Endocrine Glands Of the Neck--Chiaia Flashcards
Endocrine glands
Glands that facilitate intercellular communication between the cells of each organ and between different organs
Why are endocrine glands necessary
They are necessary to maintain:
1) homeostasis of nutrients and vital substances in the cellular environment
2) coordination of growth/development
3) adaptation to environmental stresses
What are the two communication systems to meet the needs of intercellular communication
- the nervous system (fast acting, specific)
- the endocrine system (slow acting, more pervasive less specific)
Hormone
- organic, secreted in small amounts by endocrine cells
- exerts effect on distant tissues
- operates at very low concentrations
Sites of action of hormones
1) circulating (distant)
2) local (paracrine, neighbor cells)
3) on self or same class of cells (autocrine)
Functions of the thyroid gland
1) regulates tissue metabolism
2) regulates tissue growth (during development)
3) 2nd regulation of plasma Ca++
Organization of thyroid gland
- two lateral lobes (right, left)
- lobes connected by narrow isthmus (over 2nd and 3rd tracheal rings)
- medial pyramidal lobe present during development
Embryological development of thyroid
- foramen cecum of tongue
- lowered by thyroglossal duct (disappears)
- functional tissue is enclosed in dense irregular CT
Follicles of the thyroid
- these are the structural and functional units
- single layer of simple cuboidal epithelial cells (follicular epithelial cells)
- cells make and release thyroid hormone into lumen where it is stored as a precursor (colloid)
What envelopes the follicles of the thyroid lobules
A dense basket-like mesh work of fenestrated capillaries
-this is how the secretions end up in the bloodstream
What are the two classes of cells that make up the thyroid follicles
1) follicular cells (simple cuboidal epithelial cells): these are the principal cells
2) parafollicular cells a.k.a “c-cells”
Hyperactive follicular cells
- take cuboidal-columnar shape
- colloid is reduced and “scalloped border” forms
Hypoactive follicular cells
- epithelial cells are inactive and have a squamous shape
- colloid is increased in follicle/lumen
What is the major function of the thyroid gland
The synthesis and secretion of thyroid hormone
What is thyroglobulin
It is a precursor glycoprotein a.k.a colloid from which thyroid hormone is made
Synthesis of thyroid hormone
1) thyroglobulin made in the rER of follicular cells and processed in the golgi
2) tyrosine residues are added to thyroglobulin and compound is sent to apical surface of cell where it is exocytosed
3) thyroperoxidase is made by follicular cells and secreted into lumen close to microvilli
4) it catalyses the iodination of thyroglobulin in the lumen
5) T3 and T4 (thyroxine) are made
Reuptake of iodinated thyroglobulin
- occurs at the apical surface to form colliod droplets (scalloped border)
- lysosomal digestion inside cell makes free T3 and T4
- **MIT and DIT serve as iodine carriers
T3 and T4 out of cell
-they diffuse out of the basal surface and enter capillary circulation where they bind to plasma proteins on way to target tissue
Anterior pituitary
- TSH released by anterior pit activates the thyroid gland
- binds to receptors on basal surface causing hypertrophy and hyperplasia of follicular cells, increased production and iodination of thyroglobulin, increased reuptake, increased secretion
Thyroid hormone regulation
1) Low blood level of T3/T4 or low metabolic rate cases release of TRH from hypothalamus
2) TRH goes to ant pit to release TSH
3) TSH released into blood stimulate follicular cells
4) T3/T4 released into blood onto target tissue
5) T3/T4 increased levels feedback to hypothalamus
Function of thyroid hormone
It is primarily concerned with cellular metabolism in all body tissues
1) metabolic effects
2) growth promoting effects
Metabolic effects of thyroid hormone
1) calorigenisis: BMR; increased energy production/oxygen consumption
2) regulation of water and ion transport
3) regulation of protein, fat and carbs metabolism
Growth promoting effects of thyroid hormone
1) acts synergistically with GH to promote normal skeletal development
2) controls moving and metamorphosis in lower vertebrates
BMR
-BMR is measure of energy expenditure and cellular metabolism at rest (60%-70%)
1) BMR increases with exercise and fitness
2) decreases with dieting and age
Hypothyroid
1) mentally and physically sluggish
2) low BMR
3) mental retardation
4) decreased glucose absorption in GI
5) weak heartbeat
Hyperthyroid
1) restless, irritable, anxious
2) elevate BMR
3) mentally alert
4) increased glucose absorption in GI tract
5) tachycardia
Cretinism
Hypothyroidism that occurs during development
Myxedema
Hypothyroidism that occurs during adulthood
-lethargy along with mental deficiency
Hashimoto’s dz
Hypothyroidism as a result of auto-immmune disorder, they kill follicular cells
What is a goiter
Enlargement of the thyroid from hypertrophy and hyperplasia of follicular cells
Can result from:
1) iodine deficiency
2) graves dz
Iodine deficiency
- less iodination so decreased T3/T4 output (hypothyroidism)
- less T3/T4 feedback so increased TRH and TSH resulting in compensatory follicular cell hypertrophy (parenchymatous goiter)
Graves dz
- plasma igG “thyroid stimulating immunoglobulin” causes overstimulation of follicular cells (hyperthyroidism)
- goiter and protruded eyes (exopthalmos)
Symptoms of graves dz
- anxiety irritability
- fine tremor of hands
- heat sensitivity
- weight loss
- goiter/exopthalamos
- rapid/irregular heartbeat
- increased T3/T4
Parafollicular cells (c-cells)
Less numerous and are of neural crest origin
- they are larger and lighter stained and are usually between follicles but are insulated from the colloid
- secrete calcitonin (not under pit control)
Calcitonin
- lowers plasma Ca++
1) inhibits osteoclasts activity (decrease bone resorption and liberation of Ca++)
2) promotes excretion of Ca++ from the kidney
Parathyroid glands
Maintenance of appropriate plasma concentration of Ca++
- 4 masses behind the thyroid
- 2 superior(thyroid)/ 2 inferior (thymus)
Parathyroid cells
- 2 types of cells
1) chief cells (principal)
2) oxyphil cells
Chief cells
1) Secrete parathyroid hormone
- its half life is only 18 min
2) Light cell (inactive)
- Dark cell (active)
3) do not store a lot of PTH
Function of PTH
-in response to low plasma Ca++, PTH released into circulation to increase Ca++
1) increases osteoclasts stimulating factor
2) promote Ca++ resorption by distal convoluted tubules of kidneys
3) promotes synthesis of 1,25 dihydroxycholecalciferol (vit D derivative) helps small intestine absorb calcium
***rate of PTH secretion directly controlled by plasma concentrations of Ca++
Oxyphil cells
- larger than chief cells though less numerous
- acidophilic numerous mitochondria
- unknown function, don’t secrete hormones
- number of cells are increased in patients with chronic kidney dz
