Thyroid and parathyroid glands

Question 1

what is the first gland to appear in the embyro? when?

Answer 1

thyroid

4th week

Question 2

what germ layer makes up the primitive thyroid gland?

Answer 2

endoderm that lines the primitive pharynx

Question 3

when does the thyroid gland begin to function prenatally?

Answer 3

end of 3rd month

Question 4

what is the role of the thyroglossal duct?

Answer 4

connects developing thyroid gland to the tongue

Question 5

the thyroid capsule is developed from what tissue?

Answer 5

deep cervical fascia

Question 6

the thyroid gland synthesizes what hormones?

Answer 6

T3
T4
calcitonin

Question 7

what is the basic structural and functional unit of the thyroid gland?

Answer 7

follicle

Question 8

the thyroid follicle is surrounded by what layers?

Answer 8

basal lamina
reticular fibers
fenestrated capillary plexus

Question 9

what is contained within the lumen of the thyroid follicle?

Answer 9

colloid

Question 10

the follicular epithelium is made up of what cell types? what are their activity levels?

Answer 10

follicular cells
principal cells

simple squamous - low activity
simple cuboidal - normal activity
simple columnar - high activity

Question 11

which cell type synthesizes and secrete calcitonin?

Answer 11

parafollicular cells

Question 12

thyroglobulin is contained in what organelle?

Answer 12

vesicles

Question 13

by which two mechanisms does calcitonin lower calcium levels?

Answer 13

• inhibits bone breakdown by osteoclasts

- promotes calcium deposition (osteoid calcification)

Question 14

colloid consists of what glycoprotein?

Answer 14

thyroglobulin

Question 15

synthesis of thyroid hormones is controlled by what mechanisms?

Answer 15

TSH

iodine levels

Question 16

what cells synthesize thyroglobulin?

Answer 16

follicular cells

Question 17

how is iodide converted to iodine in the thyroid gland?

Answer 17

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

Question 18

how are MIT and DIT made?

Answer 18

iodine iodinates tyrosine residues of each thyroglobulin molecule to form MIT and DIT

Question 19

what cells secrete TSH?

Answer 19

basophils (thyrotropes)

Question 20

what organelle is the site of thyroglobulin synthesis?

Answer 20

RER

Question 21

what organelles are the sites of thyroglobulin glycosylation?

Answer 21

RER and GA

Question 22

where does oxidation of iodide take place?

Answer 22

in the active sites of thyroid peroxidase (membrane bound enzyme) on the follicular APICAL cell membrane - facing the colloid

Question 23

what is needed for oxidation of iodide?

Answer 23

hydrogen peroxide

Question 24

where does oxidation of iodide occur?

Answer 24

colloid

Question 25

what are the two pathways by which thyroglobulin can be processed following endocytosis? which is the main pathway?

Answer 25

lysosomal (main)

transepithelial

Question 26

lysosomal pathway

Answer 26

• 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

Question 27

T4 is only produced by which cell?

Answer 27

follicular cell

Question 28

T3 is mainly produced how?

Answer 28

conversion from T4 by kidney, liver, and heart

Question 29

transepithelial pathway

Answer 29

• 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

Question 30

inside the cell T3 and T4 bind to what receptor type?

Answer 30

nuclear thyroid hormone receptor proteins

Question 31

the parathyroid glands arise from which pharyngeal pouches?

Answer 31

inferior parathyroid glands - 3rd

superior parathyroid glands - 4th

Question 32

what are the cell types of the parathyroid glands?

Answer 32

chief cells
oxyphil cells
intermediate cells

Question 33

which parathyroid cell has secretory granules containing PTH?

Answer 33

chief cells

Question 34

what is the main role of PTH?

Answer 34

regulation of calcium and phosphate levels in blood

Question 35

PTH has influence on what organs?

Answer 35

bone
kidney
GI (vitamin D)

Question 36

how does PTH regulate bone physiology?

Answer 36

• PTH attaches to osteoblast receptors

- causes osteoblasts to release osteoclast-stimulating factor - frees calcium from bone

Question 37

how does PTH regulate kidney physiology?

Answer 37

• PTH prevents calcium loss in urine

- PTH promotes phosphate loss in urine

Question 38

how does PTH regulate GI physiology?

Answer 38

• regulated formation of vitamin D in kidney
• controls rate of calcium absorption from GI
• vitamin D facilitates calcium absorption from GI

Question 39

what would be the effect of complete removal of parathyroid glands?

Answer 39

• drop in calcium level

- tetanic contraction of muscles (laryngeal / respiratory muscles leads to death)