Anterior Pituitary (Adenohypophysis)

1. Overview

The anterior pituitary, also known as the adenohypophysis, is the glandular front portion of the pituitary gland responsible for producing and secreting a wide range of hormones that regulate growth, metabolism, reproduction, and stress response. It functions under the influence of releasing and inhibiting hormones from the hypothalamus and forms a crucial part of the hypothalamic-pituitary axis.

2. Location

The anterior pituitary is located within the sella turcica, a bony depression of the sphenoid bone at the base of the skull. It sits beneath the hypothalamus and is connected to it via the infundibulum (pituitary stalk). The entire pituitary gland lies inferior to the optic chiasm and superior to the sphenoid sinus.

3. Structure

The anterior pituitary is derived embryologically from oral ectoderm (Rathke’s pouch) and is composed of glandular epithelial tissue. It can be histologically and functionally divided into the following parts:

• Pars distalis: The largest and most hormone-productive portion.

• Pars intermedia: A narrow band between the anterior and posterior lobes; rudimentary in humans.

• Pars tuberalis: Wraps around the pituitary stalk and may assist in seasonal hormonal regulation.

The pars distalis contains several hormone-producing cell types, identified by the hormones they produce and histochemical staining:

• Somatotrophs – produce growth hormone (GH)

• Lactotrophs – produce prolactin (PRL)

• Corticotrophs – produce adrenocorticotropic hormone (ACTH)

• Thyrotrophs – produce thyroid-stimulating hormone (TSH)

• Gonadotrophs – produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

4. Function

The anterior pituitary synthesizes and secretes six major hormones, each with distinct targets and physiological roles:

• Growth Hormone (GH): Stimulates growth of bone and soft tissues, increases protein synthesis, and promotes lipolysis.

• Prolactin (PRL): Stimulates milk production in the mammary glands postpartum.

• Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to produce cortisol and androgens.

• Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce T3 and T4 hormones.

• Follicle-Stimulating Hormone (FSH): Promotes follicular development in ovaries and spermatogenesis in testes.

• Luteinizing Hormone (LH): Triggers ovulation and corpus luteum formation in females; stimulates testosterone production in males.

5. Physiological role(s)

The anterior pituitary plays a master regulatory role in the endocrine system, affecting multiple physiological processes:

• Growth and Development: GH regulates linear growth in children and protein metabolism throughout life.

• Metabolism and Stress: ACTH-mediated cortisol release ensures glucose availability and immune modulation during stress.

• Reproductive Function: FSH and LH regulate menstrual cycle, ovulation, spermatogenesis, and sex steroid production.

• Lactation: Prolactin supports milk synthesis during and after pregnancy.

• Thyroid Regulation: TSH ensures proper thyroid function, critical for metabolic rate, thermogenesis, and energy balance.

6. Clinical Significance

Disorders involving the anterior pituitary can result from hormone excess or deficiency, tumors, or developmental abnormalities:

• Pituitary Adenomas: Benign tumors that may be functional (hormone-secreting) or non-functional. Depending on hormone affected:

  • Prolactinomas: Cause galactorrhea, amenorrhea, and infertility.

  • Somatotroph adenomas: Lead to gigantism in children or acromegaly in adults.

  • Corticotroph adenomas: Cause Cushing’s disease via excess ACTH.

• Hypopituitarism: Partial or complete loss of anterior pituitary function, often due to ischemia, trauma, surgery, or tumor compression. Leads to deficiencies in one or more pituitary hormones.

• Sheehan's Syndrome: Postpartum pituitary necrosis due to hemorrhage, causing panhypopituitarism.

• Empty Sella Syndrome: Herniation of subarachnoid space into the sella turcica compresses the pituitary; may be asymptomatic or cause hormone deficits.

• Pituitary Apoplexy: Sudden hemorrhage into the pituitary gland, presenting with headache, visual changes, and acute hypopituitarism—an endocrine emergency.

• Craniopharyngioma: A developmental tumor that may compress the pituitary and hypothalamus, disrupting hormonal function and vision.