The pituitary gland, often referred to as the master gland, plays a vital role in regulating a wide array of bodily functions through the hormones it secretes. Though small in size, this pea-shaped structure is a central player in the endocrine system, controlling the activity of other hormone-producing glands throughout the body. Understanding the anatomy of the pituitary gland helps explain how it performs such powerful regulatory functions, influencing everything from growth and metabolism to reproduction and stress response.
Location and General Structure
The pituitary gland is located at the base of the brain, nestled within a small, bony cavity known as thesella turcicaof the sphenoid bone. It is connected to the hypothalamus above it by a slender stalk called theinfundibulum. This close anatomical relationship is essential for the regulation of hormone production via signals from the hypothalamus.
In adults, the pituitary gland typically measures about 10 millimeters in diameter and weighs approximately 0.5 to 1 gram. Despite its small size, it has a highly organized structure and is divided into two main lobes, each with distinct developmental origins and functions.
Main Divisions of the Pituitary Gland
The pituitary gland is composed of two primary parts:
- Anterior pituitary (adenohypophysis)
- Posterior pituitary (neurohypophysis)
Each part originates from different embryological tissues and serves separate functions, though they work together to regulate physiological processes.
Anterior Pituitary (Adenohypophysis)
The anterior pituitary makes up about 75% of the entire gland and is responsible for synthesizing and secreting a variety of hormones. It originates from an upward growth of oral ectoderm called Rathke’s pouch during embryonic development.
The anterior lobe is further divided into three parts:
- Pars distalis: The largest and most active part, where most hormone production occurs
- Pars intermedia: A thin layer between the anterior and posterior lobes, present mainly during fetal development and producing melanocyte-stimulating hormone (MSH)
- Pars tuberalis: Surrounds the infundibulum and helps support vascular connections
Posterior Pituitary (Neurohypophysis)
The posterior pituitary develops from a downward extension of neural ectoderm from the hypothalamus. Unlike the anterior lobe, it does not produce hormones itself. Instead, it stores and releases hormones that are synthesized in the hypothalamus.
The posterior pituitary consists of the following parts:
- Pars nervosa: The main functional component containing axons from the hypothalamus
- Infundibulum: The stalk that connects the gland to the hypothalamus and acts as a conduit for nerve fibers and blood vessels
Blood Supply of the Pituitary Gland
The pituitary gland has a rich blood supply that is essential for hormone distribution and communication with the hypothalamus. Two major circulatory systems are involved:
Hypophyseal Portal System
This unique system connects the hypothalamus to the anterior pituitary and allows releasing or inhibiting hormones from the hypothalamus to reach the anterior lobe directly, avoiding systemic circulation. It consists of:
- Superior hypophyseal arteries: Supply blood to the median eminence and upper infundibulum
- Primary capillary plexus: Located in the median eminence, receives hypothalamic hormones
- Secondary capillary plexus: Located in the anterior pituitary, where hormones exert their effects
Posterior Pituitary Circulation
The posterior pituitary is supplied mainly by theinferior hypophyseal arteries, and the hormones released here enter the systemic circulation directly through venous drainage.
Hormones of the Anterior Pituitary
The anterior pituitary secretes six key hormones that influence many target organs throughout the body:
- Growth Hormone (GH): Stimulates growth of bones and tissues
- Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland
- Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to produce cortisol
- Follicle-Stimulating Hormone (FSH): Stimulates production of eggs in ovaries and sperm in testes
- Luteinizing Hormone (LH): Triggers ovulation and testosterone production
- Prolactin (PRL): Promotes milk production in breastfeeding women
These hormones are regulated by releasing and inhibiting hormones from the hypothalamus, transported through the hypophyseal portal system.
Hormones of the Posterior Pituitary
The posterior pituitary releases two major hormones that are synthesized in the hypothalamus and transported down axons into the posterior lobe:
- Antidiuretic Hormone (ADH): Also called vasopressin, regulates water balance by acting on the kidneys
- Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding
These hormones are released in response to neural signals and enter the bloodstream through capillaries in the posterior pituitary.
Cell Types in the Pituitary Gland
Each hormone in the anterior pituitary is produced by a specific type of cell, identifiable under the microscope by staining techniques:
- Somatotrophs: Produce growth hormone
- Lactotrophs: Produce prolactin
- Corticotrophs: Produce ACTH
- Thyrotrophs: Produce TSH
- Gonadotrophs: Produce FSH and LH
In contrast, the posterior pituitary contains mainly unmyelinated nerve fibers, glial cells called pituicytes, and Herring bodies where hormones are stored before release.
Functional Importance of the Pituitary Gland
Through its secretion of critical hormones, the pituitary gland influences numerous physiological processes including:
- Growth and development
- Metabolism and energy balance
- Reproductive cycles and fertility
- Stress response and immune regulation
- Fluid and electrolyte balance
Dysfunction in the pituitary gland can lead to a variety of disorders such as gigantism, dwarfism, Cushing’s disease, hypothyroidism, infertility, and diabetes insipidus, depending on which hormones are over- or under-produced.
The anatomy of the pituitary gland reveals a small yet extraordinarily complex and essential structure. Positioned at the heart of the endocrine system and closely connected to the hypothalamus, the pituitary regulates vital processes that affect nearly every organ and tissue in the body. Divided into anterior and posterior lobes with distinct embryologic origins and functions, it secretes hormones that control growth, metabolism, reproduction, and fluid balance. An understanding of the pituitary gland’s structure, blood supply, and hormone production is critical for anyone studying human biology, medicine, or healthcare. This small organ truly lives up to its nickname as the body’s master gland.