Thyroid, Parathyroid and Adrenal Glands Flashcards Preview

Histology > Thyroid, Parathyroid and Adrenal Glands > Flashcards

Flashcards in Thyroid, Parathyroid and Adrenal Glands Deck (31):

Thyroid Gland

- located at the lower part of the neck
- one gland with two lobes
- joined together by an isthmus
- 4 nodules - parathyroid glands behind the thyroid

- highly vascularized
- frequent capillary openings
- endothelial is fenestrated - pores that facilitate the transport of substances that do not have to go through the membrane via diffusion or active transport


Thyroid Gland Development

- Develops from foramen cecum
- Thyroid diverticulum comes from the same region where the tongue comes from
- By 7th week, the thyroid is in the proper location


Thyroid Follicles

- surrounded by thin layer of epithelial cells
- filled with colloid (gelatin like substance)
- has a basal layer - on the outside, secreting its product towards the inside of the follicle
- Follicles are separated via septa which are capsules of CT


Follicular Cells

- Layer of epithelial cells surrounding the follicle
- produce thyroid hormone
- have abundance of rough ER
- prominent Golgi
- contain microvilli


Parafollicular Cells

- called C cells - look clearer
- produce calcitonin
- can be embedded in the epithelial layer
- also form bunches in between follicles
- Come from a different embryological origin


T3 (triiodothyronine) and T4 (thyroxine)

- Thyroid Hormones
- iodine has been incorporated covelantly on modified versions of tyrosine
- Produced, stored, absorbed and secreted by follicular cells
- main function is to increase metabolic rate
- increase oxidative phosphorylation by increasing mitochondria and the number of cristae
- T3 is more active than T4



- produced by C cells
- regulates blood calcium
- inhibits the activity of osteoclasts (bone resorption), lowering blood calcium
- this occurs when there is too much calcium in the blood


Iodine Absorption

- to produce T3 and T4, the follicular cells have iodine pumps (transporters) that concentrate iodine in the thyroid gland


Synthesis of Thyroid Hormones

1. synthesis of Thyroglobulin by follicular cells
- then secreted to the outside (inside of the follicle)

2. Na/I symporter
- located at basal portion of follicular cells
- brings iodine into the cell

3. Iodine is oxidized once it is inside the cell by thyroid peroxidase
- once oxidized, it is channeled to the colloid by pendrin

4. oxidized iodine interacts with tyrosine residues on thyroglobulin to create T3 and T4


Thyroid Feedback Loop

- Levels of T3 and T4 in the blood become high and feedback on the hypothalamus and the pituitary to stop releasing TRH and TSH


Thyroid Hormone Secretion

- TSH receptor on the basal portion of follicular cells
- TSH binds and the cell starts resorbing the colloid
- encocytic vesicles filled with colloid fuse with lysosomes
- Proteases cleave the bonds between iodinated tyrosine residues and the thyroglobulin molecule
- T4 and T3 are liberated and released into the cytoplasm and into the blood stream
- mainly T4
- T3 is much more active in inducing a response, most hormone activity is attributable to T3
- they are internally cleaved and recyled


Variation in Follicular Activity

Normal Activity
- the amount of thyroglobulin in the colloid is going to be used at a low rate

Hyperactive Follicle
- amount of colloid and therefore thyroglobulin decreases
- cells in the epithelium become larger
- more of the columnar shape

Hypoactive Follicle
- epithelial cells become squamous
- colloid is much bigger - a lot more pressure on the epithelium


Parathyroid Glands

- The back of the thyroid, adjacent to the thyroid
- Different morphology and therefore different functions

- surrounded by capsule made of CT
- CT sends branching septa towards the inside

- highly vascularized - secretions need to go directly into the blood stream

- accumulation of fat cells and oxyphil cells with age


Parathyroid Gland Develpment

- derived from the parapharyngeal pouches (also where the thymus is formed)
- Inferior glands from the 3rd pouch
- superior glands from the 4th pouch
- not uncommon to find the parathyroid glands in a different position next to the thymus because they develop with one another


Chief Cells

- also called principal cells
- secrete PTH
- arranged in cords
- surround blood vessels (fenestrated type) for easy secretion into the blood stream


