Structure, Functions and Disorders of Pineal Gland and Thymus

Pineal gland

Structure

In mammals and humans, pinacocytes and interstitial cells both produce hormones within the pineal gland. Neurotransmitters are chemical messengers released by nerve cells that influence pinealocytes. The pineal stalk carries nerve fibers to the gland. These fibers contain substances including:

• Vasopressin
• GABA
• Histamine
• Oxytocin
• Serotonin
• Orexin

All these neurotransmitters have receptors on pinealocyte cells, indicating that they are also influenced by these other chemicals present in the brain. The sympathetic superior cervical ganglia and parasympathetic sphenopalatine and otic ganglia affect a large number of neurons outside the brain of humans and other mammals. This nerve center relays information from the pineal gland to the hypothalamus through the suprachiasmatic nucleus (SCN). SCNs are vitally important because they control the rhythm of the circadian rhythm in the body based on light perception and communication along the retino-hypothalamic tract.

Function

Since it regulates the body's circadian rhythms, the pineal gland is essential to the body's internal clock. Circadian rhythms are the body's daily rhythms, such as the signals that predict when someone feels tired, sleeps, wakes up and feels alert. Hormones like melatonin, which play an important role in regulating circadian rhythms, are found in the pineal gland. The amount of light a person is exposed to affects their melatonin production. Pineal glands release more melatonin when it is dark, suggesting melatonin is a sleep-enhancing hormone. There are many manufacturers of supplements that advertise melatonin as a "natural" sleep aid.

Disorders

Insomnia is closely associated with disorders of the circadian rhythm and melatonin production in the pineal gland. Melatonin production may worsen insomnia, for example, in delayed sleep phase syndrome. Atopic dermatitis, also known as atopic dermatitis, can contribute to the symptoms of seasonal affective disorder, or winter depression. Additionally, pineal gland tumors may lead to brain surgery when they affect the gland.

Circadian rhythm disorders

There can be synchrony between wakefulness and sleep that is not in harmony with societal norms or the natural cycle of darkness and light. In addition to experiencing irregular sleep schedules and improperly timed sleepiness, the affected person will also experience insomnia. There are several types of circadian sleep problems:

• Delayed sleep phase problem - It is a condition where night owls have trouble going to sleep and waking up early because they have difficulty falling asleep and staying asleep
• Advanced sleep phase problem - Sleep phase syndrome: characterized by early sleep onsets and waking up early in the morning.
• Free running or non – 24 - Most often found in blind individuals who do not see light, the timing of sleep may slowly shift over a few weeks or months.
• Unregular sleep-wake rhythm - shorter sleep intervals during 24 hours rather than a prolonged overnight sleep period.

Seasonal affective disorder (SAD)

The seasonal affective disorder may result from the prolonged darkness that occurs in the northern hemisphere during the winter months. Winter depression may be associated with weight gain, reduced physical activity, and other symptoms. A lightbox or light therapy glasses that artificially apply light may be helpful for phototherapy. The timing of the light is usually in the morning, but a medical professional should be consulted beforehand.

Pineal gland tumors

Rarely, the pineal gland can be affected by cancer. Pineal gland tumors, though rare, occur in children between the ages of 3 to 8.2 Generally, pineal gland tumors in young adults, between the ages of 20 and 40, occur more frequently. The pineal gland may be affected by a small number of tumors in the brain. Three types of tumors of the pineal gland can be distinguished. They are as follows:

• Pineocytoma - Usually classed as a grade II tumor, pineocytomas are slow-growing.
• Pineoblastoma - It is classified as either a grade III or grade IV malignant form of pineoblastoma, depending on its degree of aggressiveness
• Mixed pineal tumor - Mixture of cell types found in mixed pineal tumors makes classification less straightforward.

Thymus

Structure

An anterior mediastinum pericardium is a pink, flattened, and asymmetrical structure that is located between the sternum and the pericardium. Infants weigh up to 70 g, whereas adults weigh about 3 g. There are two lobes to the thymus, which are joined by aeroli. The two thymic lobes are surrounded by a thin cap of connective tissue. Trabeculae and septa are fibrous extensions of the thymus capsule that divide it into lobes.

