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The anterior pituitary gland, also known as the adenohypophysis, is the front portion of the pituitary gland, a small but highly important endocrine gland located at the base of the brain in a bony structure called the sella turcica. It is connected to the hypothalamus by the pituitary stalk (infundibulum) and plays a central role in regulating various physiological processes by secreting hormones directly into the bloodstream.

This part of the pituitary is primarily composed of glandular epithelial cells and functions under the control of hypothalamic releasing and inhibiting hormones, which reach it through the hypophyseal portal system. The anterior pituitary produces several key hormones such as growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and the gonadotropins—follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones influence growth, metabolism, reproduction, lactation, and stress responses, making the anterior pituitary a master regulator of many vital body functions.

Function of the Anterior Pituitary Gland

The anterior pituitary gland, also known as the adenohypophysis, is a major endocrine gland responsible for producing and secreting several hormones that regulate growth, metabolism, reproduction, and various physiological processes. Its functions are controlled primarily by releasing and inhibiting hormones from the hypothalamus, and it works in close coordination with other endocrine organs through feedback mechanisms.

The anterior pituitary performs its role by producing specific hormones, each with distinct target organs and physiological effects:

Growth Hormone (GH) or Somatotropin

Growth hormone stimulates growth and development of the body, particularly bones and muscles. It increases protein synthesis, promotes cell division, and influences metabolism by mobilizing fats for energy and reducing glucose uptake in tissues. In children, GH is essential for normal physical growth, while in adults, it helps maintain muscle mass and bone density.

Thyroid-Stimulating Hormone (TSH) or Thyrotropin

TSH regulates the activity of the thyroid gland. It stimulates the thyroid to produce thyroxine (T4) and triiodothyronine (T3), hormones that control the rate of metabolism, energy production, and overall growth and development. The secretion of TSH is regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus and feedback from circulating thyroid hormones.

Adrenocorticotropic Hormone (ACTH)

ACTH stimulates the adrenal cortex to secrete glucocorticoids, mainly cortisol. These hormones play a vital role in controlling metabolism, immune responses, and the body’s adaptation to stress. ACTH release is controlled by corticotropin-releasing hormone (CRH) from the hypothalamus and regulated by cortisol levels through negative feedback.

Follicle-Stimulating Hormone (FSH)

FSH is involved in reproduction. In females, it promotes the growth and maturation of ovarian follicles, leading to the development of eggs. In males, it stimulates spermatogenesis in the testes by acting on Sertoli cells. FSH secretion is regulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus and feedback from sex hormones like estrogen and testosterone.

Luteinizing Hormone (LH)

LH works alongside FSH to control reproductive functions. In females, it triggers ovulation and stimulates the formation of the corpus luteum, which produces progesterone essential for maintaining pregnancy. In males, LH stimulates Leydig cells in the testes to produce testosterone. Its release is also regulated by GnRH and influenced by sex hormone levels.

Prolactin (PRL)

Prolactin primarily promotes milk production in the mammary glands after childbirth. It also plays a role in regulating reproductive functions by influencing gonadotropin secretion. Prolactin secretion is mainly inhibited by dopamine from the hypothalamus and stimulated by factors like suckling during breastfeeding.

Endorphins and Other Peptides

The anterior pituitary also secretes small amounts of endorphins, which act as natural painkillers, and other peptide hormones that contribute to immune modulation and stress adaptation.

Anatomy of the Anterior Pituitary Gland

The anterior pituitary gland, or adenohypophysis, forms the front portion of the pituitary gland and accounts for about two-thirds of its total mass. It is primarily a glandular structure responsible for hormone synthesis and secretion, and its anatomy is specialized to allow close communication with the hypothalamus for precise hormonal regulation.

Location and Relation

The anterior pituitary lies in the sella turcica, a depression in the sphenoid bone at the base of the skull. It is connected to the hypothalamus by the pituitary stalk (infundibulum) and is separated from the posterior pituitary by a narrow cleft called the pars intermedia. Above it lies the optic chiasma, and it is protected below by the bony floor of the sella.

Origin and Development

Embryologically, the anterior pituitary originates from an upward growth of the ectodermal lining of the primitive oral cavity called Rathke’s pouch. This developmental origin explains its epithelial glandular nature, distinguishing it from the neural origin of the posterior pituitary.

Subdivisions of the Anterior Pituitary

The anterior pituitary is divided into three main regions:

• Pars distalis: The largest and most active portion where most hormone-producing cells are located.
• Pars tuberalis: A thin extension of tissue that wraps around the infundibulum; it may play a role in seasonal and circadian rhythm regulation.
• Pars intermedia: A narrow zone between the anterior and posterior lobes, containing cells that produce melanocyte-stimulating hormone (MSH) in some species; in humans, it is vestigial but may retain some endocrine function.

Histological Structure

The anterior pituitary is made up of epithelial cords and clusters of hormone-secreting cells supported by a rich network of capillaries. The blood supply comes mainly through the superior hypophyseal arteries, which form the primary capillary plexus in the median eminence of the hypothalamus. Blood from this plexus drains into portal veins that reach the secondary capillary plexus in the anterior pituitary, enabling direct delivery of hypothalamic hormones.

Cell Types

Specialized endocrine cells in the anterior pituitary are classified according to the hormones they produce:

• Somatotrophs: Secrete growth hormone (GH).
• Lactotrophs: Secrete prolactin (PRL).
• Corticotrophs: Secrete adrenocorticotropic hormone (ACTH).
• Thyrotrophs: Secrete thyroid-stimulating hormone (TSH).
• Gonadotrophs: Secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Blood Supply and Portal System

The anterior pituitary has no direct neural connection to the hypothalamus; instead, it communicates via the hypophyseal portal system, which ensures that hypothalamic releasing or inhibiting hormones reach the adenohypophysis quickly and in high concentration without being diluted in the general circulation.

Protective Structures

It is encased in the dura mater, forming a dural fold called the diaphragma sellae, which has a small opening for the passage of the infundibulum. This structure provides both mechanical protection and a barrier against excessive brain movement.