Endocrinology: Hypothalamus and Pituitary Flashcards
Location of hypothalamus
- Central location within the brain
- Sits in a cavity: bone = sella turcica
- Located a few cm behind the eyes
- Pituitary gland is essentially attached to hypothalamus through pituitary stalk
Function of the hypothalamus
- Link between the nervous and endocrine system: receives lots of information from higher brain centres (electrical signals = nervous system) and secretes hormones (endocrine) to regulate the anterior pituitary
- Has many neural connections to other areas of the brain
- Major control centre for autonomic-mediated functions
What does the hypothalamus regulate?
o Hormone production and release from pituitary; in turn influences thyroid gland (metabolism), adrenal gland (stress), gonads (reproduction)
o Contraction of uterus during labour
o Milk production and release
o Kidneys (water retention, sodium, and potassium balance
o Growth and development
o Biological rhythms
o Body temperature
o Circulation
o Food-water intake
What are hypothalamic control centres?
- Hypothalamic control centres are clusters of neurons. Each cluster is identified as a nucleus, e.g., paraventricular nucleus, supraoptic nucleus
Link between hypothalamus and pituitary
- Extensive neural connections exist between the hypothalamus and the rest of the brain.
- Hypothalamus is surrounded by the limbic lobe (associated with emotions)
- The pituitary is mainly controlled by the hypothalamus
- Physically connected via pituitary stalk/ infundibulum
- Many hormones released from the pituitary influence secretion from the other endocrine glands
Development of the pituitary
- Anterior pituitary: up growth from oral cavity; has vascular connection with the hypothalamus
- Posterior pituitary: down growth from the brain; has neural connection with the hypothalamus
- Two glands distinct from each other, front; secretary epithelial cells, up growth from root of mouth, back: neural tissues, down growth
- As (front) moves upwards, becomes physically connected to the other portion of the gland; connections develop
Histology of posterior pituitary
- Nervous tissue: axons of neurosecretory cells
- Pituicytes are a kind of glial cells: support
- Capillaries are fenestrated: no blood-brain-barrier in posterior pituitary
Histology of anterior pituitary
- Chromophils (cells which take up stain; actively secreting hormones)
o Acidophils (somatotrophs ~40-50%, lactotrophs (aka mammotrophs ~10-15%)
o Basophils (corticotrophs ~15-20%, thyrotrophs ~3-5%, gonadotrophs ~10-15%) - Chromophobes (resting/ degranulated chromophils)
Neural connections between hypothalamus and pituitary
- Paraventricular nucleus and supraoptic nucleus of the hypothalamus: terminal endings in the posterior pituitary
- Hypothalamus communicates with pituitary gland via neural connections involving axonal projections
- Specialised neurons in the hypothalamus produce and release specific regulatory hormones (hypothalamic releasing/ inhibiting hormones)
- These hormones travel along axons through a tract
- Axons of hypothalamic neurons terminate in the median eminence; at the base of hypothalamus, where they release their hormones into the blood vessels of the hypothalamic-pituitary portal systems
Vascular connections between the hypothalamus and pituitary
- Anterior lobe of pituitary is larger, more vascular, glandular epithelial cells, anterior posterior hormones synthesised
- The hypothalamic-pituitary portal system is a complex network of blood vessels that allows direct communication between the hypothalamus and the anterior pituitary gland
- Consists of a network of capillaries in the median eminence, which receive hypothalamic hormones released by neurons
- Capillaries coalesce to form portal veins, which carry the hypothalamic hormones to a second capillary bed in the anterior pituitary gland
- In the anterior pituitary gland, the hypothalamic hormones act on specific cells, known as endocrine cells or pituitary cells, to regulate the secretion of pituitary hormones
- Anterior pituitary gland then releases its own hormones into systemic circulation, where they exert their effects on target organs throughout the body
Posterior pituitary role
- An ‘extension’ of the hypothalamus
- Doesn’t synthesis hormones
- Stores hormones formed in the hypothalamic neurons
- Secretes hormones directly into the blood
