Anterior Pituitary Gland Functions

Anterior Pituitary Gland

• The anterior pituitary, also known as the adenohypophysis, is one of two lobes of the pituitary gland.
• It secretes five major hormones: growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and adrenocorticotropic hormone (ACTH).
• ACTH also has melanocyte-stimulating hormone (MSH) activity, affecting melanin production.

Hypothalamic-Hypophysial Connections

• Vascular connection (Hypothalamic-Hypophysial portal vessels): Portal vessels connect the primary capillary plexus in the hypothalamus to a secondary capillary plexus in the anterior pituitary. Hypothalamic neurons secrete releasing and inhibitory hormones. These hormones travel via portal vessels to regulate anterior pituitary hormones.
• Nervous connection (Hypothalamic-Hypophysial tract): Cell bodies of hypothalamic neurons (supraoptic and paraventricular) send axons to the posterior pituitary. Posterior pituitary hormones are synthesized in the hypothalamus and stored for release upon stimulation.

Hypothalamic Releasing and Inhibitory Hormones

• These hormones regulate anterior pituitary hormone secretion.
• Major hormones include corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), growth hormone-releasing hormone (GHRH), somatostatin (SS), gonadotropin-releasing hormone (GnRH), and dopamine (PIH).
• These hormones have specific effects, like CRH stimulating ACTH secretion.

Growth Hormone (GH)

• Chemical nature: A single polypeptide chain containing 191 amino acids.
• Indirect actions:
  • Accelerates skeletal muscle and soft tissue growth (including abdominal and thoracic viscera).
  • Stimulates linear growth of long bones by increasing bone matrix, and calcium and sulfate uptake.
• Direct actions:
  • Anabolic hormone, stimulating protein synthesis in all body cells.
  • Mobilizes free fatty acids (FFAs) from adipose tissue, providing energy during hypoglycemia, fasting, and stress.
  • Increases blood glucose by inhibiting glucose uptake and utilization by tissues (anti-insulin or diabetogenic effect).
• Regulation of GH secretion:
  • Hypothalamic regulation: GHRH stimulates GH synthesis and release while somatostatin (SS) inhibits them.
  • Negative feedback by somatomedins: Somatomedins inhibit GH secretion directly and stimulate hypothalamus to produce SS.
  • Sleep: Deep sleep increases GH secretion; bursts decrease with age.
  • Hypoglycemia, exercise, and stress: These stimulate GHRH secretion, increasing GH output.

Growth Hormone Disorders

• Dwarfism: GH deficiency before puberty, leading to height below 1-1.2 meters. Causes include anterior pituitary lesions, hypothalamic dysfunction, or GH receptor abnormalities.
• Gigantism: Excess GH secretion before epiphyseal closure in childhood, resulting in abnormally high stature (exceeds 2 meters). Caused by anterior pituitary tumors.
• Acromegaly: Excess GH secretion after epiphyseal closure. Causes bone growth in hands, feet, and facial features but does not affect linear growth. Also characterized by soft tissue overgrowth, possible kyphosis, and increased risk of diabetes.

Prolactin (PRL)

• Chemical nature: Single polypeptide chain, molecular weight ~22,000.
• Actions: Stimulates milk production and breast development during pregnancy (with estrogen), while inhibiting ovulation.
• Regulation:
  • Hypothalamic inhibition by dopamine (PIH). TRH stimulates PRL secretion.
  • Negative feedback: PRL inhibits its own secretion by stimulating PIH release.

PRL Disorders

• PRL deficiency: Failure of lactation.
• PRL excess: Caused by prolactin-secreting tumors. In females, this can result in galactorrhea (excessive milk production) and infertility due to ovulation failure. In males, this can inhibit spermatogenesis.

Tropic Hormones

• These hormones (TSH, FSH, LH, and ACTH) stimulate hormone production in other endocrine glands.
• TSH: Stimulates thyroid hormone secretion and growth.
• FSH: Stimulates ovarian follicle growth in females and spermatogenesis in males.
• LH: Stimulates ovulation in females and testosterone secretion in males.
• ACTH: Stimulates steroid hormone production in the adrenal cortex. Has MSH activity.

Test your knowledge on the anterior pituitary gland and its hormone secretion, including growth hormone and adrenocorticotropic hormone. This quiz also covers the hypothalamic connections that regulate the anterior pituitary. Challenge yourself with questions on this essential endocrine system topic.