Histochemistry of the Brain Stem
Different regions of the brain stem are characterized by different contents in chemical substances. The delimitation of areas according to their chemical composition is called chemoarchitectonics. Substances can be demonstrated by quantitative chemical analysis after homogenization of the brain tissue, or by treating histological sections with certain chemicals that make it possible to show the exact localization of a substance in the tissue. The methods com-plement each other.
Iron was one of the first substances forwhich different distributions were demon-strated. By means of the Berlin blue reac-tion, a high iron content can be demon-strated in the substantia nigra (A1) and in the pallidum, while a lower iron content is found in the red nucleus (A2), in the dentate nucleus of the cerebellum, and in the stri-atum. The iron is contained in neurons and glial cells in the form of small particles. This high iron content is a characteristic of the nuclei that make up the extrapyramidal system.
Neurotransmitter substances and theenzymes required for their synthesis and degradation show marked regional varia-tions. While catecholaminergic and sero-toninergic neurons form specific nuclei inthe tegmentum, the motor nuclei of cranial nerves are characterized by a high content in acetylcholine and acetylcholineesterase. Quantitative chemical analysis ofbrain tissue yields a relatively high content of norepinephrine in the tegmentum of the midbrain (B3), but a considerably lower content in the tectum (B4) and in the teg-mentum of the medulla oblongata (B5). The content of dopamine is particularly high in the substantia nigra (B1) and very low in the rest of the brain stem.
Metabolic enzymes (C) also show regionalvariations in their distribution. Activity of oxidative enzymes is generally higher in graymatter than in white matter. In the brainstem, activity is particularly high in the cranial nerve nuclei, the lower portion of the olive, and the pontine nuclei. The differ-ences refer not only to the individual areas but also to the localization of enzyme activ-ity within the cell bodies (somatic type) or in the neuropil (dendritic type).
Neuropil. The substance between the cellbodies, which appears amorphous in Nissl-stained material, is called the neuropil. It consists mainly of dendrites and also of axons and glial processes. The majority of all synaptic contacts are found in the neuropil.
The distribution in the medulla oblongata of succinate dehydrogenase (an enzyme of thecitric acid cycle) serves as an example for different localizations of an oxidative meta-bolic enzyme within the tissue: in the oculo-motor nucleus (C6), its activity in the peri-karya and in the neuropil is high, while it is low at both locations in the solitary nucleus (C7). In the posterior nucleus of the vagusnerve (C8), the cell bodies contrast with theneuropil owing to their high activity. By comparison, the highly active neuropil in the gracile nucleus (C9) lets the poorly re-acting perikarya appear as light spots. Fiber tracts (for example, the solitary tract) (C10) show very low activity. The distribution of enzymes is characteristic for each nuclear area and is referred to as the enzyme pattern.