The Pituitary Gland

Overview of the Pituitary Gland

The pituitary gland is connected to the hypothalamus and secretes nine hormones that regulate body homeostasis.

Learning Objectives

Summarize the structure and function of the pituitary gland

Key Takeaways

Key Points

  • The pituitary gland, or hypophysis, is an endocrine gland about the size of a pea. Although located at the base of the brain and often considered to be part of the brain, the pituitary gland is in fact a separate organ, and is not part of the brain.
  • The pituitary gland is divided into two parts, the anterior pituitary and the posterior pituitary. The anterior pituitary receives signalling molecules from the hypothalamus, and in response, synthesizes and secretes seven hormones.
  • The posterior pituitary does not produce any hormones of its own; instead, it stores and secretes two hormones made in the hypothalamus.

Key Terms

  • pituitary gland: An endocrine gland, about the size of a pea, that sits in a small, bony cavity at the base of the brain whose secretions control the other endocrine glands and influence growth, metabolism, and maturation.
  • hypothalamus: A region of the forebrain located below the thalamus, forming the basal portion of the diencephalon, and functioning to regulate body temperature, some metabolic processes, and governing the autonomic nervous system.

The pituitary gland, or hypophysis, is an endocrine gland about the size of a pea. Although located at the base of the brain and often considered to be part of the brain, the pituitary gland is in fact a separate organ. It protrudes off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity.

This illustration shows where the pituitary gland is located in the brain. It protrudes off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity.

Pituitary location: The location of pituitary gland in the human brain.

The pituitary is functionally connected to the hypothalamus by a small tube called the infundibular stem, or, pituitary stalk. The pituitary gland secretes hormones that regulate homeostasis.

The pituitary gland is divided into two parts, the anterior pituitary and the posterior pituitary.

  • The anterior pituitary receives signaling molecules from the hypothalamus, and in response, synthesizes and secretes seven important hormones including thyroid-stimulating hormone and growth hormone.
  • The posterior pituitary does not produce any hormones of its own, rather, it stores and secretes two hormones made in the hypothalamus— oxytocin and
    anti-diuretic hormone.
In this image that shows the location of the pituitary gland, it is referred to by its other name, the hypophysis.

The pituitary gland: In this image, the pituitary gland is referred to by its other name, the hypophysis.

Control of the Pituitary Gland by the Hypothalamus

The pituitary gland consists of the anterior pituitary and the posterior pituitary.

Learning Objectives

Explain how the hypothalamus controls the pituitary gland

Key Takeaways

Key Points

  • While the pituitary gland is known as the master endocrine gland, both of its lobes are under the control of the hypothalamus: the anterior pituitary receives its signals from the parvocellular neurons, and the posterior pituitary receives its signals from the magnocellular neurons.
  • The pituitary gland is connected by a system of blood vessels to the hypothalamus. This system of blood vessels is known as the hypophyseal portal system, and it allows endocrine communication between the two structures.
  • The mechanism for hormone transport via hypothalamoportal vessels involves cells that are regulated by different nuclei in the hypothalamus; for instance, neurons that release neurotransmitters as hormones in the connective link between the pituitary and the brain.

Key Terms

  • pituitary gland: An endocrine gland, about the size of a pea, that sits in a small, bony cavity at the base of the brain whose secretions control the other endocrine glands and influence growth, metabolism, and maturation.
  • hypothalamus: A region of the forebrain located below the thalamus, forming the basal portion of the diencephalon, that regulates body temperature, some metabolic processes, and governs the autonomic nervous system.
  • hypophyseal portal system: The system of blood vessels that link the hypothalamus and the anterior pituitary in the brain.

The pituitary gland consists of two components: the anterior pituitary and the posterior pituitary, and is functionally linked to the hypothalamus by the pituitary stalk (also named the infundibular stem, or simply the infundibulum).

Whilst the pituitary gland is known as the master endocrine gland, both of the lobes are under the control of the hypothalamus: the anterior pituitary receives its signals from the parvocellular neurons, and the posterior pituitary receives its signals from magnocellular neurons.

The anterior lobe of the pituitary receives
hypothalamic-releasing hormones from the hypothalamus that bind with receptors on endocrine cells in the anterior pituitary that regulate the release of adrenal hormones into the circulatory system. Hormones from the hypothalamus are rapidly degraded in the anterior pituitary, which prevents them from entering the circulatory system.

The posterior lobe of the pituitary gland develops as an extension of the hypothalamus. As such, it is not capable of producing its own hormones; instead, it stores hypothalamic hormones for later release into the systemic circulation.

This is a drawing of the skull with the parts of the brain identified. In particular, the anterior and posterior pituitary gland are called out. They are referred to as the anterieor and posterior hypophysis in the drawing.

Pituitary gland: The anterior and posterior lobes of the pituitary (hypophysis) gland are shown.

The Anterior Pituitary

The anterior pituitary secretes seven hormones that regulate several physiological processes, including stress, growth, and reproduction.

Learning Objectives

Identify the location and the hormones produced by the anterior pituitary

Key Takeaways

Key Points

  • A major organ of the endocrine system, the anterior pituitary, also called the adenohypophysis, is the glandular, anterior lobe of the pituitary gland.
  • The anterior pituitary regulates several physiological processes, including stress, growth, reproduction, and lactation.
  • The anterior pituitary gland secretes 7 hormones: follicle -stimulating hormone, luteinizing horomone, adrenocorticotropic horomone, thyroid -stimulating horomone, prolactin, endorphins, and growth hormone.

