The structure, distribution, and quantitative relationships of the glia in the abdominal ganglia of the horse leech, Haemopis sanguisuga.

The glial cells in abdominal ganglia of the horse leech Haemopis sanguisuga were studied by electron microscopy and analysed quantitatively to evaluate the suitability of this easily obtainable carnivorous species for physiological studies. Each abdominal ganglion contains eight giant glial cells, 12,000-14,000 small glial cells, and approximately 300 neurons. The giant glial cells constituted 44.6% and the small glial cells 6.4% of the ganglion's volume. The giant glial cells contain glycogen and bundles of filaments that are chiefly located in their periphery, close to the neurons into which they send processes. The small glial cells are frequently surrounded by the giant glial cells but also occur around neuronal perikarya and axon tracts, as well as against the basal lamina and connective tissue layers. The small glial cells contain lysosomes and sometimes form a trophospongium with the neurons. A system of extracellular channels, which is continuous with the basal lamina, indents the giant glial cells and extends around parts of the neurons. The extracellular channels contain a matrix that appears very similar to the basal lamina and to the cytoplasm in the processes of the small glial cells. Some of the extracellular channels contain collagen fibrils. Hemidesmosomes join the matrix-filled extracellular channels to both the neurons and the giant glial cells. Ionic lanthanum has a free access to the neurons and glial cells via the extracellular spaces and matrix-filled channels. Areas of synaptic intermingling rarely contain glial cell processes.