Pituitary | Anatomy2Medicine
Pituitary

Pituitary

    • Hypothalamic-Pituitary Relationships
      • Pituitary gland, is called the hypophysis,
        • posterior pituitary is also called the neurohypophysis
        • anterior pituitary is also called the adenohypophysis.
    • Relationship of the hypothalamus to the posterior pituitary

 

  • The connections between the hypothalamus and the posterior lobe of the pituitary are neural

 

      • a hormone-secreting neuron has its cell body in the hypothalamus and its axons in the posterior lobe of the pituitary.
      • the hormones secreted by the posterior lobe (ADH and oxytocin) are actually neuropeptides; in other words, they are peptides released from neurons.
      • The cell bodies of ADH and oxytocin-secreting neurons are located in supraoptic and paraventricular nuclei within the hypothalamus.
        • ADH is primarily associated with surpraoptic nuclei (MCQ)
        • oxytocin is primarily associated with paraventricular nuclei. (MCQ)
    • Relationship of the hypothalamus to the anterior pituitary
      • The anterior pituitary secretes six peptide hormones: thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone, prolactin, and adrenocortico- tropic hormone (ACTH).

 

  • The hypothalamus and anterior pituitary are linked directly by the hypothalamic-hypophysial portal blood vessels, which provide most of the blood supply to the anterior lobe.
  • There are two important implications of the portal blood supply to the anterior lobe of the pituitary

 

        • The hypothalamic hormones can be delivered to the anterior pituitary directly and in high concentration
        • the hypothalamic hormones do not appear in the systemic circulation in high concentrations. The cells of the anterior pituitary, therefore, are the only cells in the body to receive high concentrations of the hypothalamic hormones.

 

  • Anterior Lobe Hormones
  • TSH, FSH, AND LH FAMILY

 

      • TSH, FSH, and LH are all glycoproteins (MCQ)
      • sugar moieties covalently linked to asparagine residues in their polypeptide chains.. The alpha subunits of TSH, FSH, and LH are identical and are synthesized from the same mRNA. (MCQ)
      • The b subunits  confer the biologic specificity (MCQ)
      • The placental hormone human chorionic gonadotropin (HCG) is structurally related to alpha chain of the TSH-FSH- LH family(MCQ)
    • ACTH FAMILY
      • The ACTH family is derived from a single precursor, pro-opiomelanocortin (POMC).
      • The ACTH family includes ACTH, gamma- and beta-lipotropin, beta-endorphin, andmelanocyte-stimulating hormone (MSH). (MCQ)
      • The preprohormone for this group, prepropiomelanocortin, is transcribed from a single gene.
      • Addison’s disease (primary adrenal insufficiency), POMC and ACTH levels are increased by negative feedback. (MCQ)
      • Because POMC and ACTH contain MSH activity, skin pigmentation is a symptom of this disorder. (MCQ)

 

  • Growth hormone

 

    • It is the single most important hormone for normal growth to adult stature. (MCQ)
    • The gene for growth hormone is a member of a family of genes for related peptides, prolactin and human placental lactogen.
    • Human placental lactogen, has 80% homology with Growth Hormone. (MCQ)
    • Regulation of Growth Hormone Secretion
      • Growth hormone is secreted in a pulsatile pattern
        • bursts of secretion occurring approximately every 2 hours. (MCQ)
        • The largest secretory burst occurs within 1 hour of falling asleep (during sleep stages III and IV). (MCQ)
      • At puberty, there is an enormous secretory burst
        • induced in females by estrogen and in males by testosterone.
    • Actions of Growth Hormone
      • Diabetogenic effect.
        • causes insulin resistance (MCQ)
        • decreases glucose uptake and utilization by target tissues such as muscle and adipose tissue.
        • increases lipolysis in adipose tissue.
      • Actions of growth hormone result from the hormone’s direct effect on target tissues such as skeletal muscle, the liver, or adipose tissue.
      • Other actions of growth hormone are mediated indirectly through the production of somatomedins (or insulin- like growth factors [IGFs]) in the liver.
      • The most important of the somatomedins is somatomedin C or IGF-1. Somatomedins act on target tissues through IGF receptors that are similar to the insulin receptor, having tyrosine-kinase activity and exhibiting autophosphorylation. (MCQ)
      • The growth-promoting effects of growth hormone are mediated largely through production of somatomedins.
      • The actions of growth hormone

 

  • causes an increase in blood insulin levels.

 

        • Increased protein synthesis and organ growth

 

  • increases the uptake of amino acids (MCQ)

 

          • stimulates the synthesis of DNA, RNA, and protein.
          • increase lean body mass
          • increase organ size.
        • Increased linear growth.

 

  • The most striking effect of growth hormone is its ability to increase linear growth.

 

        • Mediated by the somatomedins, growth hormone alters every aspect of cartilage metabolism (MCQ)
          • stimulation of DNA synthesis
          • RNA synthesis
          • protein synthesis.
        • In growing bones, the epiphyseal plates widen, and more bone is laid down at the ends of long bones.
        • There also is increased metabolism in cartilage-forming cells and proliferation of chondrocytes.

 

  • Prolactin

 

 

    • Actions of Prolactin
      • At puberty, prolactin, with estrogen and progesterone, stimulates proliferation and branching of the mammary ducts. (MCQ)
      • Breast development.

 

  • During pregnancy, prolactin (again with estrogen and progester- one) stimulates growth and development of the mammary alveoli, which will produce milk once parturition occurs.

 

      • Lactogenesis (milk production).
        • The major action of prolactin is
          • stimulation of milk production
            • by inducing the synthesis of the components of milk, including lactose casein and lipids.
          • secretion in response to suckling.
      • Inhibition of ovulation.

 

  • In females, prolactin inhibits ovulation by inhibiting the synthesis and release of gonadotropin-releasing hormone (GnRH) (MCQ)

 

        • Inhibition of GnRH secretion and, secondarily, inhibition of ovulation account for the decreased fertility during breast-feeding.

 

  • In males with high prolactin levels (e.g., due to a prolactinoma), there is a parallel inhibitory effect on GnRH secretion and spermatogenesis, resulting in infertility. (MCQ)

 

        • Why the lactation does not occur  even though prolactin levels are very high during pregnancy, (MCQ)
          • because the high levels of estrogen and progesterone down-regulate prolactin receptors in the breast and block the action of prolactin.
          • At parturition, estrogen and progesterone levels drop precipitously, and their inhibitory actions cease.Prolactin can then stimulate lactogenesis, and lactation can occur.
    • Posterior Pituitary Hormones
      • originates primarily in the paraventricular nuclei of the hypothalamus (MCQ)
      • causes milk flow (ie, letdown) from the breast.
      • Antidiuretic hormone (ADH)
        • synthesized in the hypothalamus
        • primarily in the supraoptic nucleus(MCQ)
        • stored and released from the posterior pituitary.
        • ADH increases water permeability of the renal collecting duct by placing water channels in the membrane. (MCQ)
      • Oxytocin

 

  • Oxytocin release is stimulated by suckling at the breast, sexual activity, or emotional factors (eg, hearing the infant cry). (MCQ)

 

      • Oxytocin causes contraction of the myoepithelial cells of the mammary gland and uterine contractions at term.