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. x250.
they serve as convenient markers in radiographic and computed tomography 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.
Because melatonin is released in the dark it regulates reproductive function in mammals by inhibiting the steroidogenic activity of the gonads (Table 20.6). Production of gonadal steroids is regulated by the inhibitory action of melatonin on neurosecretory neurons located in the hypothalamus (arcuate nucleus) that produce GnRH. Inhibition of GnRH causes a decrease in the release of FSH
table 20.6. Hormones of the Pineal Gland Hormone Composition Source
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 and LH from the anterior lobe of the pituitary gland. In addition to melatonin, extracts of pineal glands from many animals contain numerous neurotransmitters such as serotonin, norepinephrine, dopamine, histamine, and hypothalamic regulating hormones such as somatostatin, and TRH. Clinically, tumors that destroy the pineal gland are associated with precocious (early-onset) puberty.
Animal studies demonstrate that information relating to the length of daylight reaches the pineal gland from photoreceptors in the retina. The pineal gland thus influences seasonal sexual activity. 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. In addition, the pineal gland may play a role in altering emotional responses to reduced day length during winter in temperate and subarctic zones (seasonal affective disorder, or SAD).
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