Histology of the Thyroid and Parathyroid Flashcards by tcarlso 3

When in gestation does the thyroid gland begin to develop?

4th week

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What is the first endocrine gland to appear in the embryo?

Thyroid gland

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What part of the developing embryo does the thyroid gland develop from?

Endoderm

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When does the thyroid gland begin to function?

End of the 3rd month

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What are the three gross anatomical units of the thyroid?

Right lobe
Left lobe
Isthmus

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Where, relative to the respiratory system, does the thyroid gland lie?

Inferior to the larynx

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What is the thyroid diverticulum?

Inpouching of the primitive oropharynx

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What is the thyroglossal duct? What happens to this in development?

The duct that connects the thyroid gland to the oropharynx

Closes in development

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What is the pyramidal lobe? IS this a part of normal anatomy?

represents the inferior end of the thyroglossal duct that persists after the remainder of the thyroglossal duct has degenerated

present in about 50% of individuals

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What is the thyroglossal duct? What happens if this does not degenerate in embryogenesis?

as the thyroid gland develops and descends into the neck, a hollow duct, the thyroglossal duct forms which temporarily connects the developing thyroid gland to the foramen cecum of the tongue. The thyroglossal duct eventually degenerates and normally disappears, however, in some individuals it may persist as a developmental remnant: a fibrous cord that extends from the fully-developed thyroid gland to the foramen cecum on the dorsum of the tongue

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What part of the cervical fascia is the thyroid capsule derived from?

Deep cervical fascia

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What type of tissue that comprises the thyroid capsule?

Dense, irregular collagenous tissue

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What gives rise to the trabeculae/septa of the thyroid gland? What do these give rise to?

The thyroid capsule

Gives rise to lobules

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What do the septa of the thyroid carry?

Blood vessels
Lymph vessels
nerve fibers

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Where are the parathyroid glands relative to the thyroid?

Within the capsule covering the posterior surface of the thyroid

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What is the function of the thyroid gland?

Synthesizes T3 and T4 and calcitonin

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What organ of the body secretes calcitonin?

Thyroid

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What is the basic structural/functional unit of the thyroid?

Thyroid follicle

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What type of histological structure surrounds the thyroid follicles? What does this consist of?

Basal lamina

Carb substance and reticular fibers

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What type of capillary network surrounds the thyroid follicle that allows for easy secretion of thyroid hormone into the serum?

Fenestrated

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True or false: the follicle enclosing a lumen is a unique histological structure to the thyroid gland

True

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What is the major difference between thyroid gland and other endocrine glands in the way that its hormones are stored/secreted?

Thyroid gland stores its secretory product in a follicle, not within secretory cells

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What are the two cells in the epithelium that surrounds the colloid?

Follicular epithelium and principal cells

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What types of epithelium surround the follicular lumen if there is low/normal/high thyroid function? (3)

i. simple squamous – low level of activity
ii. simple cuboidal – normal level of activity
iii. simple columnar – highly active

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What is the function of calcitonin?

- Inhibits bone breakdown by osteoclasts | - Promotes Ca deposition

What cells in the thyroid synthesize calcitonin? Do these cells participate in the colloid formation/secretion?

Parafollicular cells Have nothing to do with colloid

What is the nucleus like in the thyroid epithelial cells?

Round to ovoid

What is the cytoplasm like in the thyroid epithelial cells (eosinophilic or basophilic)?

Basophilic

What part of the thyroid cells contain thyroglobulin?

Vesicles

What is the function of the microvilli of the thyroid follicles?

Increase free cell surface and absorption

What are "clear" / "C" cells of the thyroid?

Parafollicular cells

What are parafollicular cells derived from?

Neural crest cells

Where are the parafollicular cells in the thyroid gland?

Wedged between follicles (NOT exposed to the colloid)

What do parafollicular cells contain?

Dense secretory granules

What type of hormone is calcitonin?

Peptide hormone

True or false: calcitonin is secreted continuously at low levels

False--only when there is a problem

Patients with hypercalcemia are treated with what endogenous hormone?

