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Histology > Thyroid, Parathyroid and Adrenal Glands > Flashcards

Thyroid, Parathyroid and Adrenal Glands Flashcards

1
Q

Thyroid Gland

A
  • 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
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2
Q

Thyroid Gland Development

A
  • 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
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3
Q

Thyroid Follicles

A
  • 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
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4
Q

Follicular Cells

A
  • Layer of epithelial cells surrounding the follicle
  • produce thyroid hormone
  • have abundance of rough ER
  • prominent Golgi
  • contain microvilli
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5
Q

Parafollicular Cells

A
  • 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
  • come from NEURAL CREST CELLS
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6
Q

T3 (triiodothyronine) and T4 (thyroxine)

A
  • 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
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7
Q

Calcitonin

A
  • 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
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8
Q

Iodine Absorption

A
  • to produce T3 and T4, the follicular cells have iodine pumps (transporters) that concentrate iodine in the thyroid gland
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9
Q

Synthesis of Thyroid Hormones

A
  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
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10
Q

Thyroid Feedback Loop

A
  • Levels of T3 and T4 in the blood become high and feedback on the hypothalamus and the pituitary to stop releasing TRH and TSH
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11
Q

Thyroid Hormone Secretion

A
  • 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
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12
Q

Variation in Follicular Activity

A

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
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13
Q

Parathyroid Glands

A
  • 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
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14
Q

Parathyroid Gland Develpment

A
  • 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
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15
Q

Chief Cells

A
  • also called principal cells
  • secrete PTH
  • arranged in cords
  • surround blood vessels (fenestrated type) for easy secretion into the blood stream
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16
Q

Oxyphil Cells

A
  • have lighter staining cytoplasm
  • function is unknown
  • have plenty of mitochondria
  • produce plenty of ATP - could help with the synthesis of lipids
17
Q

Parathyroid Hormone Feedback Loop

A
  • decreased blood calcium stimulates parathyroid hormone secretion into the blood

Bone

  • 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)

Kidney
- PTH stimulates Tubule cells

Intestine

  • PTH stimulates absorption of Ca via biosynthesis of Vit D
  • More of an indirect way
18
Q

Adrenal Glands

A
  • 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
19
Q

Adrenal Gland Development

A

Medulla

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

Cortex

  • Develops from the celomic epithelium
  • derived from mesoderm
20
Q

Adrenal Cortex Layers

A

Capsule

Zona Glomerulosa
- 15% of the thickness

Zona Fasciculata
- 65% in the middle

Zona Reticularis
- 7%, adjacent to medulla

21
Q

Adrenal Gland Vascularization

A
  • highly vascularized
  • arterial and venous blood vessels
  • Suprarenal vein connects everything with the circulation through the body
22
Q

Zona Glomerulosa

A
  • outer layer
  • Secretes aldosterone (mineralocorticoid)
  • stimulated by angiotensin II
  • Stimulates Na reabsorption in the kidney
  • arranged in cords that are bent (many different arrangments)
23
Q

Zona Fasciculata

A
  • 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
24
Q

Zona Reticularis

A
  • 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
25
Q

Endocrine Control of Adrenal Cortex

A

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
26
Q

Adrenal Medulla

A
  • 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
27
Q

Adrenal Medulla - Secretory Role

A
  • Prominent Golgi
  • Prominent Rough ER
  • Cholinergic Stimulation Stimulates secretion of epinephrine and norepinephrine
  • -both are derived from dopamine
  • -dopamine is derived from tyrosine
28
Q

Histophysiology of Adrenal Glands

A

Aldosterone
- 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

29
Q

Fetal Adrenal Gland

A
  • Fetal adrenal glands are very large relative to their body weight
30
Q

Adrenal Gland and Stress

A
  • 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
31
Q

Effect of stress levels on Structure of Adrenal Cortex

A

Hypophyectomy

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

Stress

  • hypothalamus is overactivated
  • thickening of cortex - hyertrophy

Histology (25 decks)

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