Pituitary Gland – Physiology Lecture ppt slideshow

Learning Objectives

• By the end of lectures, students should be able to:
• State the location of pituitary gland and hypothalamus
• Enumerate the hormones secreted by pituitary gland
• Enumerate the hormones/factors secreted by hypothalamus
• Summarize the functions of hormones of pituitary gland
• Summarize the functions of hormones/factors secreted by
  hypothalamus
• Explain the interrelationships of functions of the hypothalamus
  and the posterior and anterior lobes of the pituitary gland
• Highlight the effects of lesions of pituitary gland

Key Terms

• Pituitary Gland: An endocrine gland, about the size of a pea, that sits
  in a small, bony cavity at the base of the brain whose secretions
  control the other endocrine glands and influence growth,
  metabolism, and maturation.
• Hypothalamus: A region of the forebrain located below the thalamus,
  forming the basal portion of the diencephalon, that regulates body
  temperature, some metabolic processes, and governs the autonomic
  nervous system.
• Hypophyseal Portal System: The system of blood vessels that link the
  hypothalamus and the anterior pituitary in the brain.

Physiological Anatomy

• Master gland
• Pituitary gland or hypophysis, an endocrine gland about pea size and
  weighing 0.5 g
• Protrusion off the bottom of the hypothalamus at the base of the
  brain, and rests in a small, bony cavity (sella turcica) covered by a
  dural fold (diaphragm sellae)

Physiological Anatomy

• The pituitary fossa, in which the pituitary gland sits, is situated in the
  sphenoid bone in the middle cranial fossa at the base of the brain.
• The pituitary gland secretes hormones regulating homeostasis,
  including tropic hormones that stimulate other endocrine glands.
• It is functionally connected to the hypothalamus by the median
  eminence via a small tube called the Pituitary Stalk.

Physiological Anatomy

Physiologically, the pituitary gland is divided into two portions

• Anterior pituitary– adenohypophysis
• Posterior pituitary– neurohypophysis
• Small, avascular zone is present in between called pars intermedia
• Embryologically
• Anterior pituitary originates from rathke’s pouch, an embryonic
  invagination of the pharyngeal epithelium– epitheloid nature
• Posterior pituitary from neural tissue outgrowth from
  hypothalamus, justifies large numbers of glial cells in this gland

Anterior Pituitary Hormones

• Secretes five endocrine hormones from different types of epithelial
  endocrine cells.
• Release of anterior pituitary hormones is regulated by hypothalamic
  hormones (releasing or inhibitory)

1. Growth hormone promotes growth of the entire body by protein
   formation, cell multiplication, and cell differentiation.
2. Adrenocorticotropin (corticotropin) controls secretions of
   adrenocortical hormones– metabolism of glucose, proteins, and
   fats.

3. Thyroid-stimulating hormone (thyrotropin) controls the rate of
   secretion of thyroxine and tri-iodothyronine by the thyroid gland–
   regulates most intracellular chemical reactions in the body.
4. Prolactin promotes mammary gland development and milk
   production and influences the secretion of estrogen/progesterone.
5. Two separate gonadotropic hormones, FSH & LH, control growth of
   the ovaries and testes and control their hormonal and reproductive
   activities respectively.
6. Beta-endorphin is a polypeptide that effects the opioid receptor,
   whose effects include the inhibition of the perception of pain.

• Posterior pituitary is neural in origin
• The hormones are stored in neurosecretory vesicles (Herring bodies)
  before being released by the posterior pituitary into the bloodstream.
• Two hormones stored by posterior pituitary and are secreted by
  hypothalamus.

1. Antidiuretic hormone (also called vasopressin) secreted by
   paraventricular nucleus– regulates water excretion, helping to control
   the concentration of water in the body fluids
2. Oxytocin secreted by supraoptic nucleus– express milk from the glands of
   breast during suckling and helps in parturition at the end of gestation

Hypothalamic Controls

• Pituitary secretions are controlled by hormonal or nervous signals
  from hypothalamus
• Secretion of anterior pituitary by hormonal signals
• Secretion of posterior pituitary by nerve signals
• Hormones from hypothalamus can be releasing as well as inhibitory
• Hypothalamo-hypophyseal portal system transport these hormones
  to pituitary gland to control its secretions

• Hypothalamic Hormones– secreted into Median Eminence

1. Thyrotropin-releasing hormone (TRH), which causes release of TSH
2. Corticotropin-releasing hormone (CRH), which causes release of
   ACTH
3. Growth hormone–releasing hormone (GHRH), which causes release
   of GH, and growth hormone inhibitory hormone (GHIH), also called
   somatostatin, which inhibits release of GH

4. Gonadotropin-releasing hormone (GnRH), which causes release of
   the two gonadotropic hormones, LH & FSH
5. Prolactin inhibitory hormone (PIH), which causes inhibition of
   prolactin secretion
6. Antidiuretic hormone (also called vasopressin) secreted by
   paraventricular nucleus– regulates water excretion, helping to
   control the concentration of water in the body fluids
7. Oxytocin secreted by supraoptic nucleus– express milk from the
   glands of breast during suckling and helps in parturition at the end
   of gestation

Posterior Pituitary Gland

• Composed mainly of cells
  called ‘Pituicytes’, which
  act as packing & supporting
  cells
• Stores & releases hormones into
  the close capillaries.
• These hormones are produced in
  hypothalamus.

