Quantitative cytology of ganglion neurons and satellite glial cells in the superior cervical ganglion of the sheep. Relationship with ganglion neuron size.

Abstract

Neurons and glial cells of the superior cervical ganglion of sheep were investigated with morphometric methods in the light and electron microscope. The nerve cell sectional area (measured on nucleated cell profiles) ranged from 165 to 2500 microns2, which corresponds to range in cell diameter from 14 to 56 microns and a range in cell volume from 1600 to 93,000 microns3, i.e. a 60-fold volume difference between smallest and largest neurons. The distribution of cell sizes appeared unimodal, with a predominance of small neurons; there were no variations in different parts of the ganglion. This wide range in nerve cell sizes is discussed in the light of the suggestion that large neurons innervate a greater amount of target tissue (e.g. smooth muscle) and are less readily excitable than smaller neurons: it is thus possible that there is differential recruitment of ganglion neurons in autonomic reflexes. The ultrastructural features of ganglion neurons in the sheep were similar to those observed in small laboratory animals. The relative volumes of perikaryal cytoplasm occupied by mitochondria and Golgi apparatus were 8.5% and 4.8%, respectively, but the average values were the same in small and large neurons. Subsurface cisternae of endoplasmic reticulum were common in the perikaryon, while in the dendrites clusters of synaptic vesicles were found beneath the plasma membrane; the absence of a glial wrapping at the latter sites suggests that they are points of (non-synaptic) release of transmitters. The extent of the capsule that satellite cells form around each neurons was compared in size-based classes of neurons. There was no difference in the size of glial nuclei, and this suggests that glial cells are probably of uniform size. However, glial cells were more densely packed over the surface of large neurons than over the surface of small neurons--in fact the packing density was proportional to the ganglion neuron volume, rather than to its surface. The average thickness of the glial capsule was significantly greater around large than around small neurons. It is suggested that the matching of glial cell number and nerve cell volume is achieved during development by glial cell mitosis taking place long after the nerve cells have ceased dividing.