3. The Nuts and Bolts of the Endocrine System: Anatomy and Histology Roadshow

Question 1

What is the endocrine system?

Answer 1

• system of hormone-secreting glands

- hormones are mediator molecules

Question 2

what is the endocrine system essential for?

Answer 2

• normal growth and development
• adaptation to internal and external environment
• contribute to the process of sexual reproduction

Question 3

What two systems interact for hormone production/action and where do they do this?

Answer 3

• Nervous system (neuroendocrine)
• endocrine system
• interact at hypothalamus (major controlling centre of the ANS) e.g. temperature circadian rhythms, hunger

Question 4

Describe the communication of the endocrine system

Answer 4

• 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

Question 5

What are the properties of endocrine cells?

Answer 5

• Ductless
• Hormone is relased to interstitial space => blood stream or lymphatic system
• highly vascularised (if cut bleeds profusely)

Question 6

What factors determine how hormones affect the metabolism of other cells?

Answer 6

• number of molecules available
• number of receptors available
• affinity of hormone and receptor

Question 7

Chemical classification of hormones

Answer 7

Steroid - oestradial; testosterone
protein/peptide - ADH, oxytocin, insulin
Amine - adrenaline, noradrenaline, dopamine
NO- both neurotransmitter and hormone

Question 8

Embryonic source of different hormones

Answer 8

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

Question 9

Endocrine sources

Answer 9

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

Question 10

Describe the hypophysis

Answer 10

• 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

Question 11

hypophysis gross anatomy

Answer 11

size of a pea
two divisions:
adenohypophysis - glandular part, outpouching of ectoderm of oral cavity
neurohypophysis - neural part, downgrowth from diencephalon of brain

Question 12

Describe the two divisions of the hypophysis

Answer 12

adenohypophysis:

• glandular part
• outpouching of ectoderm of oral cavity
• produces FSH, LH, TSH,

neurohypophysis:

• neural part, downgrowth from diencephalon of brain
• produces oxytocin, ADH

Question 13

Hypophysis blood supply

Answer 13

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

Question 14

hypophysis veins

Answer 14

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

Question 15

hypophysis control

Answer 15

Signals from the hypothalamus are responsible for hormone release from both adenohypophysis and neurohyphosis

Question 16

describe thyroid gland

Answer 16

• 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

Question 17

Thyroid gland development and role

Answer 17

• 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)

Question 18

Thyroid gland blood course

Answer 18

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)

Question 19

Describe the parathyroid glands

Answer 19

• 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

Question 20

Parathyroid glands - VAN

Answer 20

Artery: primarily inferior thyroid (also superior)
Vein: superior, middle & inferior thyroid
Nerve: from middle and inferior cervical ganglions

Question 21

Adrenal glands

Answer 21

• 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

Question 22

Cortex of adrenal glands

Answer 22

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

Question 23

Medulla of adrenal glands

Answer 23

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

Question 24

Adrenal glands - blood supply

Answer 24

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

Question 25

Adrenal glands - blood course

Answer 25

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)

Question 26

Adrenal glands - activity

Answer 26

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