Oxyphil Cells

- have lighter staining cytoplasm
- function is unknown
- have plenty of mitochondria
- produce plenty of ATP - could help with the synthesis of lipids


Parathyroid Hormone Feedback Loop

- decreased blood calcium stimulates parathyroid hormone secretion into the blood

- PTH causes release of Ca from bone
- targets osteoblasts
- which stimulate osteoclast simulating factor
- mobilizes calcium which goes into the blood
- Calcitonin does the opposite effect (inhibits osteoclasts)

- PTH stimulates Tubule cells

- PTH stimulates absorption of Ca via biosynthesis of Vit D
- More of an indirect way


Adrenal Glands

- 2 glands, one on each side
- cap on top of the kidney
- Has a capsule that surrounds each of the two glands - formed by CT, which sends septa towards the inside to form portions called trabeulae

- Contains 2 layers - outer cortex and central medulla


Adrenal Gland Development

- derived from neural crest cells (ectoderm)
- has to do with the development of the nervous system

- Develops from the celomic epithelium
- derived from mesoderm


Adrenal Cortex Layers


Zona Glomerulosa
- 15% of the thickness

Zona Fasciculata
- 65% in the middle

Zona Reticularis
- 7%, adjacent to medulla


Adrenal Gland Vascularization

- highly vascularized
- arterial and venous blood vessels
- Suprarenal vein connects everything with the circulation through the body


Zona Glomerulosa

- outer layer
- Secretes aldosterone (mineralocorticoid)
- stimulated by angiotensin II
- Stimulates Na reabsorption in the kidney

- arranged in cords that are bent (many different arrangments)


Zona Fasciculata

- middle layer

- Secrete mainly hydrocortisone (cortisol)
- cortisol is a glucocorticoid that regulates blood glucose
- increases gluconeogenesis
- increases blood sugar

- Very organized columns with capillaries between them
- plenty of lipid drooplets
- Mitochondria have vesicular cristae
- Some lysosomes
- Prominent golgi


Zona Reticularis

- inside layer
- Not well arranged
- able to produce some of the glucocorticoids
- Mainly secretes androgens
- DHEA - weak androgen
--converted to testosterone in other tissues in the body
--can also be converted to estrogens


Endocrine Control of Adrenal Cortex

Zona Fasciculata/Reticularis
- Hypothalamus is stimulated which releases CRH
- CRH acts on anterior pituitary to release ACTH
- ACTH goes to Adrenal gland and induces it to release glucocorticoids

Zona Glomerulosa
- Liver produces angiotensiongen which is converted to angiotensin I by renich and then converted to angiotensin II
- angiotensin II causes the zona glomerulosa cells tro release aldosterone


Adrenal Medulla

- mainly postganglionic sympathetic neurons
-- lost its axons and processes - specialized in secretory function

- secretes epinephrine (mainly) and norepineprhine
- also secretes ATP, chromogranins, Dopamine-B-hydroxylase and enkephalins

- highly vascularized


Adrenal Medulla - Secretory Role

- Prominent Golgi
- Prominent Rough ER
- Cholinergic Stimulation Stimulates secretion of epinephrine and norepinephrine
--both are derived from dopamine
--dopamine is derived from tyrosine


Histophysiology of Adrenal Glands

- Na and water conservation

Cortisol and Androgens
- multiple metabolic effects, and immunosuppression
- also weak masculine effects and libido

Epinephrine and Norepinephrine
- acute stress response (fight or flight)

Medullary vein is accessible by every capillary in the adrenal gland


Fetal Adrenal Gland

- Fetal adrenal glands are very large relative to their body weight


Adrenal Gland and Stress

- stress stimulates hypothalamus to release ACTH
- ACTH induces the adrenal gland
- Stimulates release of long term and short term responses
- Long term response
--glucocrticoids and mineralocrorticoids
- Short term response
--epinephrine and norepinephrine


Effect of stress levels on Structure of Adrenal Cortex

-- Part of hypophysis is removed
-- going to have atrophy of cortex

- hypothalamus is overactivated
- thickening of cortex - hyertrophy