FIGURE 21.11 • Photomicrograph of infant pineal gland. This H&E–stained section is from a median cut through the pine cone–shaped gland. The conical anterior end of the gland is at the top of the micrograph. The arrows indicate the part of the gland that connects with the posterior commissure. The gland is formed by an evagination of the posterior portion of the roof of the third ventricle (diencephalon). The dark areas indicated by asterisks are caused by bleeding within the gland. 25.
organelles along with numerous, dense-core, membrane-bounded vesicles in their elaborate, elongated cytoplasmic processes. The processes also contain numerous parallel bundles of microtubules. The expanded, clublike 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. 21.12). 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
FIGURE 21.12 • Photomicrograph of human pineal gland. This higher-magnification photomicrograph shows the characteristic concretions called brain sand or corpora arenacea. Pinealocytes (chief cells of the pineal gland) account for the majority of the cells seen in the specimen. They are arranged in clumps or cords. Those blood vessels (BV ) that contain red blood cells are readily apparent; numerous other blood vessels are also present but are not recognized at this magnification without evidence of the blood cells. 250.
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, they serve as convenient markers in radiographic and computed tomography (CT) studies.
The human pineal gland relates light intensity and duration to endocrine activity.
The pineal gland is a photosensitive organ and an important timekeeper and regulator of the day/night cycle (circadian rhythm). It obtains information about light and dark cycles from the retina via the retinohypothalamic tract, which connects in the suprachiasmatic nucleus with sympathetic neural tracts traveling into the pineal gland. During the day, light impulses inhibit the production of the major pineal gland hormone, melatonin. Therefore, pineal activity, as measured by changes in the plasma level of melatonin, increases during darkness and decreases during light. In humans, these circadian changes of melatonin secretion play an important role in regulating daily body rhythms.
TABLE Hormones of the Pineal Gland21.6 Hormone Composition Source Major Functions Melatonin Indolamine (N-acetyl-5-methoxytryptamine) Pinealocytes Regulates daily body rhythms and day/night cycle (circadian rhythms); inhibits secretion of GnRH and regulates steroidogenic activity of the gonads particularly as related to the menstrual cycle; in animals, influences seasonal sexual activity