Introduction

The central nervous system (CNS) consists of the brain and spinal cord. Both structures have little connective tissue support. Both neuronal cells and fibers are supported by other cells types called glial cells or neuroglia. In addition to holding the neurons and their processes in place, glial cells have other functions, e.g., myelin formation by oligodendrocytes.

Sections through the brain and spinal cord show two distinct areas: gray matter and white matter. The gray matter of the central nervous system consists of cellular elements (neuronal and glial) and nerve fibers. Bundles of fibers in the CNS are described as tracts, and concentrations of neuronal cell bodies in the CNS are called nuclei (in contrast to the peripheral NS, where clusters of nerve cell bodies are called ganglia and where bundles of fibers are termed nerves.

The position of gray matter and white matter differ in the brain and spinal cord. In the spinal cord, the gray matter is internal to the white matter. In the cerebral cortex and cerebellar cortex, these positions are reversed. Thus, the outer regions (cortex) of the cerebrum and cerebellum consist of gray matter which has white matter beneath it. The tissue of both cortices is folded into gyri for the cerebrum and into folia for the cerebellum Nerve cell of various types are disposed into layers lying parallel to the pial surface of both the cerebrum and the cerebellum.

There are several different histological stains used to stain CNS tissue sections. Each reveals a specific and different aspect of nervous tissue than that shown by the others. Silver staining demonstrates nerve cell bodies, large cell processes, and fiber tracts by reacting with the neurofibrillar content. Cytoarchitecture and fibroarchitecture are revealed, as well as individual axon size and axon distribution into tracts, but silver does not stain myelin or glial cell processes.

Heidenhain's method for myelin sheath with metanil yellow counterstain is specific for the myelin sheath and is used primarily to show the number and arrangement of bundles of nerve fiber (tracts) as well as thickness of the myelin sheath. Windle's thionin stain (Nissl stain) demonstrates the basophilic components of cells, particularly the nuclei, nucleoli, and ribonucleoprotein granules of Nissl bodies found in the cytoplasm. It does not show fibers (either axons or dendrites), myelin, nor synapses. It is used primarily to demonstrate the grouping of nerve cell bodies into nuclei or layers, and offers some information about the size of the cell bodies, the density of cell packing, and the state of development of the Nissl bodies.

Home - Labs - Tests - Help - Search - Buggz - Contacts - Links