Pituitary Gland: Anatomy, Functions, Hormones, Disorders

Pituitary Gland is found in the Sella turcica, a bony structure near the base of the human brain, and is known as the "Master gland." This pea-sized gland, also known as the hypophysis, is in charge of regulating the activity of other hormone-secreting glands. It secretes hormones, which govern the requirements of the human body and transmit messages to various glands and organs.

Pituitary Gland: Definition

The hypophysis, or pituitary gland, is a pea-sized endocrine gland located at the base of our brain. Because it generates some of the body's most vital hormones, it's known as the "Master Gland." It is located in the Pituitary Fossa, a bony structure directly below the hypothalamus and near to the optic nerve. The lobes of the pituitary gland are separated into three sections:

• Anterior pituitary
• Intermediate pituitary (Absent in adult human beings)
• Posterior pituitary

The pituitary gland is a ductless endocrine gland that secretes hormones into the bloodstream directly. The pituitary gland's position on the underside of the brain is referred to as hypophysis (from the Greek meaning "lying under"). Because its hormones regulate other important endocrine glands, such as the adrenal, thyroid, and reproductive glands (e.g., ovaries and testes), and in some cases have direct regulatory effects in major tissues, such as those of the musculoskeletal system, the pituitary gland is known as the "master gland."

Chemical Coordination and Integration: Important Notes

Pituitary Gland Hormones

The anterior and posterior halves of a healthy adult human's pituitary gland are separated. During pregnancy, the Intermediate Pituitary regresses, and in adult humans, it is absent. Hormones have the following major functions:

Anterior Pituitary Hormones

The anterior pituitary hormones are proteins made up of one or two long polypeptide chains. TSH, LH, and FSH are classified as glycoproteins because they contain glycosides, which are complex carbohydrates. Each of the three hormones is made up of two glycopeptide chains, one of which, the alpha chain, is the same in all three. 

The other chain, the beta chain, has a different structure for each hormone, which explains why TSH, LH, and FSH have varied activities. The anterior pituitary hormones, like all protein hormones, are produced as giant inactive molecules termed prohormones in the cytoplasm of the cells. These prohormones are kept in granules, where they are split into active hormones and released into the bloodstream.

Peptides produced from the hypothalamic's median eminence neurons into the hypophyseal-portal veins, which travel a short distance to the pituitary microvasculature, control the generation and secretion of each of the major anterior pituitary hormones. 

Thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone, gonadotropin-releasing hormone, and growth-hormone-releasing hormone are some of the peptides involved. Dopamine and somatostatin, which are powerful inhibitors of prolactin and GH, are also produced in the hypothalamus.

Pituitary-hormone signalling is complicated by feedback loops involving pituitary hormones and their target glands. Thyroid hormone, for example, suppresses TRH secretion while simultaneously inhibiting TRH's action on thyrotrophs. Negative feedback loops like these aid in maintaining a healthy equilibrium between pituitary hormone release and hormone secretion by pituitary target glands. Physiological disturbances, such as stress's impact on the pituitary-adrenal axis and neuroendocrine cycles, can upset this equilibrium.

The anterior pituitary is in charge of producing and secreting various important hormones in the body. The following hormones are among them:

Human Growth Hormone (HGH): This hormone is in charge of the body's cell growth and repair.

Thyroid Stimulating Hormone (TSH): Inspires the thyroid gland to produce its own hormone, thyroxine. Thyrotropin is another name for TSH.

Adrenocorticotropic Hormone (ACTH): Induces the release of Cortisol, also known as the "stress hormone," by the adrenal gland. Corticotropin is another name for ACTH.

Luteinising Hormone (LH) and Follicle-Stimulating Hormone (FSH): Luteinising Hormone (LH) and Follicle-Stimulating Hormone (FSH) are two hormones that control male and female sexual and reproductive characteristics.

Prolactin (PRL) is a hormone that causes the breasts to produce milk. Though it is present at all times, it secretes more during PREGNANCY

Posterior Pituitary Hormones

Vasopressin (antidiuretic hormone) and oxytocin are two important neurohypophyseal hormones that are produced and integrated into neurosecretory granules in the nuclei's cell bodies. Those hormones are made up of a precursor protein that also contains one of the hormones and a protein called neurophysin. 

The precursor protein is broken after synthesis and inclusion into neurosecretory granules, resulting in distinct hormone and neurophysin molecules that are loosely connected to one another. The granules are transported via the axons and stored in the pituitary gland's posterior lobe.

The granules fuse with the cell wall of the nerve endings when the nerve cells are stimulated by internal or external events (e.g., breast suckling in the case of oxytocin-secreting neurons), the hormone and neurophysin dissociate from one another, and both the hormone and the neurophysin are released into the bloodstream. The hormones are released into the bloodstream in response to nerve impulses passed from the hypothalamus to the posterior pituitary lobe.

