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Question 2

Thyroid Gland Embryology

Answer 2

• Midline diverticulum of foregut (endoderm -> follicular cells)- Ultimobranchial bodies – 4th branchial pouch (invade follicle -> parafollicular cells)

Question 3

parathyroid embryology

Answer 3

•Branchial Pouch III (inferior)•Branchial pouch IV (superior)

Question 4

Thyroid Cell Types

Answer 4

• Follicular cells (A-cells or Acinar) -> surround colloid• Simple squamous to cuboidal epithelium• Secrete thyroid hormones, T3 and T4- Parafollicular Cells (C cells or calcitonin)• Located b/w spaces b/w follicles• Secrete calcitonin -> calcium homeostasis

Question 5

Thyroid Hormone (T3 and T4) Synthesis

Answer 5

• Thyroglobulin synthesis (137 tyrosines) -> w/in follicular cells -> secreted into follicle lumen• Transport of iodide by sodium/iodide symporter• Iodine pumped across apical membrane into lumen and converted to iodine by thyroid peroxidase• Iodination of tyrosine residues of thyroglobulin • Formation of T3 and T4  oxidative coupling of iodinated thyroglobulins- Thyroglobulin Resorption (in response to TSH)• Release of T3 and T4

Question 6

Parathyroid Gland: cell types

Answer 6

• Chief cell (embryonically differentiate during embryonic development)• Small, high nucleus/cytoplasm ratio• Secrete, store and synthesize PTH (increases blood Ca2+)- Oxyntic cells (differentiate later in puberty)• Large eosinophilic cytoplasm• No known function• Diagnostic for identifying parathyroid gland• More eosinophilic/red

Question 7

PTH mechanism

Answer 7

• Triggered if low Ca concentration in blood -> release of PTH - Causes: • efflux of Ca from bone• decreased loss of Ca from urine (kidney)• increased absorption of Ca by intestines• conversion of 25-(OH)2-Vit D3  1,25-(OH)2 Vit D3 (hormonally active)

Question 8

Adrenal Gland Blood supply

Answer 8

• Subcapsular plexus -> supplies capsule- Sinusoidal capillaries (fenestrated) -> supplies cortex and drains into medullary capillary sinusoids- Medullary arterioles -> supply medullary capillary sinusoids- Medulla has a dual blood supply -> arterial from medullary arterioles and venous from sinusoidal capillaries

Question 9

Adrenal Cortex:

Answer 9

• Cells synthesize steroid hormones• Cells have numerous lipid droplets, mitochondria and SER• Divided into 3 layers: Zona glomerulosa (outer) Zona fasciculata Zona reticularis (inner)

Question 10

Zona glomerulosa

Answer 10

most peripheral layer just under capsule (reddish appearance)Cells grouped in ovoid configurationProduces aldosterone (mineralcorticoids)

Question 11

Zona Fasciculata

Answer 11

• middle zone- Radial columns of cells separated by capillaries (medullary – arterioles and cortical sinusoids)- Produces glucocorticoids (cortisol)- Secretion controlled by ACTH (adrenohypophysis)- pale staining- Abundant SER- Mitochondria w/ tubular cristae- Lipid droplets in cytoplasm

Question 12

Zona reticularis

Answer 12

• Cells organized in a loose meshwork• Produces weak androgens (DHEA) and some glucocorticoids• Secretion controlled by ACTH (adrenohypophysis)

Question 13

Adrenal Medulla

Answer 13

Surrounded by adrenal cortex• Cells arranged in cords around medullary sinusoids• Chromaffin cells are major cell type

Question 14

Chromaffin Cells

Answer 14

• Produces epinephrine (adrenaline; 80%) and norepinephrine (20%)• Enzyme phenylethanolamine N-methyl transferase (PNMT) converts norepinephrine to epinephrine• Glucocorticoids (from Cortex) up-regulate expression of PNMT (increases amount of epinephrine released)