Ultrastructure of neurons in the nucleus basalis of Meynert in squirrel monkey.

The nucleus basalis of Meynert in the squirrel monkey exhibits numerous labeled neurons following the retrograde transport of horseradish peroxidase from occipital cortical injection sites. The typically large, often clustered, labeled cells are seen most frequently in association with the fibrous bordering structures of the substantia innominata and in the internal and external laminae of the globus pallidus. Ultrastructurally the copious cytoplasm of nucleus basalis neurons abounds with organelles. Large, vacuolated lipofuscin granules proliferate as a function of age and are not evident in younger monkeys. Approximately 4% of the somal surface is occupied by symmetrical synapses with either flat or pleomorphic vesicles. The remainder is covered mostly by neuroglial processes. Somatic spines bearing synapses are occasionally observed. In the neuropil surrounding nucleus basalis somata, the synapses onto dendrites and spines are mostly asymmetrical with large, round vesicles. Labeled nucleus basalis cells in the substantia innominata immediately lateral to the optic tract are larger and rounder than cells in the internal and external pallidal laminae. However, no remarkable ultrastructural differences were observed between nucleus basalis somata in the substantia innominata and external pallidal lamina, or between horseradish peroxidase-labeled and unlabeled large cells.