LG 2

Pituitary Function and Control of Pituitary Hormones

Overview

• The pituitary gland plays a crucial role in the endocrine system, often referred to as the "master" gland.

• However, its functions are heavily regulated and influenced by the hypothalamus, a small brain region located below the thalamus.

Learning Outcomes

• Understand the relationship between the hypothalamus and pituitary gland.

• Identify the hormones and functions of the anterior and posterior pituitary.

Anatomy of the Pituitary Gland

• Structure:

  • Pea-shaped; measures 1–1.5 cm.

  • Located in the hypophysial fossa of the sella turcica, which is part of the sphenoid bone.

  • Connected to the hypothalamus via the infundibular stalk.

  • Divided into two portions: anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).

Historical Perspective

• Historically regarded as the “master” endocrine gland but is now understood to be regulated by the hypothalamus, which oversees its functions.

• Role of the Hypothalamus:

  • Synthesizes at least nine hormones.

  • Pituitary gland secretes seven hormones, pivotal in regulating growth, development, metabolism, and homeostasis.

Pituitary Gland Composition

• Anterior Pituitary:

  • Accounts for 75% of pituitary gland weight.

  • Comprised of epithelial tissue.

• Posterior Pituitary:

  • Composed of neural tissue.

• Pars Intermedia:

  • A third region present in some animals; atrophies during human fetal development.

Hypothalamic Control of the Anterior Pituitary

• Releasing Hormones:

  • Five releasing hormones stimulate secretion of anterior pituitary hormones:

  1. Growth Hormone-Releasing Hormone (GHRH):

     • Also known as somatocrinin; stimulates secretion of growth hormone (GH).

  2. Thyrotropin-Releasing Hormone (TRH):

     • Stimulates secretion of thyroid-stimulating hormone (TSH).

  3. Corticotropin-Releasing Hormone (CRH):

     • Stimulates secretion of adrenocorticotropic hormone (ACTH).

  4. Prolactin-Releasing Hormone (PRH):

     • Stimulates secretion of prolactin (PRL).

  5. Gonadotropin-Releasing Hormone (GnRH):

     • Stimulates secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

• Inhibiting Hormones:

  • Two inhibiting hormones suppress anterior pituitary hormones:

  1. Growth Hormone-Inhibiting Hormone (GHIH):

     • Known as somatostatin; inhibits secretion of growth hormone.

  2. Prolactin-Inhibiting Hormone (PIH):

     • Dopamine; inhibits secretion of prolactin.

General Functions of the Hypothalamus

• Regulatory Functions:

  1. Controls body temperature.

  2. Regulates cardiovascular system.

  3. Manages food intake and body weight.

  4. Controls thirst and water balance.

  5. Involved in sleep and wakefulness.

  6. Influences emotional reactions.

  7. Responds to stress.

  8. Regulates ovarian secretions during the ovarian cycle.

  9. Regulates testicular secretions.

Anterior Pituitary Hormones and Their Secretory Cells

1. Somatotrophic Cells:

   • Secrete growth hormone (GH, somatotropin).

   • Stimulates general body growth and regulates metabolism.

2. Thyrotrophic Cells:

   • Secrete thyroid-stimulating hormone (TSH, thyrotropin).

   • Controls thyroid gland activities.

3. Gonadotrophic Cells:

   • Secrete FSH and LH.

   • In men, stimulate spermatogenesis and testosterone production.

   • In women, stimulate follicular maturation, estrogen and progesterone secretion.

4. Prolactin Cells:

   • Secrete prolactin (PRL) for milk production initiation.

5. Corticotrophic Cells:

   • Secrete ACTH, which stimulates the suprarenal cortex to produce glucocorticoids like cortisol.

   • Can also secrete melanocyte-stimulating hormone (MSH).

Hormones and Cells of the Anterior Pituitary

Hypophyseal Portal System

• Pituitary glands are connected to the hypothalamus via the hypophyseal portal system, allowing hypothalamic hormones to influence anterior pituitary secretions.

