NERVOUS TISSUE LABORATORY

The objective of this laboratory is to familiarize you with the appearance of nervous tissue - neurons, neuroglia and nerve - in different locations in the body. Detailed study of the specific organization of the brain and spinal cord occurs in your neuroanatomy course.

CNS

Spinal Cord

Excellent examples of neuron cell bodies, axons, and myelin are found in Slide 132 of spinal cord. Motoneurons of the ventral horn of the spinal cord are multipolar, and are very large and well-developed. Using the following diagram, look for and identify all of the features labelling the diagram on any of the three spinal cord cross sections on your slide. Be aware that in addition to H&E, luxol fast blue stain has been added which colors myelin blue. Thus all areas of "white Matter" are royal blue; "gray matter" is light purple to pink. Begin by holding the slide to a white background and orienting yourself to the following features: dorsal vs ventral; gray and white matter; dorsal and ventral roots of spinal nerves.

Brain

Continue studying features of the CNS in Slide 130, temporal lobe, with the same stain as that of spinal cord. Hold the slide to a white background. An outer pink rind or cortex consists of gray matter; a deeper purple layer is the underlying white matter.

Follow the convex surface of the cortex and identify numerous cross sections of blood vessels surrounded by delicate connective tissue strands. These vessels lie in the subarachnoid space (arachnoid = spider) on the surface of the brain. The surface of the brain itself is lined by a delicate simple squamous epithelium, the pia mater. This layer will be difficult to appreciate, since you may only see a few spindle-shaped pial cells on the cortex. Look into the substance of the brain for blood vessels there also. These vessels are filled with amorphous-appearing red blood cells, thus a lumen is frequently not present to help in their identification and they will appear as large purple blobs at low magnifications. These blood vessels, having been derived from the vessels on the surface, are surrounded by the Virchow- Robin space, which is in continuity with the subarachnoid space. In life, this is not a real space but a potential space. It is represented in your sections by an large artifactual space caused by tissue shrinkage. This space helps you identify the blood vessels.

Follow the surfaces of your tissue around in search of the lateral ventricle, which you will recognize since it is the one surface which is clearly lined by cuboidal ependymal cells. Hanging from the roof of each ventricle is a choroid plexus, where irregularly-shaped clusters of connective tissue and blood vessels are covered by ependymal cells. The choroid plexus is the site of secretion of cerebrospinal fluid.

Following identification of these major features, you are now ready (?) to identify several different cell types in the brain.

Neurons Look for the largest cells possible; these will always be multipolar neurons. As in spinal cord, they are extremely large cells with a triangular or irregular outline. The nucleus will be centrally located, paler than the cytoplasm, and contain a single nucleolus if it is present in the plane of section. Although the cytoplasm is very basophilic, Nissle substance is less easily resolved in this slide than in the spinal cord. Artifactual shrinkage spaces surround all the cells.

Neuroglia Only the nuclei of the glial cells will be stained. Their cytoplasm is not visualized in this prepration, and only a clear area will be seen around each nucleus. For this reason, you will only be able to identify the best examples of some of the neuroglia, but will not be able to identify each with certainty.

Astrocytes: The nuclei of astrocytes are oblong or football-shaped, thus may appear circular or elliptical, depending on their orientation in the plane of section. The nuclei are very light and large relative to other glial cells, so to identify them simply look for the largest, lightest nuclei, and with the help of the lab instructor, you can make a positive identification, particularly when they are compared with an adjacent oligodendrocyte.
Oligodendrocyte: The nuclei of these cells appear similar to those of lymphocytes. They are smaller and much denser than astrocyte nuclei, and are always spherical.

(In some areas underneath the pia mater you will see homogeneous-appearing, lavender circular bodies, and if you haven't reached overload yet, you will be wondering what they are. They are called corpora amylacea, and represent the dilated foot processes of the astrocytes, filled with a starch-like material.)

PNS
Slide 82 has aorta, vena cava, and peripheral nerve and ganglia embedded in loose c.t. These are autonomic ganglia, specifically collateral ganglia, containing cell bodies of postganglionic sympathetic neurons. Both ganglia and nerves occur as compact, well- circumscribed pale eosinophilic bundles. The ganglia can be recognized by searching any of these bundles for typical neuron cell bodies as previously described. Each bundle is surrounded by a fairly dens fibrous c.t. capsule. Within the ganglia, clear spaces can be seen which are capillary lumens. Examine the multipolar neurons for Nissle substance, often located in the peripheral cytoplasm. The neuron cell bodies are intimately surrounded by satellite cells, which are seen as nuclei flattened around the cell bodies of the neurons. (What is the embryologic origin of these neurons and satellite cells?)
Each small nerve seen as a bundle in this section represents the peripheral branching of a fascicle out of the main nerve trunk. Thus each fascicle consists of a group of axons, supporting cells, endoneurium, and a surrounding layer of dense c.t. probably derived from both the epineurium and perineurium. Although nomenclature may vary for this layer, we will refer to the c.t. layer around these fascicles as perineurium to maintain consistency. "Holes" in the nerve represent empty capillaries in the endoneurium. Observe that nerve stains light pink relative to the more orangy surrounding collagen.
Distinguishing nerve from smooth muscle is an art which requires a practiced eye, and this is an excellent slide to compare and contrast nerve and smooth muscle, found in the large adjacent aortic wall. Keep in mind that some nerve profiles are cross sections, while others are longitudinal sections.
The only differences in these paraffin sections between nerve vs. smooth muscle will be the presence in the nerve of (1) a c.t. capsule, (2) capillaries, and (3) slightly different coloration than the muscle. Additionally, with longitudinal sections, you will see the fibers of nerve have a wavy pattern, whereas in smooth muscle the fibers are fairly straight. So find a longitudinal strip of nerve adjacent to the aorta at this time and compare the two. Be sure you can recognize smooth muscle, nerves and ganglia before leaving this slide.
On Slide 125, jejunum, there are other good examples of ganglia. Terminal ganglia are found in the walls of various organs, some representing cell bodies of postganglionic parasympathetic neurons. Throughout the gut, between the outermost two layers of smooth muscle, which are circular and longitudinal in orientation, is found a layer of ganglia and nerves known as Auerbach's plexus. More difficult to find are neuronal cell bodies in the submucosa, part of a submucosal (Meissner's) plexus. These two plexuses are constant features throughout the gut. Note that this neuronal tissue is lighter than the muscle, aiding in its identification.
Now move to Slide 23 of tongue and look for large nerves in cross and longitudal section, mixed in amongst striated muscle and collagen. A few ganglia are present, but don't waste alot of time looking for these.
In Slide 135, human toe, one finds a number of Pacinian corpuscles. These are sensory deep pressure receptors which, because of concentric rings of fibers and cells encapsulating a nerve ending, have the appearance of a cut onion.