This micrograph shows a low-magnification view of the cerebral cortex (CC). It includes the full thickness of the gray matter and a small amount of white matter at the bottom of the micrograph (WM). The white matter contains considerably fewer cells per unit area; these are neuroglial cells rather than nerve cell bodies that are present in the cortex. Covering the cortex is the pia mater (PM). A vein (V) can be seen enclosed by the pia mater. Also, a smaller blood vessel (BV) can be seen entering the substance of the cortex. The six layers of the cortex are marked by dashed lines, which represent only an approximation of the 394 boundaries. Each layer is distinguished on the basis of predominant cell types and fiber (axon and dendrite) arrangement. Unless the fibers are specifically stained, they cannot be utilized to further aid in identification of the layers. Rather, the delineation of the layers, as they are identified here, is based on cell types, and more specifically, the shape and appearance of the cells. The six layers of the cortex are named and described as follows:
I: The plexiform layer (or molecular layer) consists largely of fibers, most of which travel parallel to the surface, and relatively few cells, mostly neuroglial cells and occasional horizontal cells of Cajal.
II: The small pyramidal cell layer (or outer granular layer) consists mainly of small pyramidal cells, and granule cells, also called stellate cells.
III: The layer of medium pyramidal cells (or layer of outer pyramidal cells) is not sharply demarcated from layer II. However, the pyramidal cells are somewhat larger and posses a typical pyramidal shape.
IV: The granular layer (or inner granular layer) is characterized by the presence of many small granule cells (stellate cells).
V: The layer of large pyramidal cells (or inner layer of pyramidal cells) contains pyramidal cells that, in many parts of the cerebrum, are smaller than the pyramidal cells of layer III but in the motor area are extremely large and are called Betz cells.
VI: The layer of polymorphic cells contains cells with diverse shapes, many of which have a spindle of fusiform shape. These cells are called fusiform cells.
In addition to pyramidal cells, granule cells, and fusiform cells, two other cell types are also present in the cerebral cortex but are not recognizable in this preparation: the horizontal cells of Cajal, which are present only in layer I and send their processes laterally, and the cells of Martinotti, which send their axons toward the surface (opposite to that of pyramidal cells).
Layer I of cerebral cortex, brain, human, Luxol fast cells appear as naked nuclei, with the cytoplasm being indistinguishable blue—PAS ×350. from the nerve fibers that make up the bulk of this layer. Also present is a small capillary (Cap). The pink outline of the vessel is due to the PAS stain-This micrograph is a higher power of layer I, the plexiform ing reaction of its basement membrane.
layer. It consists of nerve fibers, numerous neuroglial cells (NN) and occasional horizontal cells of Cajal. The neuroglial
Layer II of cerebral cortex, brain, human, Luxol fast blue—PAS ×350.
This micrograph shows layer II, the small pyramidal cell layer. Many small pyramidal cells (PC) are present. Granule cells (GC) are also numerous, though difficult to identify here.
Layer IV of cerebral cortex, brain, human, Luxol fast prominent. The micrograph also reveals a number of capillaries. Note how blue—PAS ×350. they travel in various directions. This micrograph shows layer IV, the granular layer. Many of the cells here are granule cells, but neuroglial cells are also
Layer VI of cerebral cortex, brain, human, Luxol fast region. Pyramidal cells (PC) are readily recognized. Other cell types present blue—PAS ×350. include fusiform cells (FC), granule cells, and Martinotti cells. This micrograph shows layer VI, the layer of polymorphic cells, so named because of the diverse shape of the cells in this