• Mechanism:

  • Blood flows from capillaries in the hypothalamus into portal veins and then to capillaries of the anterior pituitary, ensuring effective communication between these two glands.

Regulation of Anterior Pituitary Secretion

• Upon release:

  1. Neurosecretory cells in the hypothalamus synthesize and package releasing/inhibiting hormones into vesicles for transport.

  2. Upon stimulation, hormones are released into primary plexus of the hypophyseal portal system.

  3. Hormones diffuse through the secondary plexus, impacting anterior pituitary cells directly.

  4. Anterior pituitary hormones then enter general circulation through hypophyseal veins.

Negative Feedback Regulation

• Hormonal secretion from the anterior pituitary is also regulated by negative feedback mechanisms from target gland hormones.

• For example:

  • ACTH promotes cortisol secretion from the suprarenal gland, leading to reduced ACTH and CRH secretion when cortisol levels are high.

Growth Hormone (GH)

• Characteristics:

  • GH is produced by somatotrophic cells and is the most abundant hormone in anterior pituitary.

  • It promotes growth in various tissues indirectly via insulin-like growth factors (IGFs).

Functions of IGFs and GH

1. Increase bone and soft tissue growth.

2. Enhance lipolysis (fat breakdown).

3. Decrease glucose uptake by tissues.

Regulation of GH

1. GHRH promotes GH secretion when stimulated by

   • Hypoglycemia, low fatty acid levels, high amino acid levels, stress, and exercise.

2. GH feeding back to negate GHRH release when levels rise, stabilizing its own secretion.

3. GHIH secreted due to high GH/IGFs, obesity, aging, etc., inhibiting GH secretion by interfering with GHRH signaling pathway.

Thyroid-Stimulating Hormone (TSH)

• Functions:

  • Stimulates synthesis/secretion of triiodothyronine (T3) and thyroxine (T4) from thyroid gland.

  • Regulated by TRH from the hypothalamus and inhibited by high levels of T3 and T4 (negative feedback).

Follicle-Stimulating Hormone (FSH)

• Functions:

  • In females: Initiates ovarian follicle development and stimulates estrogen secretion.

  • In males: Stimulates sperm production in testes.

  • Controlled by GnRH from the hypothalamus and suppressed by estrogen/testosterone.

Luteinizing Hormone (LH)

• Functions:

  • In females: Triggers ovulation, stimulates corpus luteum and progesterone secretion.

  • In males: Stimulates testosterone production from testes.

  • Regulated by GnRH.

Prolactin (PRL)

• Functions:

  • Initiates and maintains milk production in mammary glands.

  • Requires a permissive effect from other hormones before milk production can occur; regulated by PIH.

Adrenocorticotropic Hormone (ACTH)

• Functions:

  • Controls secretion of cortisol and other glucocorticoids from the suprarenal cortex.

  • Stimulated by CRH from hypothalamus and stress-related stimuli.

Melanocyte-Stimulating Hormone (MSH)

• Functions:

  • Increases melanocyte activity in response to stress, potentially affecting brain activity; minimal presence in humans.

Posterior Pituitary

• Composition:

  • Does not synthesize hormones; it stores and releases oxytocin and antidiuretic hormone (ADH).

Control of Posterior Pituitary Secretion

1. Neurosecretory cells in hypothalamus synthesize oxytocin and ADH, packaged into vesicles.

2. Vesicles transported to posterior pituitary and stored.

3. Upon stimulation, hormones are released into systemic circulation.

4. Target tissues receive these hormones for physiological effects.

Summary of Posterior Pituitary Hormones

Clinical Connections

• Oxytocin and Childbirth:

  • Nursing after childbirth promotes oxytocin release to help with uterine contraction and placental expulsion.

  • Synthetic oxytocin (Pitocin) used clinically to induce labor and control post-delivery hemorrhage.

Questions and Comments