Slide DMS099 [Cerebral cortex; cat]. This is a thin (1.5 µm) section of well-fixed, plastic resin embedded tissue stained by an H & E technique. As indicated in the above diagrams, the section is a small vertical (perpendicular) slice of one of the ridges (gyri) of the cerebral cortex of the cat brain. First, survey the section using your 4x objective. From the periphery of the section inward identify: vascularized pia mater, gray matter, and white matter. The latter occupies the central most part of the section and displays faint linear striations (due to numerous nerve fibers coursing in this area to and from the gray matter).

Now switch to low power and study the three identified regions of the section in more detail. The pia mater is the innermost and most delicate of the three fibrous meningeal coverings of the CNS. It is closely adherent to the brain tissue (impossible to strip off without destroying brain tissue) and is composed of one to several layers of flattened, modified fibroblasts and the ECM (collagen and elastic fibers, etc.) they produce. (What is the embryonic origin of these cells)? Mast cells, macrophages, lymphocytes, etc. are also found amongst the pial fibroblasts.

The pia mater and the next meningeal layer, the arachnoid, are so intimately related that they are often considered a single membrane, the pia-arachnoid (leptomeninx). [See the diagram]. The two meningeal layers are joined by cobweb like trabecular processes. (Hence, name arachnoid, spider). In the spaces between the trabeculae (subarachnoid spaces) CSF circulates. The inner surface of the pia forms a firm interface with a special layer of brain glial cells to form a pial-glial (external limiting) membrane.

Numerous blood vessels (arteries and veins) are associated with the pia-arachnoid, for the most part lying within the subarachnoid spaces. As blood vessels enter and leave the brain they carry with them arachnoid and pia-glia tissues which form a cuff around the vessel. Only processes of glial cells (astrocytes and oligodendroglia) surround the basement membrane of the capillary endothelium. The capillaries of the CNS are distinctive structurally and functionally. Review the concept of the blood-brain barrier.

Now study the gray matter , first with low and then with progrressively higher power. You will see many basophilic nuclei and occasional capillaries in a field of granular eosinophilic material. Although it cannot be appreciated in this preparation the eosinophilic material consists of an amazing network of the many processes of nerve and glial cells. (It is sometimes called the neuropil). The largest nuclei [usually with a prominent nucleolus] belong to nerve cells [neurons]. Except for the largest neurons [giant pyramidal cells] not much of the surrounding cytoplasm (perikaryon, cell bodies, soma) will be seen. The more numerous smaller nuclei mostly belong to glial and capillary endothelial cells. (Recall that glial cells outnumber nerve cells by about 10 to 1 in the CNS).

The neurons of the mammalian cerebral cortex are not randomly distributed. Although it may not be readily apparent to you from looking at this section, experts have shown that they are actually arranged in six layers or laminae parallel to the brain surface. It is not necessary at this time that you know the names and features of these layers. Do appreciate, however, that the stratification you see in the sections does indeed correspond to different population of cells and their processes.

Lastly, study the centrally located white matter in the section at low and high magnification. Remember that, as a rule, white matter contains no neuronal cell bodies. It consists here mostly of myelinated nerve fibers [going to or from the gray matter neurons], supportive glial cells, and blood vessels. The short, eosinophilic streaks that you see in the white matter at high magnification are portions of large axons.

Cerebral cortex: Note the pia and arachnoid as well as the cortical blood vessel on the surface of the cortex.