Outer cortex

• An outer layer of lymphocytes compartmentalized by elongated epithelial cells has a dark stain.
• T-cells form in immature form.
• These cells proliferate and select one another.

Inner cortex

• Greater number of epithelial reticular cells in the central zone, but fewer lymphocytes.
• Mature T-cells make up the zone.
• It consists primarily of epithelial tissue into which cortical lymphocytes migrate before being transported via venules and lymphatics.
• As well, the cortico-medullary junction may be the final stage of selection.

Similarly, the medial part of the thymus has thymic corpuscles as well, another name for Hassall's corpuscles. They are round-shaped structures made up of flattened epithelial cells. It is believed that they are old and degenerated cells.

• There are two lobes of the thymus gland, each with a lobulated surface that has been divided into the outer cortex and inner medulla.
• A connective tissue septum separates each lobe of the gland within its dense capsule of connective tissue.
• It is composed mainly of star-shaped reticular cells that take up the bulk of the gland's mass.
• Depending on factors such as antigenic expression, ultrastructure, and their ability to produce thymic hormones, the epithelial cells in the thymus gland can be subdivided into four distinct subtypes.
• Subtypes include the subcapsular cortex, the inner cortex, the medullary cortex, and Hassall's corpuscles.
• Lymphocytes are loosely packed in the outer cortex of the gland, and reticulocytes rich in the cytoplasm are found in the medulla.
• Besides Hassall's concentric corpuscles, the structure also contains thymic corpuscles, which are squamous cells organized in concentric arrays.
• Infants and young children have a large thymus, which reaches its largest size during puberty. Afterward, the gland slowly begins to coalesce, replacing itself with adipose tissue.
• The inferior thyroid artery, internal thyroid artery, and intercostal arteries provide blood to the thymus gland.
• Two bilateral muscles attach the gland to the sternum: the sternohyoid muscle and the sternothyroid muscle.

Functions

A mature T cell develops from the bone marrow to the thymus/thymocyte where it generates antigen specificity, undergoes thymic education, then migrates to the arterial bloodstream.
As a primary lymphoid organ, the thymus performs the following functions:

• There are at least a few T cells that are potentially specific to each foreign antigen in the environment in every individual (generate diversity) to produce sufficient numbers (millions) of different T cells.
• The goal is to select T cells for survival in a way that minimizes the possibility of an auto-immune reaction.

Production of hormones

In addition to the endocrine system, the thymus plays an important role. The development and maturation of mature T cells are likely dependent upon the hormones thymosin and thymopoietin produced by thymic epithelial cells together with cytokines (such as IL-7).

• As mediators of cellular immunity, T cells are responsible for inducing the development, activation, and differentiation of the thymus gland.
• Prothymocytes are stimulated to become thymocytes and T lymphocytes by thymic hormones, thymosin, and thymopoietin.
• Besides producing cytokines, the gland also controls the development of T cells through cytokine release.
• By the 12th week of pregnancy, the thymus is an essential part of fetal immunity, and it remains active until childhood.
• The thymus is an endocrine gland that produces human growth hormone, which plays a crucial role in the growth and development of the body.

Disorders

Following are some diseases and disorders associated with the thymus gland;

Thymic cysts

• Some congenital cysts are present in the thymus gland, causing the thymocytes to stop proliferating.
• Usually, these cysts are benign and can be removed. As the immune system of the individual is compromised by thymic cysts, symptoms such as coughing and upper respiratory tract infections are perceived.

Involution

• Malnourished, neglected, or abused infants are more likely to suffer from thymic involution.
• Furthermore, it can also happen with chemotherapy, radiation, and steroid therapy because the thymus is especially sensitive to stress.
• It is much more common for newborns and infants of early childhood to develop an involution of the thymus.
• It is possible to interpret early involution as a sign of a social problem, such as abuse.

Hypoplasia

• In neonatal conditions, hyperplasia results in a decrease in the number of cells in an organ.
• The third and fourth pharyngeal pouches are faulty in development resulting in thymus hyperplasia.
• Alcohol and organic acids are thought to disturb the differentiation of the pharyngeal pouch by damaging the neural crest.
• Affected individuals may exhibit a variety of clinical manifestations according to their level of hypoplasia and genetic defect.

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