3. The Nuts and Bolts of the Endocrine System: Anatomy and Histology Roadshow Flashcards
What is the endocrine system?
- system of hormone-secreting glands
- hormones are mediator molecules
what is the endocrine system essential for?
- normal growth and development
- adaptation to internal and external environment
- contribute to the process of sexual reproduction
What two systems interact for hormone production/action and where do they do this?
- Nervous system (neuroendocrine)
- endocrine system
- interact at hypothalamus (major controlling centre of the ANS) e.g. temperature circadian rhythms, hunger
Describe the communication of the endocrine system
- communication via the release of hormones
- mediator molecules are hormones
- much slower than nervous system (seconds/minutes/day)
- act anywhere in the body via the circulatory system
- each hormone elicits a response from any cell carrying a receptor sensitive for that hormone
What are the properties of endocrine cells?
- Ductless
- Hormone is relased to interstitial space => blood stream or lymphatic system
- highly vascularised (if cut bleeds profusely)
What factors determine how hormones affect the metabolism of other cells?
- number of molecules available
- number of receptors available
- affinity of hormone and receptor
Chemical classification of hormones
Steroid - oestradial; testosterone
protein/peptide - ADH, oxytocin, insulin
Amine - adrenaline, noradrenaline, dopamine
NO- both neurotransmitter and hormone
Embryonic source of different hormones
steroid hormones (ovaries, testes, adrenal cortex) - embryonic mesodermal tissue
protein/peptide:
pituitary - ectodermal tissue of oral cavity
Thyroid; parathyroid;pancreas - ectodermal origin
scattered endocrine cells in epithelium of GI tract and lunds - endoderm
Amine: thyroid; adrenal medulla - ectodermal origin
Endocrine sources
Discrete glands (seperate/distinct) = hypophysis, thyroid, parathyroid, adrenal Larger glands with exocrine and endocrine function= kidneys, ovaries/testes, placents Diffuse neuroendocrine system= scattered nerve cells with endocrine function
Describe the hypophysis
- pituitary gland - undergrowth
- compound gland suspended fromthe hypothalamus by infundibulum
- located in the sella turcica in the sphenoid bone
- master endocrine gland, produces several hormones that influence activity of other endocrine glands
hypophysis gross anatomy
size of a pea
two divisions:
adenohypophysis - glandular part, outpouching of ectoderm of oral cavity
neurohypophysis - neural part, downgrowth from diencephalon of brain
Describe the two divisions of the hypophysis
adenohypophysis:
- glandular part
- outpouching of ectoderm of oral cavity
- produces FSH, LH, TSH,
neurohypophysis:
- neural part, downgrowth from diencephalon of brain
- produces oxytocin, ADH
Hypophysis blood supply
Supplied by two blood vessels which are branches of the internal carotid:
-superior hypophyseal, supplies the median eminence and upper part of stalk
-inferior hypophyseal, supplies neurohypophysis and lower part of stalk
Arteries supplying median eminence and stalk end as capillary plexuses
hypophysis veins
Capillary plexus in median eminence and stalk ending are drained by portal veins
these portal veins pass to anterior lobe of pituitary
veins then form a secondary capillary plexus
this setup provides a route for neurosecretory substances released from the hypothalamus to also reach the anterior lobe
hypophysis control
Signals from the hypothalamus are responsible for hormone release from both adenohypophysis and neurohyphosis
describe thyroid gland
- bilobed gland in the neck , 4 x 2 cm in size, 30g
- 2 lateral lobes, connected by an isthmus that courses anterior to the trachea
- from just below oblique line of thyroid cartilage to 5/6 tracheal ring; immediately anterior to trachea
Thyroid gland development and role
- derived from endoderm, develops as growth from pharynx floor, near base of tongue
- under control from hypothalamus (thyrotropin-releasing hormone) and hypophysis (thyroid stimulating hormone)
- essential role in regulating tissue metabolism, growth and development via T3 (trioiodothyronine) and T4 (thyroxine)
Thyroid gland blood course
blood supply:
-superior thyroid (from external carotid)
inferior thyroid (from subclavian)
blood drainage (veins):
-from an extensive plexus into the internal jugular vein
-Brachiocephalic vein (care in surgery e.g. cricothyrotomy and recurrent nerve from vagus)
Describe the parathyroid glands
- 4 parathyroid glands, 2 superior and 2 inferior, embedded in capsule of thyroid, posterior aspect
- very small, about size of a grain of rice, 40mg
-develop from cells originating from 3rd/4th pharyngeal pouches, migrate caudally with thymus. Inferior parathyroids travel further to lie lower compared to superior parathyroids
- role: secrete parathyroid hormone which regulates calcium and phosphate levels within homeostasis
- must be preserved during thyroidectomy, if removed blood calcium levels fall -> muscles, including resp. and laryngeal, go into tetanic contraction -> death
Parathyroid glands - VAN
Artery: primarily inferior thyroid (also superior)
Vein: superior, middle & inferior thyroid
Nerve: from middle and inferior cervical ganglions
Adrenal glands
- hormones released in response to stress
- location: superior pole of kidneys T12; retroperitoneal
- coronal cut: outer capsule, cortex and medulla
-shape: pyramidal (R), semilunar (L)
2-3 x 1 cm; 3.5-5g
- cortex and medulla differ very much in origin, structure and function
- origin: cortex from mesoderm (mesothelium), medulla from neural crest cells
Cortex of adrenal glands
adrenal cortex secretes corticosteroids
cells subjacent to capsule secrete mineralocorticoids e.g. aldosterone, function to maintain electrolyte balance
cells in the deeper part of the cortex secrete glucocorticoids e.g. hydrocortisone affect metabolism and electrolyte balance
Cortex also produces adrenal androgens e.g. testosterone
Medulla of adrenal glands
secretory cells of medulla - chromaffin cells - develop from same embryonic tissue as sympathetic ganglia
medulla is regarded as a modified sympathetic ganglion (ANS)
medulla cells secrete adrenaline and smaller amounts of noradrenaline i.e. catecholamines
fight-or-flight response: affects heart rate, blood pressure and smooth muscle of viscera
Adrenal glands - blood supply
Supplied by:
- superior (from inferior phrenic)
- middle (from aorta)
- inferior suprarenal arteries (from renal)
Arteries branch before entering capsule and the rami supply the cortex and medulla of gland
Cortical arteries give rise to subcapsular plexus and in turn to cortical sinusoids that distributes blood to cortical cells
Adrenal glands - blood course
medullary arteries pass through cortex to supply the medulla, so the medulla receives blood from two sources
so hormones produced by cortical cells influence activity of cells in the medulla e.g. cortisol mediates noradrenaline conversion to adrenaline
venous drainage: adrenal veins, drain into IVC (R) or renal vein (L)
Adrenal glands - activity
Activity of adrenal glands is under control of the:
- hypothalamus e.g. ACTH(Adrenocorticotrophic hormone) release from hypophysis (stimulates secretion of corticosteroids
- Sympathetic division of ANS: coeliac plexus and splanchnic nerves
Adrenal gland medulla is under direct control from ANS (sympathetic) leads to fast response
-release is initiated by impulses from preganglionic sympathetic fibres that end in the chromaffin cells of the medulla. chromaffin cells release catecholamines into adjacent circulation
