Pituitary gland (The master gland)

  • The endocrine system is made up of tissues or organs (collectively called glands) which secrete a chemical substance called hormone.
  • Hormone is a specialized chemical substance secreted by endocrine glands (ductless glands) and released into the blood stream.
  • They travel to all parts of the body through blood but are effective in extremely small amount to produce a dramatic response in the target cells which are located far away from the glands.
  • The study of endocrine glands, the hormones they secrete and their mode of action is called endocrinology.
  • The endocrine system:
    • Regulates the concentration of chemicals in body fluids and the metabolism of proteins, lipids and amino acids.
    • Co-ordinates with the nervous system to help the body react to stress and stimuli properly.
    • Regulates growth and development, including sexual development and reproduction.

Pituitary gland:

  • It is located directly just below the hypothalamus of brain and is held by an infundibular stalk.
  • It is about 1 cm long, 1 to 1.5 cm wide and 0.5 cm thick (about the size and shape of a pea).
  • It is also known as the master gland because the hormones released by this gland regulates and controls the secretions of other endocrine glands in our body.
  • Pituitary gland itself is under the control of hypothalamus, hence also called the hypophyseal gland.
  • The pituitary gland has two distinct lobes:
    • The anterior lobe (Adenohypophysis)
    • The posterior lobe (Neurohypophysis)
  • Between these two lobes is a small zone called the pars intermedia.
  • The anterior lobe is larger with more than 75% of the total weight of the gland and abundance of functional secretory cells.
  • The posterior lobe has a greater supply of large nerve endings.

Hormones of the Neurohypophysis:

  • It doesn’t secrete hormone but stores hormones secreted by the hypothalamus which is brought to it by nerve fibers (hypothalamus-hypophyseal tract).
  • It has spindle shaped cells called pituicytes. The hormones are:

1. Antidiuretic hormone (ADH):

  • It is a polypeptide and is also known as vasopressin.
  • It causes the contraction of arterial walls increasing blood pressure.
  • It also promotes the reabsorption of water from the distal convoluted tubules of nephrons in kidneys.
  • Its deficiency (hyposecretion) causes diabetes insipidus, i.e. micturition increases (frequent urination).

2. Oxytocin:

  • It is a polypeotide.
  • It stimulates parturition (the contraction of pregnant uterus), so called a birth hormone.
  • It also induces lactation (milk ejection from breasts after childbirth), so called milk ejection hormone.
Tropic and non-tropic hormones secreted by the pituitary gland

Hormones of the Adenohypophysis:

  • It synthesizes 5 tropic or stimulating hormones (whose primary target is another endocrine gland).
  • The 2 non tropic hormones are growth hormone and prolactin, which are involved with growth and milk production respectively.

1. Growth hormone (Somatotropin):

  • It is a polypeptide (protein).
  • It stimulates the growth rate of body by increasing the tissue mass (protein synthesis), maintaining epiphyseal growth of bones and stimulating cell division.
  • Hyposecretion in young person causes dwarfism (marked closure of epiphyseal disks of bones and no growth of body).
  • Hypersecretion in young person causes gigantism (the person continues to grow to 7 to 8 feet, looking like a giant).
  • After normal growth has stopped, overproduction of growth hormone causes acromegaly (bones in head, hands, and feet start thickening rather than lengthening).

2. Prolactin:

  • It is a polypeptide.
  • It promotes breast development during pregnancy.
  • It stimulates milk production from the mammary glands of mother after childbirth.

*milk ejection (not milk production) is stimulated by oxytocin.

3. Thyroid stimulating hormone (TSH):

  • It is glycoprotein in nature and stimulates the production and secretion of thyroid hormones (Thyroxine).

4. Adrenocorticotropic hormone (ACTH):

  • It is also called corticotropin.
  • It stimulates the secretion of adrenal cortex steroids (glucocorticoids).

5. Luteinizing hormone (LH):

  • It is glycoprotein in nature and is also called a gonadotropin.
  • In females, it stimulates the development of corpus luteum, release of oocyte (ovum) and the production of progesterone and estrogen.
  • In males, it stimulates the secretion of testosterone and the development of interstitial tissue of testes.

6. Follicle stimulating hormone (FSH):

  • It is glycoprotein in nature.
  • In females, it stimulates the growth of ovarian follicles and ovulation.
  • In males, it stimulates the production of sperm.

7. Melanocyte stimulating hormone (MSH):

  • It is also known as intermidin.
  • It is apparently involved with the distribution of melanin (skin pigment produced by melanocytes) in combination with the ACTH.

Pituitary gland (The master gland in the endocrine system)