Calcitonin

What can cause a calcitonin deficiency?

Removal of the parathyroid cells/follicles

True or false: a loss of calcitonin is not associated with any disease

True

What cells synthesize thyroglobulin? Where does this go?

Follicular cells Released into the follicular lumen

Each molecule of thyroglobulin has how many amino acid residues? Which amino acid is this?

120 Y residues

What is the form of I that is in the serum? What about in the follicle?

Iodide (I-) in the serum | Iodine (I) in the follicle

What happens to the iodide as it is pumped from the bloodstream into follicular cells? What does this?

Oxidized on the apical (microvillar) cell membrane facing the colloid

The iodine produced by follicular cells iodinates the thyroglobulin residues to form what?

MIT | DIT

What happens to the MITs and DITs in the thyroglobulin molecule?

MIT + DIT = tri iodinated tyrosine DIT + DIT = tetra iodinated tyrosine

What are the cells in the pituitary that secrete TSH? Are these basophils or acidophils?

Thyrotropes--basophils

What is the fate of the TSH released by the thyrotrope pituitary cells?

Enters general circulation and travels to the thyroid gland, where it binds to the TSH receptor

Where in the cell is thyroglobulin synthesized? Where does it go from there?

RER Goes to the golgi for glycosylation

Where in the cell is thyroglobulin glycosylated? What occurs after this happens?

Golgi apparatus Formation of transport vesicles that move to the luminal aspect of the cell membrane

Where do the vesicles that contain thyroglobulin and are made by the golgi apparatus go once synthesized?

Transported to the luminal aspect of the cell membrane, and released into the lumen by exocytosis

What is the form of I that we take in from out diet? Where is this changed?

Iodine--reduced to iodide in the GI lumen

How does iodide enter the follicular cells? Is this active or passive transport?

Active transport via Na/I symporters located on the basal membrane of the follicular cells

What is the enzyme that oxidizes iodide to iodine, and where is this found?

Thyroid peroxidase, which is bound to the apical side of the follicular cell membrane

What is the cofactor that is needed to oxidize iodide to iodine?

H2O2

Where in the thyroid gland does the oxidation of iodide occur?

In the colloid

What is the form of I that can bind to thyroglobulin?

Iodine

What is the enzyme that iodinizes thyroglobulin tyrosine residues? Where does this occur?

Thyroid peroxidase occurs where luminal (microvillar) aspect of follicular cell contacts the colloid

Iodinated thyroglobulin is stored in the follicular lumen as what?

Colloid

True or false: thyroglobulin is not a hormone

True--it is the inactive storage form of the thyroid hormones

What are the cells in the pituitary that release TSH? Are they basophilic or acidophilic?

Thyrotropes | Basophilic

Where does TSH bind to?

Adheres to TSH receptors on basal membranes

What forms the luminal surface of the follicular cells?

Filopodia

How is colloid brought into the thyroid cells?

Via endocytosis

What are the two pathways through which thyroglobulin may be processed?

Lysosomal | Transepithelial

What is the first step of the lysosomal pathway?

Vesicles enclosing colloid unite with endosomes in the cytoplasm that contain proteases

What happens to the thyroglobulin in the lysosomal pathway once it unites with the endosomes that contain proteases?

Proteases split the iodinated tyrosine residues from thyroglobulin

Once the iodinated tyrosine residues are split from thyroglobulin in the lysosomal pathway, what happen?

The iodinated tyrosine residues are released into the *cytoplasm* as MITs or DITs

What is thyroxine?

Once MITs and DITs are released into the cytoplasm in the lysosomal pathway, what happens next?

The enzyme iodotyrosine dehalogenase splits the iodine from MIT and DITs

What is the enzyme that splits iodine from MITs and DITs in the cytoplasm of thyroid cells?

Iodotyrosine dehalogenase

What happens to the iodine and the tyrosine that are released in the cytoplasm in the lysosomal pathway?

Recycled

T3 and T4 are liberated from follicular cell membrane on which side?

