The hypophyseal gland, more commonly known as the pituitary gland, is a small, pea-sized endocrine gland located at the base of the brain. It serves as a central regulator within the endocrine system, orchestrating the functions of many other hormone-producing glands throughout the body. Because of its wide-reaching influence on numerous bodily processes, including growth, metabolism, reproduction, and stress response, it is frequently referred to as the “master gland”. Hormones secreted by the pituitary gland travel through the bloodstream, carrying messages that help maintain the body’s internal balance.
Anatomy of the Pituitary Gland
The pituitary gland is nestled within a protective bony cavity at the base of the skull called the sella turcica, a saddle-shaped depression in the sphenoid bone. This small gland, weighing less than a gram, is broadly divided into two distinct parts: the anterior pituitary, also known as the adenohypophysis, and the posterior pituitary, or neurohypophysis.
The anterior and posterior lobes have different developmental origins, reflecting their varied functions. The anterior pituitary develops from an upward invagination of the oral ectoderm, while the posterior pituitary originates from a downward extension of neural tissue from the developing brain. This dual origin underlies how each lobe interacts with the hypothalamus and releases hormones.
The Hypothalamic-Pituitary Connection
The hypothalamus, a region of the brain situated directly above the pituitary gland, exerts significant control over pituitary function through two distinct mechanisms. This close anatomical and functional relationship is fundamental to the body’s endocrine regulation.
For the anterior pituitary, control is primarily achieved through a specialized network of blood vessels known as the hypothalamic-hypophyseal portal system. Hypothalamic neurons release specific “releasing” and “inhibiting” hormones into this system. These hormones travel directly to the anterior pituitary, where they either stimulate or suppress the production and release of its hormones. This direct vascular connection ensures efficient communication and precise regulation.
The posterior pituitary, on the other hand, is controlled by a direct neural connection. Neurons in the hypothalamus synthesize hormones that travel down their axons through the pituitary stalk directly into the posterior pituitary. The hormones are then stored and released into the bloodstream upon neural stimulation from the hypothalamus.
Hormones of the Anterior Lobe
The anterior pituitary is responsible for producing and secreting six major hormones that regulate a wide array of bodily processes. These hormones, often referred to as tropic hormones, primarily stimulate other endocrine glands to release their own hormones.
Growth Hormone (GH) plays a direct role in growth and development, particularly in children. In adults, GH helps maintain healthy muscles and bones and influences body fat distribution. Thyroid-Stimulating Hormone (TSH) targets the thyroid gland, prompting it to produce and release thyroid hormones, which are involved in regulating metabolism and energy levels.
Adrenocorticotropic Hormone (ACTH) stimulates the adrenal glands to produce cortisol, often called the “stress hormone,” which is involved in metabolism, blood pressure regulation, and reducing inflammation. Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) are gonadotrophic hormones that regulate the function of the ovaries and testes. In females, FSH stimulates egg development and estrogen production, while LH triggers ovulation and progesterone production. In males, FSH promotes sperm production, and LH stimulates testosterone synthesis. Prolactin (PRL) primarily stimulates milk production in the mammary glands after childbirth and can also influence menstrual periods and sexual function.
Hormones of the Posterior Lobe
The posterior pituitary gland does not produce hormones itself but serves as a storage and release site for two specific hormones synthesized in the hypothalamus. These hormones are Antidiuretic Hormone (ADH) and Oxytocin.
Antidiuretic Hormone (ADH), also known as vasopressin, regulates water balance by increasing water reabsorption in the kidneys’ collecting ducts, thereby limiting urine production. ADH also contributes to blood pressure regulation by constricting blood vessels.
Oxytocin plays a role in childbirth by stimulating uterine contractions and is responsible for the milk “let-down” reflex during breastfeeding. Beyond its reproductive functions, oxytocin influences social bonding and can have a role in male reproductive processes, such as sperm movement.
Conditions Affecting the Pituitary Gland
Dysfunction of the pituitary gland can lead to a range of health issues, broadly categorized as either hyperpituitarism, an overproduction of hormones, or hypopituitarism, an underproduction of hormones. These imbalances can affect various bodily systems, given the pituitary’s widespread regulatory role.
Pituitary adenomas, which are benign (noncancerous) tumors, are a common cause of pituitary disorders. These growths can secrete excessive hormones, leading to hyperpituitarism, or, if large, compress the gland or surrounding structures, causing hormone deficiencies or vision problems. An adenoma overproducing Growth Hormone in adults can lead to acromegaly, causing enlarged hands, feet, and facial features, along with metabolic issues. In children, excess Growth Hormone can result in gigantism, causing rapid and excessive growth.
Conversely, hypopituitarism occurs when the pituitary gland does not produce enough hormones, often due to damage from surgery, radiation, or a tumor. An example is diabetes insipidus, caused by insufficient Antidiuretic Hormone (ADH) release from the posterior pituitary. This deficiency leads to the kidneys producing excessive dilute urine, leading to increased thirst and potential dehydration. Other forms of hypopituitarism include growth hormone deficiency in children, leading to stunted growth, or central hypothyroidism, caused by too little Thyroid-Stimulating Hormone.