Oxytocin increases uterine contractions, which are necessary for labour and delivery, as well as milk ejection during breastfeeding. Vasopressin lowers blood pressure and enhances water reabsorption from the kidneys, preserving bodily fluid and preventing dehydration. Increased serum osmolality, which is a sign of dehydration, stimulates vasopressin release.

Pituitary Gland Disorders

Panhypopituitarism is a dangerous and occasionally fatal condition characterised by decreased anterior and posterior pituitary hormone output. When just anterior pituitary hormones are lacking, the term panhypopituitarism is usually used. Panhypopituitarism is characterised by adrenal insufficiency, hypothyroidism, and gonadal failure, as well as inadequate stress responses. 

Panhypopituitarism can be caused by pituitary vascular insufficiency, autoimmune, infections, or neoplasms. If you have central diabetes insipidus, the lesion usually affects both the posterior and anterior pituitary. Isolated deficiencies of one or two pituitary hormones can also happen, and they're usually heritable. 

Those are extremely uncommon circumstances. Infertility can be caused by a lack of LH and FSH in some people. A common type of isolated deficiency is proportionate congenital growth failure caused by GH insufficiency.

Individual anterior pituitary hormone-secreting tumours have been identified. Acromegaly from GH-secreting tumours and Cushing syndrome from ACTH-secreting tumours are two of the most common illnesses caused by functioning pituitary tumours, while both are rare. 

Because pituitary tumours tend to interfere with prolactin-suppressing signals from the hypothalamus (through tissue compression), autonomous hypersecretion of prolactin is a common characteristic. In both men and women, excess prolactin is linked to variable degrees of gonadal failure and, in some circumstances, spontaneous breast-milk secretion (galactorrhea).

Excess vasopressin or oxytocin-secreting tumours in the posterior pituitary are uncommon; however, functional states of excess vasopressin (inappropriate vasopressin secretion) and temporary vasopressin shortage have been documented.

Anatomy of Pituitary Gland

The pituitary gland is located in the middle of the base of the skull, behind the sella turcica, a bony structure located below the nose and immediately beneath the hypothalamus. A stalk of neuronal axons and the so-called hypophyseal-portal veins connects the pituitary gland to the hypothalamus. It weighs between 500 and 900 mg in normal adult individuals (0.02 to 0.03 ounce).

The anterior, middle, and posterior lobes of the pituitary gland are separated into three lobes in most animals (also called the neurohypophysis or pars nervosa). The intermediate lobe does not exist as a separate anatomic entity in humans, but rather as a collection of cells scattered throughout the anterior lobe. Despite this, the pituitary's anterior and posterior lobes are functionally, physically, and embryologically separate. The anterior pituitary contains many hormone-secreting epithelial cells, whereas the posterior pituitary is mostly made up of unmyelinated (non-insulated) secretory neurons.

Anterior Pituitary

Rathke's pouch, an outpouching of the roof of the pharynx, is where the cells of the anterior pituitary are formed embryologically. Under a light microscope, the cells appear to be quite uniform, yet there are at least five different types of cells, each secreting a different hormone or hormones. 

Thyrotropin (thyroid-stimulating hormone; TSH) is synthesised and secreted by thyrotrophs; gonadotrophs secrete both luteinizing hormone (LH) and follicle-stimulating hormone (FSH); corticotrophs secrete adrenocorticotropic hormone (ACTH; corticotropin); somatotrophs secrete growth hormone (GH; somatotropin); and lactotrophs

Somatotrophs are abundant in the anterior pituitary gland, accounting for around 40% of the tissue. They are mostly found in the gland's anterior and lateral parts, and each day they secrete one to two milligrammes of GH.

Posterior Pituitary

Axons shoot out from two pairs of large clusters of nerve cell bodies (nuclei) in the hypothalamus to form the posterior lobe of the pituitary gland. The supraoptic nuclei are found directly above the optic tract, whereas the paraventricular nuclei are found on either side of the brain's third ventricle. The nuclei, axons of the cell bodies of nerves that make up the nuclei, and nerve terminals in the posterior pituitary gland make up the neurohypophyseal system.

From those nuclei, neuronal connections travel to other parts of the brain, including areas that sense osmolality (solute concentrations) and regulate thirst.

Things to Remember:

• Pituitary disorders impair the pituitary gland's function, increasing or lowering the amount of hormone release. This is usually caused by a non-cancerous tumour known as a pituitary adenoma.
• A pituitary macroadenoma (tumour greater than 10 mm) can reduce blood flow to the gland. It can either create an overflow or entirely block blood flow into the gland. Pituitary apoplexy is the medical term for this condition.
• The pituitary gland's primary role is to secrete hormones into the bloodstream. The anterior lobe, intermediate lobe, and posterior lobe are the three pituitary glands. Adults, on the other hand, do not have an intermediate pituitary.
• The posterior pituitary is in charge of the storage and secretion of two crucial hormones:

           1. Antidiuretic Hormone (ADH): Controls the body's water balance by influencing water reabsorption by the kidneys.

           2. Oxytocin: This hormone regulates various aspects of pregnancy and childbirth, including uterine contractions and milk production.