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The pituitary gland

The pituitary gland (hypophysis) is attached to the hypothalamus at the base of the brain via the hypophyseal stalk, and secretion of pituitary hormones is regulated by the hypothalamus. The pituitary gland is divided into three parts: the anterior lobe, the intermediate lobe, and the posterior lobe. The first two together form a true endocrine gland.

The posterior pituitary

The posterior lobe, which is essentially neural tissue, acts more as an extension of the hypothalamus. Axons of hypothalamic neurosecretory cells reach into the posterior pituitary forming a part of the hypophyseal stalk called the infundibulum, and their bodies are located in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. The nuclei synthesize two hormones, oxytocin and antidiuretic hormone (vasopressin), and release them via the posterior pituitary into the posterior hypophyseal artery, and the circulation.

The anterior pituitary

The anterior pituitary is attached to the hypothalamus via a part of the hypophyseal stalk called pars tuberalis, which contains hypophyseal portal system. Neurosecretory cells of the hypothalamus  which terminate at the median eminence release their hormones into the primary plexus (first capillary bed) of the hypophyseal portal system. The messengers travel via the pars tuberalis into the secondary plexus (second capillary bed). From here, they stimulate the release of anterior pituitary hormones, which enter the secondary plexus and travel into the circulation.

Pituitary hormones

The pituitary gland releases a total of 9  major hormones.

2 from the posterior pituitary:

  • Oxytocin
  • Antidiuretic hormone (vasopressin)

6 from the anterior pituitary:

  • Follicle-stimulating hormone
  • Luteinizing hormone
  • Thyroid-stimulating hormone
  • Adrenocorticotropic hormone
  • Growth hormone
  • Prolactin

 and one from the intermediate lobe:

  • Melanocyte stimulating hormone

Both anterior and intermediate lobe also release beta-endorphins, an opioid peptide.

The hormones released by the posterior pituitary are synthesized in PVN and SON of the hypothalamus, and stored in the posterior lobe.

Four of the hormones released by the anterior pituitary are tropic (FSH, LH, TSH, ACTH), two nontropic (prolactin and MSH), and one mixed (GH).