the characteristic electron dense matrix, vesicles, shape and arrangement of the presynaptic elements, and the pale, vesicle-containing dendritic postsynaptic elements.The volume of the glomerular layer showed an overall increase from 66 x 103 cu. μm at 1 month of age to 82 x 106 cu. μm at 12 months (although there was almost equally large intergroup variation probably not associated with age). The number of synapses per unit area was between 0.114 and 0.131 per sq. μm and unrelated to age. The average length of the synaptic appositions increased from0.32 ± 0.01 μm at 1 month to0.36 ± 0.02 μm at 12 months (the major increase occurring between 1 and 4 months). The calculated total numbers of synapses in the glomerular layer were between 20 and 21 millions at 1, 4 and 8 months and showed a statistically insignificant increase to slightly over 23 millions at 12 months. Since there are about 140 thousand vomeronasal axons, each axon makes on the average about 150 synapses.New vomeronasal neurosensory cells are continually being formed during life and the new cells develop axons which grow to the bulb. However, both the overall number of cells in the epithelium and the total number of synapses in the bulb are constant after about 8 months of age. This could be achieved without synaptic turnover if the newly formed cells die before making synapses. Alternatively, it could be achieved if the entire population of neurosensory cells and their synapses are undergoing continual replacement. |
