Slide #DMS094 [Peripheral nerve - cross section] In this slide, study the funicular arrangement of nerve fibers, identifying the connective tissue sheaths: epineurium, perineurium and endoneurium (see picture). (Only a portion of the epineurium may be present.) What are the structural differences in these sheaths? What functions, besides protection, do these sheaths serve?
Within a funiculus, identify myelinated nerve fibers and note size of axon, thickness of myelin and presence of the neurilemmal (Schwann cell) sheath . Correlate these structures with the development and structure of myelin as observed with the electron microscope. Keep in mind that the shape and position of the axon are usually distorted by fixation. In the transverse section of a nerve trunk note the apparent spiral ("whorled") arrangement of fibers. In the longitudinal section identify nodes of Ranvier and internodal segments. Nonmyelinated nerve fibers are also present in the funiculus, but these are much smaller and difficult to distinguish.
Compare the ultrastructure of myelinated and unmyelinated nerves in the EM module of this lesson and study the fine structure of the node of Ranvier from the illustrations in your textbook.
This is a very low power image of a cross-section through a nerve trunk comparable to the previous slide, but stained with H&E. At this magnification, one should identify the connective tissue that is the epineurium as well as the perineurium surrounding each individual fascicle.
At this low power one sees parts of several fascicles, each surrounded by perineurium. The individual nerve fibers or axons together comprising the fascicle are just apparent at this power and seen to better effect in the next image.
This high power image of a cross-section through a portion of a fascicle reveals the numerous individual nerve fibers, each surrounded by a thin connective tissue layer, the endoneurium (stained light brown). The purplish material at the center of each fiber represents the axon itself, surrounded by a myelin sheath (extracted and appearing clear), and then bound by the endoneurial connective tissue.
This is a low power view of a longitudinal section through a portion of a peripheral nerve. Again, the individual fascicles are seen to be bound by a layer of connective tissue, the perineurium.
A high power view of a longitudinal section through part of a nerve fascicle illustrates nicely the morphology of the nerve fibers and its associated layers. Axons are seen as the purplish-colored fibers, surrounded by a clear zone representing the extracted remnants of the myelin sheath, outside of which may be found the delicate connective tissue covering, the endoneurium. Nodes of Ranvier are also apparent in longitudinal sections such as these.