== Nerve Fibers And Nerves ==

Osmium stains myelin, therefore the myelin sheaths will be black, unmyelinated fibers are vaguely outlined, other structures do not stain.

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Connective tissue stains blue, nuclei are dark blue, and myelin is orange-red.

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Check the same features as previously described, in this cross-section, to see their appearance with a routine stain.

The pia is a delicate layer of epithelial cells associated with loose fibrocollagenous tissue. It lies external to a basement membrane which completely surrounds the CNS.

With H&E, the meninges as well as the various layers of the cerebral cortex are visible. Distinguish the general regions of the cerebral cortex:

*White matter.

*Gray matter (cerebral cortex).

*Meninges consisting of the pia mater.

The outer portion of the cerebellum, the cortex, is made up of a molecular layer and a granular layer. At the junction between these two layers are the large cell bodies of Purkinje cells. These cells are characteristic of cerebellum. They contain numerous dendrites that extend into the molecular layer and a single axon.

The choroid plexuses are located in the ventricular system of the brain and produce cerebrospinal fluid. Each choroid plexus consists of a vascular stroma covered by columnar epithelial cells, which form large frond-like masses. The cerebrospinal fluid produced in the ventricles flows out through exit foramina at the base of the brain and circulates in the subarachnoid space. It is reabsorbed by the venous sinuses in the dura.

On this trichrome stained section of choroid plexus note the multiple fronds of tissue. Each is covered with ependymal cells and has a connective tissue infrastructure.

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[[File:HistologicChapter6SpinalCord.jpg|thumb|200px|Spinal Cord]]

The spinal cord is composed of two principal parts extending the length of the cord, the inner core of gray matter and an outer layer of white matter. The entire cord is supported in a framework of neuroglia. In the most peripheral part of the cord, processes of fibrous astrocytes condense to form the marginal glial membrane, in which nerve fibers are absent.

When dorsal root fibers enter the cord and ventral root fibers first leave the cord, they lie in the subarachnoid space. As they move laterally to exit through the intervertebral foramina, the dura mater continues over them as epineurium. The arachnoid fades out quickly.

With H&E, details of neuron cell bodies are seen quite well; other tissues stain much as in other organs.

Distinguish the general regions of the cord:

*The H-shaped gray matter with its dorsal gray columns, the broad ventral columns, and the intermediate gray area.

*The white matter with its three divisions.

*The delicate dorsal medial (posterior) septum of neuroglia and the ventral fissure.

*The thick dura mater (dense fibrous connective tissue) - the outermost layer of the meninges.

*The central canal in the gray commissure. It is lined with ependymal cells, which have a columnar shape; some may have flagella.

*Note innumerable myelinated fibers, mostly in cross sections, since these are mainly ascending and descending tracts. Myelin is not preserved; a clear space remains. The axon is unstained or faintly seen, or it may not be present.

*Thin connective tissue septa and small blood vessels can be seen. Look for perivascular spaces around the blood vessels - clear spaces that are actinically enlarged due to shrinkage of tissues.

*The numerous small nuclei seen are mainly those of fibrous astrocytes and oligodendroglia.

*Identify the marginal glial membrane. It forms the most peripheral part of the spinal cord, is formed by processes of fibrous astrocytes, and lacks nerve fibers.

*Note the uppermost large pale area (substantia gelatinosa), the more fibrillar base of the dorsal horn, and myelinated fibers sweeping into these areas from the dorsal white columns (incoming sensory fibers).

*The small nuclei are astrocyte and oligodendroglia nuclei. Small neuron cell bodies are not easily seen.

*Some of the sections may show the actual entrance of the dorsal roots.

These are the post-ganglionic visceral efferent neurons. Axons of neurons in the lateral sympathetic nucleus of the thoracic cord had come to the sympathetic chain via the white rami communicantes. Many have synapsed on these ganglion cells. (Others pass through the chain to synapse elsewhere.)

This ganglion has an epineurium but many ganglia do not show such a distinct capsule.

These are in the walls of organs. Preganglionic parasympathetic fibers go from the cell bodies in the brain stem or sacral spinal cord to the organs to be innervated and synapse on small, typical multipolar neuron cell bodies in ganglia (post-ganglionic neurons). Postganglionic fibers (axons) of these are then distributed to smooth muscle and glands.

Between the two muscle layers of the esophagus and all subsequent parts of the digestive tract, is a chain of parasympathetic ganglia and vagus nerve fibers (myenteric plexus of Auerbach). The fibers are finely myelinated or unmyelinated. Groups of ganglion cells are seen intermittently within the chain. Look for lightly stained areas between the muscle layers and note: