Pituitary Gland Flashcards
What is a hormone?
A messenger molecule carried from the organ where they are produced to the organ which they affect by means of the bloodstream.
What are the two types of hormone molecules and differences in synthesis, storage and receptors?
The two types are peptide and steroid. A peptide is synthesised as a prohormone which requires further processing such as cleavage to activate. It is stored in vesicles and undergoes regulatory secretion. It binds receptors on the cell membrane and transduces signals using 2nd messenger systems. Steroid hormones are synthesised in a series of reactions from cholesterol and released immediately through constitutive secretion. They bind to intracellular receptors to change gene expression directly.
What neurons regulate anterior pituitary function?
Hypothalamic parvocellular neurons regulate anterior pituitary function. They are short and terminate on median eminence. Release hypothalamic releasing/inhibitory factors into capillary plexus in median eminence. These hypothalamic regulatory factors carried by portal circulation to anterior pituitary.
Describe the adenohypophysis
This is the anterior pituitary and anatomically distinct from the hypothalamus. It is made up of endocrine cells such as: Somatotrophs, Lactotrophs, Corticotroph, Thyrotrophs and Gonadotrophs. The anterior pituitary is regulated by hypothalamic releasing/inhibiting factors via the hypophyseal-pituitary portal system.
How does the hypothalamo-pituitary portal system work?
- Axon terminals of hypothalamic neurosecretory cells release hormones (RHs and IHs) into the hypothalamo-pituitary portal system.
- The RHs and IHs travel in the portal system to the anterior pituitary.
- The RHs and IHs stimulate or inhibit the release of hormones (black dots) from anterior pituitary cells.
- Anterior pituitary hormones leave the gland via the blood.
How is thyroid hormone production regulated?
- Axon terminals of hypothalamic neurosecretory cells release Thyrotrophin Releasing Hormone (TRH) into hypothalamo-hypophysial portal system
- TRH travels in the portal system to the anterior pituitary
- TRH stimulates the release of Thyroid Stimulating Hormone (Thyrotrophin) from anterior pituitary thyrotrophs
- TSH leaves the gland via the blood to travel to the thyroid gland to stimulate thyroid hormone release (thyroxine)
What are the different anterior pituitary cells & their hormones?
Somatotrophs: Growth hormone (somatotrophin)
Lactotrophs: Prolactin
Corticotroph: Adrenocorticotrophic hormone
(ACTH, corticotrophin)
Thyrotrophs: Thyroid stimulating hormone (TSH, Thyrotrophin)
Gonadotrophs: Luteinising hormone (LH)
Follicle stimulating hormone (FSH)
How does hypothalamo-pituitary regulation work for the different hormones?
Growth hormone is stimulated and released by growth hormone-releasing hormone and inhibited by somatostatin.
Prolactin is inhibited by dopamine.
TSH is released by thyrotrophin releasing hormone.
LH and FSH are released by gonadotrophin-releasing hormone.
ACTH is released by the corticotrophin-releasing hormone.
Where are receptors for the hormones produced by the anterior pituitary gland?
Growth hormone has receptors across general body tissues, especially the liver. Prolactin receptors located in the breasts, esp in lactating women. Thyrotrophin receptors found in thyroid gland. Gonadotrophin receptors found in testes and ovaries. Adrenocorticotrophic hormone receptors found in adrenal cortex.
What causes bitemporal hemianopia?
Fibres from the nasal (medial) retinae cross at the optic chiasm. Compression of the optic chiasm by a pituitary tumour/suprasellar tumour prevents transmission of sensory information from lateral visual fields to the occipital lobe.
Describe neuroendocrine reflex pathway for milk production
- Mechanical stimulation of nipple & surrounding area activates afferent pathways.
- Afferent signals integrated in the hypothalamus and inhibit dopamine release from dopaminergic neurons.
- Less dopamine in the hypothalamic-pituitary portal system causes less inhibition of anterior pituitary lactotrophs.
- Increased plasma prolactin increases milk secretion in mammary glands.
What are the 2 mechanisms of growth hormone action?
First method is the direct action of somatotrophin produced by the anterior pituitary glands on body tissues, such as bones and muscles. These have somatotrophin receptors on the surface which the peptide hormone can bind to and stimulate growth. The second pathway involves somatotrophin stimulating the liver to produce IGF-1 or Insulin-like Growth Factor 1 (Somatomedin). Muscles and bones also have IGF-1 receptors which then stimulates growth too. IGF-2 is present mainly in the developing fetus.
How can an excess of somatotrophin manifest?
During puberty, the epiphyseal plates aren’t fused and hence an excess of somatotrophin results in pathologic growth of height. However, post-puberty it causes acromegaly which features many other symptoms but not an increase in height since the epiphyseal plates are fused so growth not possible in adulthood.
What are the symptoms of acromegaly?
Coarsening of facial features, Macroglossia (large tongue), Prominent nose, Large jaw - prognathism, Increased hand and feet size, Sweatiness, Headache, Can usually see a suprasellar pituitary gland stretching the optic chiasm.
CMNPLSH
Describe structure of neurohypophysis
The neurohypophysis is the posterior pituitary gland. Anatomically continuous with the hypothalamus. Has long hypothalamic magnocellular neurons running through it which originate in the supraoptic (AVP) and paraventricular (oxytocin) nuclei. These neurons pass from the nuclei through the stalk and finally into the posterior pituitary from where it is released into the bloodstream. Hormones made in the neuronal soma.
