A light and electron microscopic study of the interstitial nucleus of Cajal in rat.

The morphology of the interstitial nucleus of Cajal (INC) in the rat was studied with the light and electron microscope. The INC was mapped throughout its rostrocaudal extent from cresyl violet-stained frozen sections cut transversely through the midbrain in the stereotaxic plane. Caudally, the INC consisted of a small number of scattered cells lying ventrolateral to the periaqueductal grey. In three of four cases studied, the caudal tip of the nucleus was located between 40 and 120 micrometers rostral to the rostral tip of the somatic cell columns of the oculomotor nucleus. Proceeding rostrally, the INC increased in size, reaching its maximal development just caudal to its most rostral extent. The INC was limited rostrally by the fibers of the fasciculus retroflexus. The mean rostrocaudal length of the INC was 1.12 mm. On the basis of light microscopic observations of cresyl violet-stained paraffin sections, two groups of neurons could be distinguished in the INC. One group consisted of large, oval to multipolar cells with mean dimensions of 33 X 23 micrometers. The second group, which included by far the greatest number of cells, consisted of small to medium neurons, round, triangular, polygonal or fusiform in shape, with mean dimensions of 19 X 14 micrometers. Injection of horseradish peroxidase into lesions in the cervical spinal cord resulted in retrograde labeling of neurons of all sizes and shapes throughout the length of the INC. Labeled neurons were also found in the red nucleus, the mesencephalic reticular formation, and the nucleus of the posterior commissure. All the morphological varieties of neurons described with the light microscope could be identified in the electron microscope. Large neurons, and some cells of the small to medium group, contained well developed Nissl bodies together with numerous cytoplasmic organelles. Many neurons in the small to medium group, however, did not contain conspicuous Nissl bodies, and had a poorly developed rough endoplasmic reticulum. Axon terminals containing either round or pleomorphic vesicles were seen in the INC. Axosomatic synapses were formed by both types of terminals. Such synapses were usually symmetrical, regardless of the shape of the vesicles within the terminal. In a number of neurons, the percentage of the surface of the neuronal somata in direct apposition to axon terminals was measured. The results of such measurements suggest that a greater percentage (more than 50%) of the surface of larger neurons is apposed by axon terminals than is the case with smaller neurons, which, on the average, were invested by axon terminals over 15% of their total surface in any given single plane of section. Axon terminals investing the surfaces of proximal dendrites were morphologically similar to those in apposition to neuronal somata.