Posterior pituitary gland releases 2 hormones:

1. Antidiuretic hormone (ADH), or arginine vasopressin

(AVP).

2. Oxytocin

• Both hormones are produced in hypothalamic nuclei:
• paraventricular nucleus → (ADH + 1/6 oxytocin)
• Supraoptic nucleus → (Oxytocin + 1/6 ADH)

1. ADH (vasopressin):

• Antidiuretic hormone (ADH), or arginine vasopressin
  (AVP), is produced mainly in Supra Optic Nucleus of
  hypothalamus.

ADH activates (2) second messenger systems:

1. cAMP
2. IP3

/Ca2+

Actions of ADH

1.  water reabsorption (retention) by distal tubules &
   collecting ducts of the kidneys → decrease osmotic
   pressure of the blood.

• Regulated by V2

receptors, through the action of cAMP.

2. Contraction of vascular smooth muscles → generalized
   vasoconstriction.

• Regulated by V1

receptors, through the action of IP3

/Ca2+

Control of ADH Release

 in osmotic pressure of the ECF ( in plasma osmolality), as in
dehydration which will stimulate osmoreceptors in the
hypothalamus →  ADH.

 blood volume ( 10%) → stimulate mechanoreceptors in the great
arteries (aorta & carotids) & right atrium →  ADH.

Hyposecretion of ADH

• Lack of ADH → Diabetes insipidus.
  2 types of DI:
  a. Neurogenic (central or cranial)
• Problem in Hypothalamus or Post pituitary gland
• Could be primary or secondary
  b. Nephrogenic (renal)
• Resistance of V2

receptors in collecting ducts of the kidney

Diabetes Insipidus

Symptoms:

Polyurea  20 L/day (N  1.5 L/d),
Polydypsia,
 specific gravity of urine (diluted urine),
 plasma osmolarity

Treatment

• ADH will be given in neurogenic Diabetes insipidus
• No ADH will be given in nephrogenic Diabetes insipidus

Hypersecretion of ADH

• ADH, ‘Schwartz-Bartter Syndrome

• Adenoma, Ectopic Kidney
• Bronchial carcinoma
  Signs & Symptoms:
• Hyponatremia i.e. [Na+]extracellular to 110 mM(140mM) resulting
  in
• Mental confusion.
• Coma
• Death

Posterior Pituitary Gland

2. Oxytocin:

• Produced mainly in the paraventricular nucleus of the
  hypothalamus

Oxytocin

• Oxytocin has a major contribution in parturition

1. In a hypophysectomized animal, the duration of labor is
   prolonged
2. Amount of oxytocin in plasma increases during labor– last
   stage
3. Stimulation of the cervix in a pregnant animal send
   nervous signals hypothalamus– increased secretion of
   oxytocin

Action of Oxytocin

1. Contraction of smooth muscles of the uterus → enhances
   labor.
2. Contraction of mammary gland myoepithelial cells of the
   alveoli & the ducts → Ejection of milk as a reflex in
   lactating women
3. In men →  ejaculation.

• Oxytocin is concerned with releasing or ejection of milk,
  while prolactin is concerned with synthesis & production of
  milk

Oxytocin– Role in Milk Ejection

• Suckling stimulus on nipple
• Sensory signals transmitted to hypothalamus
• Release of oxytocin by posterior pituitary gland
• Reaches the breast by blood– contraction of
  myoepithelial cells
• Milk begins to flow after suckling
• Milk letdown or milk ejection

Sheehan Syndrome

• Sheehan syndrome is a condition that happens when the pituitary

gland is damaged during childbirth. It’s caused by excess blood loss

(hemorrhage) or extremely low blood pressure during or after labor. A

lack of blood deprives the pituitary of the oxygen it needs to work

properly.

Pituitary Adenomas

• Pituitary adenomas arise from the pituitary gland account for 15% of

primary brain tumors and are the third most common intracranial

tumor after meningiomas and gliomas.

• The great majority (over 90%) of pituitary adenomas are benign slow

growing tumors, approximately 5-10% are somewhat more aggressive

(atypical) and less than 1% qualify as pituitary carcinoma.

• Symptoms and signs of a mass lesion: headaches, altered appetite,

thirst, visual field defects—particularly bitemporal hemianopia

• Treatment– surgical removal and radiotherapy