The locus coeruleus of cat

Summary The locus coeruleus of cat is populated by two types of neurons: medium sized ones, with plump cell bodies and relatively short dendrites; and small ones, with triangular bodies and relatively long dendrites. The former type is regarded here as typical of the centre, whereas the second type could simply represent displaced neurons from the adjacent griseum centrale. Electron microscopy failed to reveal any outstanding richness in pigment granules in kittens up to five weeks old. Very characteristic somatic appendages were found, mostly in the medium sized neurons. These somatic spines communicate with the perikaryon by means of a narrow neck region. A complex, multilayered, glial sheath surrounds the cells. This glial sheath is pierced by the somatic appendages, which are not surrounded by glia and make contact with axonal knobs. Typical dendritic spines appear to be absent. Axodendritic synapses are made on medium sized dendritic trunks. By and large, most of the synaptic vesicles present in the centre are of the small, clear-centered type. However, dense core vesicles extremely variegated in size and appearance were found, both in presynaptic and postsynaptic profiles. The possibility that dense core vesicles should be regarded as atypical lysosomes rich in by-products of the metabolism of catecholamines (melanine) has been considered.Supported by grant MA 4183 from the Medical Research Council of Canada.