Pineal gland (epiphysis)
The pineal gland, or epiphysis is a small cone-like structure that comprises a part of the diencephalon. It is a neuroendocrine gland that secretes the hormone melatonin and several other polypeptide hormones that have a regulatory function on other endocrine glands.
The pineal gland projects posteriorly from the wall of the third ventricle above the quadrigeminal plate, resting in the groove between the two superior colliculi. The gland has several functions, the most important of which is maintaining the body’s circadian rhythm and regulating the sleep-wake cycle. In addition, the pineal gland plays a role in modulating the onset of puberty and the development of the reproductive system.
This article will discuss the anatomy and function of the pineal gland.
|Definition||A small neuroendocrine organ that comprises the epithalamus|
|Function||Circadian rhythm, sexual and reproductive development|
- Location and structure
- Hormones and function
- Melatonin: Pineal gland hormone
- Clinical relations
Location and structure
The pineal gland is a neuroendocrine organ that comprises a part of the epithalamus, one of the three divisions of the diencephalon. Other components of the epithalamus are the stria medullaris, habenular nuclei, posterior commissure and paraventricular nuclei.
The pineal gland, also called the pineal body, develops as an outward projection from the posterior wall of the third ventricle, below the splenium of corpus callosum. It sits in the groove between the two superior colliculi, and is bilaterally related to the posterior aspects of the two thalami.
The pineal gland is attached to the rest of the brain by the pineal stalk, which divides into two laminae, the inferior and superior laminae. The inferior and superior laminae contain the posterior commissure and the habenular commissure, respectively. The space between the laminae is filled with an extension of the cavity of the third ventricle known as the pineal recess of the third ventricle.
The parenchyma of the pineal gland mostly consists of cords and clusters of pinealocytes, and supporting astrocyte-like neuroglial cells. The pinealocytes are highly modified neurons that secrete the hormone melatonin into the surrounding network of fenestrated capillaries. The neuroglial cells are the main component of the pineal stalk.
Another structural component of the pineal gland are the corpora arenacea or brain sand, which are deposits of calcium, phosphates and carbonates that form multilaminar corpuscles. These accumulate with age and are mostly a byproduct of secretory activity. The corpora arenacea are clinically significant because they are sometimes used as landmarks during radiologic examinations. They also help the microscopic identification of the pineal gland, which is otherwise histologically unremarkable.
Hormones and function
The pineal gland is classified as one of the secretory circumventricular organs of the brain, meaning that it has direct access to the bloodstream via fenestrated capillaries. The most important function of the pineal gland is to gather information about the state of the day-night cycle from the outside environment and use this information to modulate its production and release of melatonin. Besides melatonin, the pineal gland also produces several other indolamine and polypeptide hormones, however their functions are poorly understood.
The hormones of the pineal gland have a highly regulatory importance in which they influence the activity of other endocrine glands, namely the pituitary gland, endocrine pancreas, adrenal gland, parathyroid gland and the gonads. The effects of the pineal gland on these glands are mostly inhibitory, by reducing the synthesis and release of the hormones produced by these glands.
To learn more about the basics of hormones and the endocrine system, check out our diagrams, videos and quizzes in the study unit below.
Melatonin: Pineal gland hormone
Melatonin is a serotonin-derived hormone that is secreted rhythmically by the pinealocytes of the pineal gland. It has two primary functions; modulation of the sleep-wake cycle by controlling the circadian rhythm, and regulation of reproductive system development.
The main factor in the production of melatonin is the amount of light that falls on the retina. When there is little to no light present, the production of melatonin is stimulated, while the presence of bright light inhibits its production. This means that its concentrations in the blood rise during the dark and fall during the day. This serves as an internal mechanism for the body to gather information about night-length, and in turn to create the circadian rhythm of the individual.
The circadian rhythm is an internal process that shows a repetitive occurrence every 24-hours and regulates the sleep-wake cycle. The circadian rhythm is largely dictated by the previously mentioned rising and falling concentrations of melatonin. It serves as an intrinsic mechanism for the body to synchronize its physiological processes (feeding, reproduction, sleep etc.) with the day and night rhythm.
The information about the day and night rhythm is transmitted to the pineal gland through the retinohypothalamic tract by the following route:
- The light falls on the retina and in response, an action potential is transmitted through the retinal fibers to the suprachiasmatic nucleus (SCN) of the hypothalamus.
- Next, the information is relayed to the paraventricular nucleus (PVN) of the hypothalamus, which in turn projects efferent fibers to the intermediolateral nucleus in the lateral column of the spinal cord.
- Finally, the postganglionic fibers of the cervical sympathetic chain transmit the impulses to the pineal gland.
Basically, the pineal gland gets its information about the amount of light from the eye via the hypothalamus, and regulates the circadian rhythm accordingly by secreting melatonin. Due to this connection to the eyes and its ability to respond to light, the pineal gland is historically referred to as the “third eye”.
Sexual and reproductive function
Studies suggest that the pineal gland inhibits the pituitary gland's secretion of sex hormones: follicle-stimulating hormone (FSH), and luteinizing hormone (LH). These hormones aid in the proper development and functioning of the ovaries and testes. Thus, it is believed that the pineal gland plays a role in regulating the onset of puberty, while it also inhibits the maturation of genitals until puberty.
Pineal gland disorders
Pineal gland function and melatonin secretion can be impaired due to disease or injury of the pineal gland. Pineal gland dysfunction can be caused by pineal tumors, craniopharyngiomas, injuries affecting the sympathetic innervation of the pineal gland, or rare congenital disorders that alter melatonin secretion.
The most prominent manifestation of pineal gland dysfunction is a change in the circadian rhythm. This is mainly followed by symptoms of insomnia, such as reduction of sleep time and quality and increase of sleep latency and night awakenings. However, changes in the circadian rhythm can also be followed by additional symptoms such as abnormal thyroid function, depression, anxiety and other mental health issues, particularly seasonal symptoms.
Production of melatonin by the pineal gland can also be impaired on a smaller scale by factors such as unnatural timing of light exposure, delayed bedtimes and rise times etc. A good example for this is jet travel when flying across five or more time zones, resulting in jet-lag syndrome. This causes a shift in the schedule of sleep and wakefulness that the internal clock cannot adjust to quickly enough. Symptoms of jet lag may last up to 10 days, and include difficulty falling asleep or remaining asleep in the new environment, along with daytime sleepiness.
In regards to the pineal gland’s function on the onset of puberty and sexual maturation, studies have suggested that low melatonin levels are associated with precocious puberty and hypergonadism, while high levels are associated with delayed puberty and hypothalamic amenorrhea.
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