A, acidophils

B, basophils

C, chromophobes Cap, capillaries

HB, Herring bodies

The pineal gland (pineal body, epiphysis cerebri) is located in the brain above the superior colliculi. It develops from neuroectoderm but, in the adult, bears little resemblance to nerve tissue.

Two cell types have been described within the pineal gland: parenchymal cells and glial cells. The full extent of these cells cannot be appreciated without the application of special methods. Those would show that the glial cells and the parenchymal cells have processes and that the processes of the parenchymal cells are expanded at their periphery. The parenchymal cells are more numerous. In an H&E preparation, the nuclei of the parenchymal cells are pale staining. The nuclei of the glial cells, on the other hand, are smaller and stain more intensely.

Although the physiology of the pineal gland is not well understood, the secretions of the gland evidently have an antigonadal effect. For example, hypogenitalism has been reported in pineal tumors that consist chiefly of parenchymal cells, whereas sexual precocity is associated with glial cell tumors (presumably, the parenchymal cells have been destroyed). In addition, experiments with animals indicate that the pineal gland has a neuroendocrine function whereby the pineal gland serves as an intermediary that relates endocrine function (particularly gonadal function) to cycles of light and dark. The external photic stimuli reach the pineal gland via optical pathways that connect with the superior cervical ganglion. In turn, the superior cervical ganglion sends postganglionic nerve fibers to the pineal gland. The extent to which these findings with laboratory animals apply to humans is not yet clear.

Recent studies in humans suggest that the pineal gland has a role in adjusting to sudden changes in day length, such as those experienced by travelers who suffer from jet lag, and a role in regulating emotional responses to reduced day length during winter in temperate and subarctic zones (seasonal affective disorder [SAD]).

Figure 1, pineal gland, human, H&E x180.

The pineal gland is surrounded by a very thin capsule (Cap) that is formed by the pia mater. Connective tissue trabeculae (CT) extend from the capsule into the substance of the gland dividing it into lobules. The lobules (L) appear often as indistinct groups of cells of varying size surrounded by the connective tissue. Blood vessels, generally small arteries 04) and veins (V), course through the connective tissue. The arteries give rise to capillaries that surround and penetrate the lobules to supply the parenchyma of the gland. In this specimen and even at this low magnification, the capillaries (C) are prominent as a consequence of the red blood cells present in their lumina.

Figure 2, pineal gland, human, H&E x360; inset x700.

This micrograph shows at higher magnification the parenchyma of the pineal gland as well as a component called brain sand (BS) or corpora arenacea. When viewed at even higher magnifications, the corpora arenacea are seen to have an indistinct lamellated structure. Typically, they stain heavily with hematoxylin. The presence of these structures is an identifying feature of the pineal gland. A careful examination of the cells within the gland at the light microscopic level reveals two specific cell types. One cell type represents the parenchymal cells. These are by far the most numerous and are referred to as pinealocytes (or chief cells of the pineal gland). Pinealocytes are modified neurons. Their nuclei are spherical and are relatively lightly stained because of the amount of euchromatin that they contain. The second cell type is the interstitial cell or glial cell that constitutes a relatively small percentage of the cells in the gland. Their nuclei are smaller and more elongate than those of the pinealocytes. The inset reveals several glial cells (G) that can be identified by their more densely staining nuclei. The majority of the nuclei of the other cells seen here belong to pinealocytes. Also seen in the inset are several fibroblasts (F) that are present within a trabecula.

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