Pineal gland

The pineal gland (pineal body, epiphysis cerebri) is an endocrine or neuroendocrine gland that regulates daily body rhythm. It develops from neuroectoderm of the posterior portion of the roof of the diencephalon and remains attached to the brain by a short stalk. In humans, it is located at the posterior wall of the third ventricle near the center of the brain. The pineal gland is a flattened, pine cone-shaped structure, hence its name (Fig. 20.10). It measures 5 to 8 mm high and 3 to 5 mm in diameter and weighs between 100 and 200 mg.

The pineal gland contains two types of parenchymal cells: pinealocytes and interstitial (glial) cells

Pinealocytes are the chief cells of the pineal gland. They are arranged in clumps or cords within lobules formed by connective tissue septa that extend into the gland from the pia mater that covers its surface. These cells have a large, deeply infolded nucleus with one or more prominent nucleoli and contain lipid droplets within their cytoplasm. When examined with the transmission electron microscope (TEM), pinealocytes show typical cytoplasmic organelles along with numerous, dense-cored, membrane-bounded vesicles in their elaborate, elongated cytoplasmic processes. The processes also contain numerous parallel bundles of microtubules. The expanded, club-like endings of the processes are associated with the blood capillaries. This feature strongly suggests neuroendocrine activity.

The interstitial (glial) cells constitute about 5% of the cells in the gland. They have staining and ultrastructural features that closely resemble those of astrocytes and are reminiscent of the pituicytes of the posterior lobe of the pituitary gland.

In addition to the two cell types, the human pineal gland is characterized by the presence of calcified concretions, called corpora arenacea or brain sand (Fig. 20.11). These concretions appear to be derived from precipitation of calcium phosphates and carbonates on carrier proteins that are released into the cytoplasm when the pineal secretions are exocytosed. The concretions are recognizable in childhood and increase in number with age. Because they are opaque to x-rays and located in the midline of the brain,

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