Part of a dorsal root ganglion stained with H&E is shown in this figure. The specimen includes the edge of the ganglion, where it is covered with connective tissue (CT). The dorsal root ganglion contains large cell bodies (CB) that are typically arranged as closely packed clusters. Also, between and around the cell clusters, there are bundles of nerve fibers (NF). Most of the fiber bundles indicated by the label have been sectioned longitudinally.
Dorsal root ganglion, cat, H&E ×350.
At higher magnification of the same ganglion, the constituents of the nerve fiber show their characteristic structure, namely, a centrally located axon (A) surrounded by a myelin space (not labeled), which, in turn, is bounded on its outer border by the thin cytoplasmic strand of the neurilemma (arrowheads). The cell bodies of the sensory neurons display large, pale-staining spherical nuclei (N) and intensely staining nucleoli (NI). Also seen in this
H&E preparation are the nuclei of satellite cells (Sat C) that completely surround the cell body and are continuous with the Schwann cells that invest the axon. Note how much smaller these cells are than the neurons. Clusters of cells (asterisks) within the ganglion that have an epithelioid appearance are en face views of satellite cells where the section tangentially includes the satellite cells but barely grazes the adjacent cell body.
Peripheral nerves are composed of bundles of nerve fibers held together by connective tissue and a specialized layer (or layers) of cells, the perineurium. The connective tissue consists of an outer layer, the epineurium, surrounding the whole nerve; the perineurium, surrounding bundles of nerve fibers; and the endoneurium, associated with individual neurons. Each nerve fiber consists of an axon that is surrounded by a cellular investment called the neurilemma, or the sheath of Schwann. The fiber may be myelinated or unmyelinated. The myelin, if present, is immediately around the axon and is formed by the concentric wrapping of the Schwann cell around the axon. This, in turn, is surrounded by the major portion of the cytoplasm of the Schwann cell, forming the neurilemma. Unmyelinated axons rest in grooves in the Schwann cell.
Peripheral nerve, cross-section, femoral nerve, H&E. ×200 and 640
This cross-section shows several bundles of nerve fibers (BNF). The external cover for the entire nerve is the epineurium (Epn), the layer of dense connective tissue that one touches when a nerve has been exposed during a dissection. The epineurium may also serve as part of the outermost cover of individual bundles. It contains blood vessels (BV) and may contain some fat cells. Typically, adipose tissue (AT) is found about the nerve.
Figure on right shows, at higher magnification, the perineurial septum (marked with arrows on left image, which is now rotated and vertically disposed (arrows)).
The layer under the epineurium that directly surrounds the bundle of nerve fibers is the perineurium (Pn). As seen in the cross-section through the nerve, the nuclei of the perineurial cells appear flat and elongate; they are actually being viewed on edge and belong to flat cells that are also being viewed on edge. Again, as noted by the distribution of nuclei, it can be ascertained that the perineurium is only a few cells thick. The perineurium is a specialized layer of cells and extracellular material whose arrangement is not evident in H&E sections. The perineurium (Pn) and epineurium (Epn) are readily distinguished in the triangular area formed by the diverging perineurium of the two adjacent nerve bundles.