Thyroid and parathyroid glands Flashcards
what is the first gland to appear in the embyro? when?
thyroid
4th week
what germ layer makes up the primitive thyroid gland?
endoderm that lines the primitive pharynx
when does the thyroid gland begin to function prenatally?
end of 3rd month
what is the role of the thyroglossal duct?
connects developing thyroid gland to the tongue
the thyroid capsule is developed from what tissue?
deep cervical fascia
the thyroid gland synthesizes what hormones?
T3
T4
calcitonin
what is the basic structural and functional unit of the thyroid gland?
follicle
the thyroid follicle is surrounded by what layers?
basal lamina
reticular fibers
fenestrated capillary plexus
what is contained within the lumen of the thyroid follicle?
colloid
the follicular epithelium is made up of what cell types? what are their activity levels?
follicular cells
principal cells
simple squamous - low activity
simple cuboidal - normal activity
simple columnar - high activity
which cell type synthesizes and secrete calcitonin?
parafollicular cells
thyroglobulin is contained in what organelle?
vesicles
by which two mechanisms does calcitonin lower calcium levels?
- inhibits bone breakdown by osteoclasts
- promotes calcium deposition (osteoid calcification)
colloid consists of what glycoprotein?
thyroglobulin
synthesis of thyroid hormones is controlled by what mechanisms?
TSH
iodine levels
what cells synthesize thyroglobulin?
follicular cells
how is iodide converted to iodine in the thyroid gland?
iodide in bloodstream is pumped into follicular cells by sodium / iodide symporters and is oxidized on the apical (microvillar) cell membrane facing the colloid into its active form - iodine
how are MIT and DIT made?
iodine iodinates tyrosine residues of each thyroglobulin molecule to form MIT and DIT
what cells secrete TSH?
basophils (thyrotropes)
what organelle is the site of thyroglobulin synthesis?
RER
what organelles are the sites of thyroglobulin glycosylation?
RER and GA
where does oxidation of iodide take place?
in the active sites of thyroid peroxidase (membrane bound enzyme) on the follicular APICAL cell membrane - facing the colloid
what is needed for oxidation of iodide?
hydrogen peroxide
where does oxidation of iodide occur?
colloid
what are the two pathways by which thyroglobulin can be processed following endocytosis? which is the main pathway?
lysosomal (main)
transepithelial
lysosomal pathway
- vesicles containing colloid unite with endosomes in cytoplasm
- endosomes contain proteases which split the iodinated tyrosine residues from the thyroglobulin
- iodinated tyrosine residues are released into the cytoplasm as MIT, DIT, T3, and T4
- iodotyrosine dehalogenase splits iodine from MIT and DIT
- ## T3 and T4 are liberated from follicular cell basolateral membrane into surrounding CT to be picked up by capillaries
T4 is only produced by which cell?
follicular cell
T3 is mainly produced how?
conversion from T4 by kidney, liver, and heart
transepithelial pathway
- thyroglobulin binds to megalin, a transmembrane protein receptor on apical follicular cell membrane facing colloid
- thyroglobulin evades / bypasses lysosomal pathway and instead endocytic vesicles are transported to basolateral membrane of follicular cell
- small amounts of T3 and T4 bound to thyroglobulin are released from basal follicular cell membrane into the blood and lymphatic capillaries
inside the cell T3 and T4 bind to what receptor type?
nuclear thyroid hormone receptor proteins
the parathyroid glands arise from which pharyngeal pouches?
inferior parathyroid glands - 3rd
superior parathyroid glands - 4th
what are the cell types of the parathyroid glands?
chief cells
oxyphil cells
intermediate cells
which parathyroid cell has secretory granules containing PTH?
chief cells
what is the main role of PTH?
regulation of calcium and phosphate levels in blood
PTH has influence on what organs?
bone
kidney
GI (vitamin D)
how does PTH regulate bone physiology?
- PTH attaches to osteoblast receptors
- causes osteoblasts to release osteoclast-stimulating factor - frees calcium from bone
how does PTH regulate kidney physiology?
- PTH prevents calcium loss in urine
- PTH promotes phosphate loss in urine
how does PTH regulate GI physiology?
- regulated formation of vitamin D in kidney
- controls rate of calcium absorption from GI
- vitamin D facilitates calcium absorption from GI
what would be the effect of complete removal of parathyroid glands?
- drop in calcium level
- tetanic contraction of muscles (laryngeal / respiratory muscles leads to death)
