Histo endocrine sys II Flashcards
What is the origin of the thyroid?
Originates from endodermal epithelium in floor of embryonic pharynx, then secondarily loses epithelial connection to pharynx
What is the histological organization of the thyroid?
–Capsule of loose CT. Extensive blood and lymphatic supply.
–Spherical structures lined by epithelia forming follicles containing central colloid.
what is the thyroglobulin?
Extracellular storage of thyroglobulinin colloid. Thyroid hormones stored in a bound (inactive) form in colloid. pwhats gonna be T3 nad T4
What is th emajor regulator of the thyroid?
Hormone thyrotropin (TSH)
which are the epithelial cell types in thyroid follicles?
Thyroid follicular (principal) cells
Parafollicularor C cells
identify
colloids and nuclei of cells
describe the function and epithelium of thyroid folicular cells
Simple epithelium lining follicle (cell height varies in relation to functional activity) from columnar (active) to squamous (inactive)
They have the characteristics of cells that synthesize, secrete, absorb and digest proteins
Produce Thyroid hormones T3 (Triiodothyronine)and T4 (Thyroxine)
idenitfy
Electron micrograph shows a single layer of epithelium containing low columnar follicular cells. The apical surfaces with visible microvilli (Mv) are in contact with the colloid, whereas basal surfaces of follicular cells rest on the basal lamina (FBL).
Accumulation of lysosomes (L) and colloid resorption droplets (CRD), extensive Golgi apparatus (G), rough endoplasmic reticulum (rER), and presence of enlarged intercellular spaces are indicative of intensive activity of follicular cells.
describe the function of parafollicular cells or C cells
Found as part of the epithelium or isolated cluster between thyroid follicles.
Secrete calcitonin (thyrocalcitonin) that lowers blood calcium inhibiting bone resorption.
Secretion is stimulated by elevation in blood calcium
identify
parafollicular cells or c cells (inacitive)
identify
parafolicular cells, with granules in colloid (active) T3 nad T4
Explain the mechanism of synthesis of thyroid hormones
1- Synthesis of thyroglobulin
The precursor of thyroglobulin is synthesized in the rER of the follicular epithelial cells. Thyroglobulin is post translationally glycosylated in the rER and the Golgi apparatus before it is packaged into vesicles and secreted by exocytosis into the lumen of the follicle.
2- The uptake of blood circulating iodide. Using ATP-dependent iodide transporters
Follicular epithelial cells actively transport iodide from the blood into their cytoplasm using ATPase-dependent sodium/iodide symporters (NIS). These cells are capable of establishing an intracellular concentration of iodide that is 30 to 40 times greater than that of the serum.
3- Iodination of thyroglobulin occurring at the microvillus surface of the follicular cell.
One or two iodine atoms are then added to the specific tyrosine residues of thyroglobulin. This process occurs in the colloid at the microvillar surface of the follicular cells and is also catalyzed by thyroid peroxidase (TPO)
4- Formation of T3 and T4 by oxidative coupling reactions
The thyroid hormones are formed by oxidative coupling reactions of two iodinated tyrosine residues in close proximity. For example, when neighboring DIT and MIT residues undergo a coupling reaction, T3 is formed; when two DIT residues react with each other, T4 is formed. After iodination, T4 and T3 as well as the DIT and MIT residues that are still linked to a thyroglobulin molecule are stored as the colloid within the lumen of the follicle
5- Upon stimulation by thyrotropin (TSH) → resorption of colloid by endocytosis → digested by lysosomal enzymes (proteases)
6- Release of T3 and T4 cross basolateral membrane and enter capillaries
Synthesis of thyroid hormones
production of T3 nad T4 is regulated through negative feedback system, explain
The follicular cells produce 20 times more T4 than T3; however, T4 is converted in the peripheral organs (e.g., liver, kidney) to a more active form of T3. Approximately 99% of T4 and T3 released from the thyroid gland into the circulation bind to specific plasma proteins. The remaining free (unbound) T4 and T3 exert negative feedback on the system and inhibit further release of T4 and T3. This inhibition occurs at the level of the anterior lobe of the pituitary gland and the hypothalamus. At the pituitary level, T4 and T3 inhibit secretion of TSH by thyrotropes
thyroid hormones are essential for:
normal fetal development
explain pathologicalc conditions involving the thyroid
- Iodine deficiency produces a condition called goiter
- Adult hypothyroidism
Can be secondary to hypothalamic failure
–Hashimoto thyroiditis presence of abnormal autoimmunoglobulinsdirected against thyroglobulin (TgAb), thyroid peroxidase (TPOAb), and the TSH receptor (TSHAb). Results in thyroid cell apoptosis and follicular destruction. - Children hypothyroidism = Cretinism
- Hyperthyroidism or thyrotoxicosis
-Graves’ disease: Immunoglobulins bind to the thyrotropin receptors mimicking thyrotropin stimulation
identify
Thyroid gland hyperactive, colloid being reabsorped
what is the parathyroid?
Small glands (4) behind the thyroid gland. Covered by the capsule. Occasionally embedded in the thyroid gland.
what cells are rpesent in the parathyroid?
- Chief cells (PRINCIPAL)
–Small polygonal cells
–Secretory granules containing parathyroid hormone (PTH) [aumenta concentracion de calcio] - Oxyphilcells
–Large cells, cytoplasm with many acidophilic mitochondria.
–Function not known
Adipose cells increase in numbers with age
identify
parathyroid gland on the right, vascular
identify
parathyroidlic cells and the violet dark are the chief cells
identify
parathyroid gland; chief cells dark pink and arrow is oxyphil
What is the function of PTH?
Promotes calcium release from bones. Binds to receptors in osteoblasts →produce osteoclast stimulating factor →promotes bone resorption →releases Ca2+ into the blood
Increases synthesis of vitamin D, that promotes reabsorption of calcium in kidney
Intestinal absorption of calcium is increased under PHT influence.
PHT and calcitonin have opposite effects in the regulation of calcium levels
explain pathologies involving PTH
Hyperparathyroidism
–Excessive high calcium in blood. Bone disease called Osteitis fibrosacystic
Hypoparathyroidism (en operaciones antes)
–Low calcium. Convulsions called Tetany
What is the embryological origin of adrenal glands?
Adrenal cortex: Coelomicintermediate mesoderm (steroid hormones)
Adrenal medulla: Neural crest (cathecholamines)