Key Terms

  • anterior pituitary gland: A major organ of the endocrine system that regulates several physiological processes including stress, growth, reproduction, and lactation.

A major organ of the endocrine system, the anterior pituitary (also called the adenohypophysis) is the glandular, anterior lobe of the pituitary gland. The anterior pituitary regulates several physiological processes including stress, growth, reproduction, and lactation.

Its regulatory functions are achieved through the secretion of various peptide hormones that act on target organs including the adrenal gland, liver, bone, thyroid gland, and gonads. The anterior pituitary itself is regulated by the hypothalamus and by negative feedback from these target organs.

Anatomy of the Anterior Pituitary Gland

The fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary. The anterior pituitary is composed of multiple parts:

  • Pars distalis: This is the distal part that comprises the majority of the anterior pituitary; it is where most pituitary hormone production occurs.
  • Pars tuberalis: This is the tubular part that forms a sheath that extends up from the pars distalis and wraps around the pituitary stalk. Its function is poorly understood.
  • Pars intermedia: This is the intermediate part that sits between the pars distalis and the posterior pituitary and is often very small in humans.

Major Hormones Secreted by the Anterior Pituitary Gland

  • Adrenocorticotropic hormone (ACTH), is a polypeptide whose target is the adrenal gland. The effects of ACTH are upon secretion of glucocorticoid, mineralocorticoids, and sex corticoids.
  • Beta-endorphin is a polypeptide that effects the opioid receptor, whose effects include the inhibition of the perception of pain.
  • Thyroid-stimulating hormone is a glycoprotein hormone that affects the thyroid gland and the secretion of thyroid hormones.
  • Follicle-stimulating hormone is a glycoprotein hormone that targets the gonads and effects the growth of the reproductive system.
  • Luteinizing hormone is a glycoprotein hormone that targets the gonads to effect sex-hormone production.
  • Growth hormone is a polypeptide hormone that targets the liver and adipose tissue and promotes growth through lipid and carbohydrate metabolism.
  • Prolactin is a polypeptide hormone whose target is the ovaries and mammary glands. Prolactin influences the secretion of estrogen/progesterone and milk production.

Regulation

Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by the hypothalamus. Neuroendocrine neurons in the hypothalamus project axons to the median eminence, at the base of the brain. At this site, these neurons can release substances into the small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophysial portal vessels).

This is an illustration of the anterior pituitary, which is linked to the hypothalamus by a portal system. The hypothalamus releases signaling molecules that incite the anterior pituitary to produce hormones.

The anterior pituitary: The anterior pituitary, in yellow, is linked to the hypothalamus by a portal system. The hypothalamus releases signaling molecules that incite the anterior pituitary to produce hormones.

The Posterior Pituitary

The posterior pituitary secretes two important endocrine hormones—oxytocin and antidiuretic hormone.

Learning Objectives

Identify the location of the posterior pituitary and the hormones associated with it

Key Takeaways

Key Points

  • The posterior pituitary (or neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system.
  • Hormones known as posterior pituitary hormones are synthesized by the hypothalamus, and include oxytocin and antidiuretic hormone.
  • The hormones are then stored in neurosecretory vesicles (Herring bodies) before being secreted by the posterior pituitary into the bloodstream.

Key Terms

  • oxytocin: A hormone that stimulates contractions during labor.
  • posterior pituitary: The posterior pituitary (or neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a true gland; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior pituitary gland.
  • Antidiuretic hormone: A hormone that stimulates water re-absorption in the kidneys.

Posterior Pituitary Gland

This is an illustration of a pituitary gland that shows the anterior pituitary and the posterior pituitary, and the hypothalamus above them.

Pituitary: Pituitary gland representation.

The posterior pituitary (or neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a gland; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior pituitary gland.

The posterior pituitary consists mainly of neuronal projections ( axons ) extending from the supraoptic and paraventricular nuclei of the hypothalamus. These axons release peptide hormones into the capillaries of the hypophyseal circulation. These are then stored in neurosecretory vesicles (Herring bodies) before being secreted by the posterior pituitary into the systemic bloodstream.

Anatomy of the Posterior Pituitary Gland

The posterior pituitary is derived from the hypothalamus and is distinct from the more fleshy, vascularized anterior lobe. The posterior pituitary is composed of two parts:

  • The pars nervosa, also called the neural lobe or posterior lobe, constitutes the majority of the posterior pituitary and is the storage site of oxytocin and vasopressin.
  • The infundibular stalk, also known as the infundibulum or pituitary stalk, bridges the hypothalamic and hypophyseal systems.

Major Hormones Secreted by the Posterior Pituitary Gland

The posterior pituitary stores two hormones secreted by the hypothalamus for later release:

  • Oxytocin, most of which is released from the paraventricular nucleus in the hypothalamus. Oxytocin is one of the few hormones that create a positive feedback loop.
  • Antidiuretic hormone (ADH, also known as vasopressin), the majority of which is released from the supraoptic nucleus in the hypothalamus. ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood.