Anatomy And Cell Biology

In humans, the pineal gland is located in the brain grossly between the thalamus and the mesencephalon, as shown in Figure 18-4. In adults the pineal is a flat, cone-shaped structure with dimensions of about 58 mm long and 3-5 mm wide and weighing around 120 mg. It is located at the posterior border of the third ventricle above the roof of the diencephalon, being connected to it by a short stalk. The pineal is covered by a layer called the pia mater. From the pia mater, connective tissue septa with blood vessels and unmyelinated nerve fibers enter the pineal to surround the cords of cells and follicles (alveoli) to form irregular lobules. Its location with respect to light stimuli is more clearly visualized in Figure 18-5. The chain of events with regard to the photoeffect is shown by light entering one of the eyes, and the signal is carried to the suprachiasmic nuclei of the hypothalamus, to the inter-omediolateral cell column of the spinal cord, then to the superior cervical ganglia, and finally to the postganglionic sympathetic neurons, probably signaling the pineal via norepinephrine release.

In infants, the pineal is large and many cells are arranged in alveoli. Before puberty, deposits of calcium, magnesium phosphate, and carbonate appear, which usually render the pineal opaque to X rays.

There are several cell types in the adult pineal organ, but the major ones are pinealocytes and interstitial cells. The pineal is developed maximally at about 7 years of age. As mentioned, salts invest the capsule and septa. The organ is supplied with many blood vessels and both myelinated and unmyelinated nerve fibers. Within the organ the capillaries are thin and fenestrated, but this may not apply to all species. The nerve fibers derive from the sympathetic autonomic nervous system. Nerve terminals may end directly on pineal cells. The neurosecretory granules containing norepinephrine are 40 nm in diameter. Serotonin is also present in the nerve endings, as well as within the pinealocytes.

The pinealocytes have cytoplasmic processes that end in swellings. The other major cell type in the pineal organ is the interstitial cell, which is irregular in shape and scattered among the pinealocytes. These cells resemble astrocytes, and some consider them to be neuroglia.

FIGURE 18-4 Medial surface of the right hemisphere of the human brain. Abbreviations: C, cerebellum; H, hypophysis; He, haben-ular commissure; P, pineal gland; Pc, posterior commissure; S, splen-ium of the corpus callosum; SC, superior colliculus; Ssc, superior subarachnoid cistern; III, third ventricle. Scale in the bottom half shows 1-mm divisions. Reproduced with permission from Shafii, M., and Shafii, S. L. (eds.) (1990). "Biological Rhythms, Mood Disorders, Light, Therapy, and the Pineal Gland." American Psychiatric Press, Inc., Washington, D.C.

FIGURE 18-4 Medial surface of the right hemisphere of the human brain. Abbreviations: C, cerebellum; H, hypophysis; He, haben-ular commissure; P, pineal gland; Pc, posterior commissure; S, splen-ium of the corpus callosum; SC, superior colliculus; Ssc, superior subarachnoid cistern; III, third ventricle. Scale in the bottom half shows 1-mm divisions. Reproduced with permission from Shafii, M., and Shafii, S. L. (eds.) (1990). "Biological Rhythms, Mood Disorders, Light, Therapy, and the Pineal Gland." American Psychiatric Press, Inc., Washington, D.C.

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