Pituitary Gland & Hypothalamus: Structure, Function & Hormones

Instructor: Cheryl Rosenfeld

Cheryl has taught veterinary and medical student for over 20 years and has a DVM and PhD degree in reproductive biology.

This lesson discusses the pituitary gland and hypothalamus, including hormones that originate from these two organs, hormones/factors that control their regulation, and effects of their hormonal products. The embryological origin of the pituitary gland is also discussed. Updated: 12/20/2020

Pituitary Gland and Hypothalamus

The pituitary gland has two separate embryological origins. The anterior pituitary gland is derived from an upward extension of oral ectoderm, whereas the posterior pituitary gland is derived from a downward extension of neuroectoderm. The hypothalamus produces hormones and other factors that regulate production of hormones by the anterior pituitary gland. Hypothalamic hormones travel down axons of neurons to reach the posterior pituitary gland where they are stored and released from.

This diagram shows the anterior and posterior lobes of the pituitary gland. Also shown is the infundibulum that extends from the brain to the posterior pituitary gland region.
hypophysis

This is another diagram to show the relationship of the hypothalamus and pituitary gland. The third ventricle extends into the posterior lobe as the infundibular recess. the largest lobe of the anterior pituitary gland is the pars distalis , which is separated by the hypophyseal cavity from the pars intermedia, another region of the anterior pituitary gland.
lobes of pituitary gland

Posterior Pituitary Gland

The posterior pituitary gland is comprised of two regions:

  • Infundibulum
  • Pars nervosa

The infundibulum region is further divided into:

  • Median eminence
  • Stem or stalk, a region that connects the median eminence to the pars nervosa region

The median eminence stores hypothalamic releasing and inhibiting factors that affects cells within the pars distalis of the anterior pituitary gland. Example hormones that are produced in the hypothalamus and then travel down the axons to the median eminence include:

  • Gonadotropin releasing hormone (GnRH): stimulates folliculotrophs and luteotrophs to produce FSH and LH, respectively
  • Cortisol releasing factor: stimulates corticotrophs to produce adrenocorticotropic hormone (ACTH)
  • Thyroid releasing hormone: stimulates thyrotrophs to produce thyroid stimulating hormone (TSH)
  • Growth hormone releasing hormone: stimulates somatotrophs to produce growth hormone (GH)

Within the median eminence, these hormones are stored as Herring bodies. These hormones reach the pars distalis region of the anterior pituitary gland via the hypothalamic-hypophyseal (pituitary gland)-portal system. Within the median eminence, these hormones are released into primary capillaries. They will then enter portal venules in the pars tuberalis region of the anterior pituitary gland. They will then enter secondary capillaries located in the par distalis where they are released to stimulate cells within this region.

Note: While the median eminence stores and releases hypothalamic hormones, it does not produce any hormones.

This diagram shows the relationship between the hypothalamus and pituitary gland and the hypothalamic-hypophyseal portal system. Nuclei within the hypothalamus will transmit their hormonal products retrograde down the axons to the median eminence. The hormones then drain into primary capillaries in the median eminence, followed by portal veins in the pars tuberalis, and lastly into the secondary capillaries located in the pars distalis region of the anterior pituitary gland. The hypothalamic hormones will be released into this area to stimulate or inhibit hormonal production by cells of the pars distalis region. The diagram also shows the hypothalamic nuclei, paraventricular nucleus (PVN) and suprapotic nucleus (SON), which are discussed further below.
pituitary gland

Pars nervosa is part of the posterior pituitary gland. Similar to the median eminence, it stores and releases hypothalamic hormones. Hypothalamic hormones stored and released from the pars nervosa include:

  • Vasopressin (anti-diuretic hormone, ADH): stimulates water reabsorption by the collecting duct of the kidney.
  • Oxytocin: stimulates uterine and mammary gland contraction at the time of birth. Contraction of the mammary gland results in 'milk let down'. This hormone also regulates maternal and paternal care.

Both vasopressin and oxytocin are nine amino acids in length, and thus, they have a short half-life in the bloodstream.

This diagram shows the hypothalamic nuclei that produce hypothalamic hormones that travel retrograde down the axons to the pars nervosa. The two primary ones of interest are: paraventricular nucleus (PVN), which contains magnocellular neurons that produce oxytocin and vasopressin. The supraoptic nucleus (SON) also produces vasopressin and a small amount of oxytocin.
PVN and SON

Support cells in the median eminence and pars nervosa are called pituicytes, but they do not produce any hormones.

Anterior Pituitary Gland

The anterior pituitary gland is derived from oral ectoderm. There are divisions to the anterior pituitary gland:

  • Pars tuberalis
  • Pars distalis
  • Pars intermedia

The pars tuberalis contains the portal veins/venules that connect the primary capillary bed in the median eminence to the secondary capillary bed in the pars distalis. Thus, this region is involved in the hypothalamic-hypophyseal portal system. No hormones though are produced from this region.

The pars distalis produces most of the hormones for the anterior pituitary gland. The cells within this region are separated into two types:

  • Chromophiles: that stain either with acidic (acidophils) or basic (basophils) dye.
  • Chromophobes: that do not stain with either acidic or basic dye, such cells may have previously been chromophiles but released their granules (degranulated).

Acidophils include two types of cells:

  • Somatotrophs produce growth hormone (GH, somatotropin) that stimulates somatomedin (insulin-like growth factor)from the liver, which results in muscle proliferation and lengthens the long bone if the growth plate is still evident. GH is inhibited by somatostatin that is produced from various organs, including the stomach and pancreas. Growth hormone releasing hormone from the hypothalamus stimulates GH production.
  • Lactotrophs produce prolactin that stimulates milk secretion. It is inhibited by dopamine from various regions in brain.

Basophils include four types of cells:

  • Folliculotrophs
  • Luteotrophs
  • Corticotrophs
  • Thyrotrophs

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