Basolateral

True or false: T4 is produced in a variety of tissues in the body

False--T4 is only produced by the thyroid. This is true for T3 however

What are the three major organs that convert T4 to T3?

Kidney Liver Heart

What is the first step in the transepithelial pathway?

Thyroglobulin binds to megalin, a transmembrane protein receptors located on the apical follicular cell membrane facing the colloid

What is the protein that thyroglobulin binds as the first step in the Transepithelial pathway of thyroid hormone secretion? What is the function of this?

Meglin | Transmembrane protein receptor that transports thyroglobulin to the basolateral side of the follicular cell

Are T3 and T4 molecules that are bound to albumin/thyroid binding globulin active or inactive?

Inactive

Where are the receptors for thyroid hormone?

Bind to intracellular proteins (nuclear thyroid hormone receptor proteins)

What is the effect of thyroid hormone on phospholipid formation? TAGs? Cholesterol formation?

Decrease phospholipid and TAG synthesis, but increase cholesterol formation

How does thyroid hormone increase cholesterol levels? (2)

- Upregulate MHMG-Coa Reductase | - Increase uptake of LDL

Thyroid hormone increases cholesterol uptake and synthesis, but hypothyroid pts have hypercholesterolemia. Explain.

They are unable to successfully manage cholesterol derived from the diet

What are the menstrual cycles like with hypothyroidism? Hyperthyroidism?

``` Hypo = long and heavy Hyper = short and light ```

What are the pharyngeal pouches that give rise to the inferior and superior parathyroid glands?

``` Inferior = 3rd Superior = 4th ```

Where are the parathyroid glands located?

Posterior to the thyroid glands

At what age are the parathyroid glands at adult size?

20 years old

What anatomical structure carries blood and lymph supply into the parathyroid gland?

The capsule's septa and reticular fibers

What characteristic of the parathyroid capillary network allows for easy secretion of PTH into the blood?

Fenestrated capillaries

In the adult, what percent of the parathyroid gland is made up of adipose tissue?

60-70%

What is the general histological arrangement of parathyroid parenchyma?

Epithelial cords/clusters

What is the function of the chief cells of the parathyroid gland? Are these eosinophilic, or basophilic? Why?

Secrete PTH | Eosinophilic d/t large amounts of mitochondria inside

What is the initial precursor to PTH? Where in the chief cells is this made? What processing does this have to go through before becoming PTH?

PreproPTH from the rER | Becomes ProPTH whilst leaving the rER, and is sent to the golgi, where it becomes PTH

How is PTH liberated from chief cells?

Exocytosis

What are the oxyphil cells of the parathyroid gland? Are they eosinophilic or basophilic?

i. function unknown, may be dormant, inactive chief cells ii. intensely eosinophilic (due to large number of mitochondria) iii. larger than chief cells

What is the function, generally, of PTH?

Regulates blood Ca and phosphate levels

What stimulates PTH release? What inhibits?

Stimulates = low Ca levels Inhibits = high Ca levels

What is the function of PTH in the bone?

Attaches to *osteoblast* receptors, and causes them to release *osteoclast* stimulating factor

What is the role of PTH In the kidney?

Increase Ca and decrease Phosphate reabsorption

What is the function of PTH in the intestines?

Regulates the formation of vitamin D in the kidneys, which in turn regulates absorption from the GI tract

What hormone has the opposite effect of PTH?

Calcitonin

What hormone increases low Ca levels?

PTH

What hormone decreases elevated Ca levels?

Calcitonin

What is the effect of the complete removal of the parathyroid glands?

Will drop Ca levels, causing tetany and respiratory arrest

What is the name of the opening of the thyroglossal duct?

Foramen cecum

What type of Immunoglobulins are produced in Grave's disease?

IgG to the TSH receptor

What causes the exophthalmos with Grave's disease? (2)

- Fibrosis of the rectus muscles | - GAG accumulation

What causes the deposition of GAGs behind the eye in hyperthyroidism?

Fibroblasts there have TSH receptors

Histology of the Thyroid and Parathyroid Flashcards

(106 cards)