Computer-assisted stereological and quantitative morphological approaches were used to analyse cerebellar glomeruli of the "simple type" in serial ultrathin sections. It was found that, of the total volume (110-200 micron3) of the glomeruli studied, 53% was occupied by granule cell dendrites, 34% by mossy terminal and 13% by Golgi axons. None of the four analysed glomeruli contained Golgi cell dendrites. The mossy terminals that were studied received, on the average, 53 granule cell dendrites. All of the dendrites originated from different granule cells and all made synaptic contacts with mossy terminal. However only about 60% of granule cell dendrites made synapses with Golgi axons. The surface of the mossy terminals occupied by synaptic junctions, was found to be 5.4-5.5%. Each granule cell dendrite emitted 3-5 terminal protrusions ("dendritic digits"). Each digit receives one or more synaptic contact from either the mossy terminal (67% of all digits), or from Golgi axon varicosities (25%). Only about 8% of all digits were contacted synaptically by both types of axonal terminals. All of the dendritic digits that were observed made synaptic connections. Each digit was, on the average, connected by symmetric attachment plaques to 4 neighbouring digits. Three-dimensional reconstructions of mossy terminal and some of contacting granule cell dendrites demonstrated that the dendrites curved around the central mossy terminal and were much longer than expected from earlier Golgi-impregnation studies. In addition to mossy terminals and Golgi axons, an axon terminal of small calibre that contained large, empty, spheroid vesicles were occasionally observed.(ABSTRACT TRUNCATED AT 250 WORDS)

The differentiation of cerebellar glomeruli was investigated by quantitative electron microscopy, starting with the period at which mossy fibers made their first appearance. Two developmental stages could be delineated in the maturation of the mossy fiber-granule cell synapse. 1. A primary growth stage (postnatal days 6--15) characterized by the rapid enlargement of mossy rosettes and the intense proliferation of post-synaptic dendrites. The synaptic perimeter of mossy terminals, i.e. percentage of membrane surface occupied by synaptic junctions, exhibited a simultaneous, rapid increase in that stage, reaching a peak at postnatal day 15. 2. Establishment and stabilization of differentiated glomeruli (15th--45th day). Because the size of mossy rosettes did not change in this period, the increase of glomerular size was due exclusively to the continuing multiplication of postsynaptic dendrites. The characteristic feature of this stage was the massive elimination o synaptic junctions. The synaptic perimeter of 14.4% at day 15 decreased to 5.7% by day 30. Since the size of individual synaptic junctions and the size of mossy terminals did not decrease while the number of postsynaptic dendrites even increased during the same period, the elimination of synaptic junctions represents a net loss of the synaptic perimeter of mossy terminals. The quantitative analysis suggests that the stabilization of the synaptic perimeter of mossy rosettes at about 6% is due to the elimination during the second developmental stage of immature synaptic junctions, produced in excess during the first growth phase. Also, the observation that synapse elimination and the subsequent stabilization of synaptic perimeter occurs in spite of a steady increase of available postsynaptic dendrites is indicative that the standard 6% value of synaptic perimeter is defined by the presynaptic mossy terminal itself. On the basis of these observations, it is also proposed that elimination of synaptic junctions may well occur without the concomitant disappearance of presynaptic and/or postsynaptic neuronal processes.

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