SFP: thyroid histology Flashcards
What is the gross structure of the thyroid?
Two lobes connected by an isthmus
Describe blood supply in the thyroid gland.
There is an extensive network of fenestrated capillaries to help facilitate transport of hormones between thyroid cells and the blood.
Describe the basic structure of the parenchyma of the thyroid.
Composed of a single layer of follicles surrounding a central lumen.
What is thyroglobulin?
A precursor for thyroid hormones found in the lumen of the follicle aka the colloid.
What are parafollicular cells?
Cells inside the basal lamina of follicular cells.
What do parafollicular cells secrete?
Calcitonin; inhibits osteoclast activity.
Describe the MOA of calcitonin.
It binds its receptors expressed by osteoclasts to decrease the number/mobilization of osteoclasts and inhibit their bone-resorptive activity. Essentially, it prevents breakdown of bone and keeps blood calcium low.
What receptor does calcitonin bind?
hCTR1
How does calcitonin impact the kidney?
Increases urinary excretion of calcium by acting on the PCT.
How does calcitonin impact bone?
Blocks osteoclast recruitment and function to decrease calcium levels in the blood.
What is TSH?
A hormone made in the anterior pituitary that controls hormone production in the thyroid.
What are the main functions of follicular cells?
- Forming thyroglobulin 2. Uptake of iodide 3. Iodide transport 4. Iodide oxidation 5. Iodination of thyroglobulin.
What transporter is associated with the uptake of iodide in follicular cells?
NIS (Na/I symporter) in the basolateral membrane.
How does iodide get into the follicular lumen?
Pendrin found in the apical membrane transports the iodide into the follicular lumen.
Describe what happens to iodide once in the lumen.
Thyroid peroxidase in the apical membrane catalyzes 2 oxidation reactions to generate the reactive ion for iodine.
What helps drive iodide oxidation?
Dual oxidases forming peroxide.
What happens to thyroglobulin in the lumen?
They get iodinated by thyroid peroxidase to form MIT or DIT.
What is organification?
Organic iodide becomes part of a protein.
How are T3 and T4 formed?
- T3 is formed by coupling of MIT and DIT by thyroid peroxidase 2. T4 is formed by two DITs coupling by thyroid peroxidase.
How is thyroid hormone released?
- Apical membrane takes up thyroglobulin w/ T3 and T4 2. Thyroglobulin is taken into an endosome that merges with a lysosome 3. Lysosome degrades thyroglobulin 4. Free T3 and T4 can be released.
What does MCT8 do?
Release T4 and T3 from the basolateral side of follicular cells and into the bloodstream.
Describe how T3 and T4 exert their effects.
- they are transported to target cells 2. THR, typically bound to TRE in DNA and corepressor proteins to repress transcription, is activated by T3 binding that causes a conformational change 3. Corepressor proteins dissociate 4. THR-RXR dimers form and recruit coactivators to activate gene transcription.
What is the location of parathyroid glands?
They are embedded in the posterior portion of the thyroid.
What are principal cells in the parathyroid gland?
Cells arranged in cords that synthesize and secrete PTH.
What are oxyphils?
Large, super eosinophilic cells that are found in clusters and increase with age.
What is the MOA of PTH?
- PTH binds PTH1R (G-protein receptor) in osteoblasts 2. PTH leads to osteoblasts secreting a chemical stimulating the formation and activity of osteoclasts 3. Osteoclasts resorb bone tissue, increasing calcium levels in the blood.
How does PTH impact the kidney?
Increases tubular resorption of calcium in the distal loop of Henle.
How does PTH impact the small intestine?
It indirectly increases calcium via stimulating enzymes (1a-hydroxylase) in the kidney that convert vitamin D to its active form, promoting calcium reabsorption in the small intestine.
What is the major regulatory mechanism for removal of phosphate?
The kidneys.
How is PTH secretion regulated?
Free calcium levels via negative feedback! Low calcium stimulates PTH secretion, while high calcium in the blood will signal via negative feedback to the parathyroid gland to stop producing PTH.
How is calcitonin secretion regulated?
Free calcium levels in the blood! High calcium increases CT secretion, while low levels will decrease CT secretion.
What is CaSR?
Receptor that C cells and chief cells use to detect calcium levels.
