Endocrine II Flashcards
How does the thyroid gland develop?
Develops as an evagination from the wall of the pharynx
What is the thyroid gland?
Lies adjacent to trachea, posterior to larynx
Consists of two lobes connected by an isthmus and surrounded by a capsule (double layers of CT).
What two cell types are found in the thyroid
Contains two types of hormone-secreting cells:
Follicle Cells = secrete thyroid hormones (T3 and T4)
Parafollicular Cells (C-cells) = secrete calcitonin
basic structural unit of the thyroid
follicle - lined by simple cuboidal epithelium of follicle cells.
What is colloid?
Colloid = acidophilic glycoprotein filling lumen of follicle, composed of thyroglobulin (thyroid hormone precursor) which undergoes extracellular iodination; serves as storage reservoir for thyroid hormones
Outline the 7 steps of synthesizing colloid and thyroid hormones.
- Synthesis of thyroglobulin
- Release into follicle lumen
- Iodination
- Storage as colloid
- Endocytosis of colloid in response to secretory stimulus (TSH from adenohypophysis)
- Hydrolysis of thyroglobulin (produces T3 and T4)
- Release of thyroid hormones into capillaries surrounding follicles
What are follicle cells
low cuboidal when hypoactive; high cuboidal/low columnar when the gland is active; Hyperactive thyroid has little or no colloid in lumen of follicles. Follicle cells contain large nuclei, basophilic cytoplasm with fine granules, and numerous mitochondria, RER and lysosomes
What are C cells
= lie adjacent to follicles but within basal lamina; larger than follicle cells with eccentrically placed nuclei, pale-staining cytoplasm, contain numerous membrane-bound granules (calcitonin)
T3 and T4
increase metabolic rate and help maintain normal function and growth in tissues
Calcitonin
decreases blood calcium by inhibiting bone resorption
Describe the location and general anatomy of the parathyroid glands
Lie on border of thyroid, usually 4 glands in humans (range is 2-6); derived from endoderm of pharyngeal pouches.
Covered with a thin capsule and also enclosed in fascia of thyroid; septa extend inward from capsule, divide gland incompletely into lobules; septa carry blood vessels, lymphatics, nerves
What cell types are in the parathyroid and what do they do?
Chief Cells = small, round, pale-staining cells with a spherical nucleus and indistinct cell borders. Secrete Parathyroid hormone
Oxyphil Cells = less common, larger with acidophilic cytoplasm, smaller darker nucleus, numerous mitochondria; increase in number after puberty. May represent non-secretory chief cells, but precise function is unknown.
What is parathyroid hormone?
antagonistic to calcitonin, promotes bone resorption, increases blood calcium levels
Describe the structure and location of the adrenal glands. Also, what hormones are secreted.
Paired glands located at cranial pole of each kidney; composed of 2 histological regions:
Cortex = outer region, secretory cells derived from mesoderm, produce corticosteroids
Medulla = inner region, secretory cells derived from neural crest tissue, produce catecholamines (epinephrine, norepinephrine)
What are the three layers of the adrenal cortex?
zona glomerulosa
zona fasiculata
zona reticularis
Describe the zona glomerulosa
Zona glomerulosa = narrow zone immediately beneath capsule (~15% of cortex), cells arranged into rounded clumps
Consists of pyrimidal or columnar cells with dark-staining spherical nuclei, acidophilic cytoplasm with some basophilic material, cytoplasm also contains lipid droplets
Produces mineralocorticoids (aldosterone and deoxycorticosterone) which control water and salt balance; aldosterone specifically increases kidney tubule resorption of Na+
Describe the zona fasiculata
Zona fasiculata = thickest layer (~78% of cortex)
Consists of irregular cuboidal/polyhedral cells arranged in long radial cords; bordered by fenestrated capillaries.
Cells contain vesicular nuclei and basophilic cytoplasm with numerous lipid droplets (droplets larger than in zona glomerulosa)
Produces glucocorticoids (cortisol, corticosterone) which regulate carbohydrate, lipid and protein metabolism; part of stress response
Describe the zona reticularis
Zona reticularis = composes ~7% of cortex volume
Cells form anastomosing network, separated by fenestrated capillaries
Cells in this region are smaller, the cytoplasm contains fewer lipid droplets and the nuclei are deeper-stained
Produces some glucocorticoids and very small amounts of sex hormones (dihydroepiandrosterone; physiologically relevant?)
Throughout the cortex, lipid droplets are associated with the synthesis of steroid hormones
What cell type is found in the adrenal medulla?
chromaffin cells
What is the function of the adrenal medulla?
Consists of anastomosing cords of cells known as chromaffin cells (due to specific staining of secretory granules with Chromium salts)
Secrete epinephrine (adrenaline), norepinephrine
Secretory cells of medulla innervated by sympathetic preganglionic fibers, so they are functionally equivalent to sympathetic ganglion cells.
Stimulation causes release of hormone by exocytosis. (Norepinephrine is the neurotransmitter of sympathetic postganglionic fibers).
Where is the pineal body
Cone-shaped body attached by a stalk to the roof of the 3rd ventricle of the brain
What does the pineal body do? What structure encapsulates it?
Produces melatonin which exerts a suppressive effect on gonadal function and regulates circadian and circannual rhythms, among other actions
Pia mater forms capsule around pineal body
What are the cell types of the pineal body?
pinealocytes
glial cells
(also extracellular concretins)
Pinealocytes
epithelial secretory cells (melatonin), irregularly shaped with branching processes; pale-staining cells with a large nucleus