- Releases 2 neurohormones: antidiuretic hormone (ADH aka vasopressin), oxytocin
Role of anterior pituitary
- Short axon neurons synthesise hypophysiotropic hormones and release them into capillaries of the hypothalamic-hypophyseal portal system
- Portal vessels carry hypophysiotropic hormones to the anterior pituitary
- Endocrine cells of the anterior pituitary are controlled by hypophysiotropic hormones
- They secrete anterior pituitary hormones into systemic blood
Another name for posterior pituitary
Neurohypophysis
Another name for anterior pituitary
Adenohypophysis
Hypophysiotropic hormones
- Top of the hormonal hierarchy
- Peptide neurohormones
- Released in very small quantities
- Two types: releasing (stimulates release) and inhibiting (suppresses release) hormones
- This is a neuroendocrine system (fast and specific)
- Part of a three hormone chain system with multiple feedback loops (short and long loop)
- Evolution has conserved use of molecules, e.g., ‘prolatin inhibiting hormone’ is dopamine
Posterior pituitary hormones
ADH (vasopressin) and oxytocin
These are peptide hormones; 9 amino acids each
ADH
o Produced by neurosecretory cells with cell bodies in the hypothalamic nuclei (SON and PVN)
o Release is stimulated by increase osmolarity, decreased blood pressure, increased stressors (adrenergic)
o Acts on kidneys: V2 receptor activation, increase cAMP, increase insertion of aquaporin in luminal membrane of collecting tubules –> increased water reabsorption
o Acts on blood vessels (including renal blood vessels): V1 receptor activation, increase IP3/DAG, increase calcium, increase vascular tone –> increase vasoconstriction
o Acts on the anterior pituitary gland: increase secretion of adrenocorticotropic hormone (ACTH), leads to increase aldosterone secretion –> increase conservation of sodium and water
Oxytocin
o Contraction of the wall of the uterus is promoted by a hormone called oxytocin
o Oxytocin is released from the posterior pituitary due to the activation of hypothalamic neurons
o These hypothalamic neurons are activated by parts of the brain which receive neural input from the stretched cervix (via sensory afferent neurons)
o Positive feedback system
o Promotes ejection (smooth muscle contraction) of milk from the mammary glands
o Involved in other behaviours (e.g., bonding attachment, coping with fearful situations): oxytocin reduce brain activity associated with fearful response
o Note that the hypothalamus is being regulated by very complex neural processes
o Role in male reproduction and ejaculation
Cells in the anterior pituitary that secrete hormones
thyrotrophs, cortiotrophs, lactotrophs, gonadotrophs, sommatotrophs
these are endocrine cells
What hormones do each of the endocrine cells in the anterior pituitary secrete? What are the percentages of each cell in the anterior pituitary?
- Thyrotroph (<10%)
o TSH –> thyroid gland –> thyroid hormone (T3 and T4) –> metabolic rate - Cortiotroph (15-20%)
o ACTH –> adrenal cortex –> cortisol –> metabolic actions; stress response - Lactotroph (10-25%)
o Prolactin –> mammary glands –> breast growth and milk secretion - Gonadotroph (10-15%)
o LH –> gonads (ovaries in females, testes in males) –> sex hormone secretion (estrogen and progesterone in females, testosterone in males)
o FSH –> gonads –> gamete production (ova in females, sperm in males) - Somatotroph (50%)
o Growth hormone –> liver –> somatomedins –> bone/ soft tissues –> growth
o Growth hormone –> many tissues –> metabolic actions
What hormones are from the thyrotrophs? What are their effect?
TSH –> thyroid gland –> thyroid hormone (T3 and T4) –> metabolic rate
What hormones are from the corticotrophs? What are their effect?
ACTH –> adrenal cortex –> cortisol –> metabolic actions; stress response
What hormones are from the lactorophs? What are their effect?
Prolactin –> mammary glands –> breast growth and milk secretion
What hormones are from the gonadotrophs? What are their effect?
LH –> gonads (ovaries in females, testes in males) –> sex hormone secretion (estrogen and progesterone, or testosterone)
FSH –> gonads –> gamete production (ova in females, sperm in males)
What hormones are from the somatotrophs? What are their effect?
Growth hormone –> liver –> somatomedins –> bone/ soft tissues –> growth
Growth hormone –> many tissues –> metabolic actions
