The Glial Cells and Glia-Neuron Relations in the Buccal Ganglia of Planorbis corneus (L.): Cytological, Qualitative and Quantitative Changes During Growth and Ageing

Buccal ganglia from snails ranging from embryos to 5.5 g adults (approximately five years old) were examined by light and electron microscopy. The glial cells are principally located around the neuron perikarya, between the blood and the neurons. Relatively few glial cells are located in the axon tracts and areas of neuropil within the core of the ganglion. The glia-neuron relations are established early in embryonic life and remain relatively constant throughout the life of the animal. The glial cells contain increased numbers of lysosomes and lipofuscin material with age. Some glial cells make up a distinct subpopulation that is specialized for the phagocytic ingestion of debris and nerve terminals in the core of the ganglion. These cells appear to degrade the ingested material to lipofuscin residues, and migrate to the edges of the ganglia where they aggregate in localized clumps. The phagocytic glial cells and the peripheral lipofuscin residues increase in numbers with age. The glial cells phagocytose nerve endings labelled with radioactive serotonin. The glial cells around the neuron perikarya contain varying numbers of glycogen particles, gliagrana and lipid droplets. Haemoglobin is present in the glial cell cytoplasm. The pigment appears identical to that present in the blood and other peripheral tissues, and may act as an oxygen store or facilitate diffusion of oxygen to the neurons when the animal is submersed. Light and electron microscopic histochemical studies of glucose-6-phosphatase and 5'-nucleotidase localization demonstrated reactive sites along the glial cell membranes, except where they face the basement membrane surrounding the ganglia. Relatively few reactive sites were present on neuronal membranes. The glial cells actively accumulate tritiated nucleosides and bind them within their cytoplasm. The bound material is not measurably associated with DNA-RNA-protein mechanisms. Glial uptake of nucleosides has been previously reported in other nervous systems although the physiological significance is not yet clear. Two of the nucleosides were employed as selective anatomical markers for the different components of the ganglia (glia, neurons, connective tissue). [^3H]Uridine labels the glial cytoplasm and connective tissue, and [^3H]adenosine, under the appropriate experimental conditions, distinguishes the neuronal and glial nuclei. Serial-section radioautographs were analysed by computer image analysis, and the components of the ganglia measured quantitatively throughout the life span of the animal. The following have been established. (i) The percentage volumes of the ganglia occupied by neuronal perikarya (33%) and glial cells (43%), and the ratio of the volumes of neuronal perikarya to glial cells remain constant. (ii) Glial cell numbers increase significantly (18%; mean number 391 cells per ganglion). The increase is chiefly due to the increase in phagocytic glial cells. There appears to be little or no turnover of glial cells. (iii) The major change responsible for the increased volume of the ganglion (approximately fourfold) and increased volumes of the neurons (approximately fourfold) is the growth and proliferation of axonal and dendritic processes. (iv) Neuronal `fallout' does not occur in the buccal ganglion (mean number 298 neurons per ganglion). The cytological, quantitative and functional properties of the glial cells and glia-neuron relations are discussed in relation to glial cells in other nervous systems.