Thyroid and Parathyroid Flashcards
1
Q
Thyroid
Location and Structure
A
- Located in the anterior neck
- Left and right lobes joined by the isthmus
- Functional subunit of the thyroid gland is the follicle which produces thyroid hormone
- Also contains parafollicular cells (C-cells) responsible for calcitonin production
- A part of the developmental thyroglossal duct may persist to form a pyramidal lobe
- Highly vascularized with fenestrated capillaries
2
Q
Thyroid Development
A
- Begins as a ventral outgrowth of the floor of the oral cavity
- Initially connected to the tongue via the thyroglossal duct
- Duct connection eventually lost and is marked by the foramen caecum on the surface of the tongue
- Occasionally (40%), the proximal part of the duct persists producing a pyramidal lobe attached to the isthmus of the gland
- Duct remnants may also produce thyroglossal duct cysts in tongue & anterior neck
- Thyroid migrates anteriorly to neck region
- Ectopic thyroid tissue may be found anywhere along the migratory course of the gland
- Thyroid parenchyma (functional gland tissue) comes from 2 sources:
- Endoderm of the floor of the oral cavity gives rise to the follicular cells
- Neural crest via the ultimobranchial body gives rise to the parafollicular cells
3
Q
Thyroid Capsule
A
- Dense fibrous connective tissue envelops the gland
- CT also divides the gland into lobules
4
Q
Thyroid Follicle
A
- Functional subunit of the thyroid gland
- Consists of a single layer follicular cells surrounding a space filled with secretory product
- Cell shape ranges depending on the functional activity of the gland from squamous (inactive) to cuboidal (active).
- Follicular space usually occupied with colloid
5
Q
Follicle cell structure
A
- Follicle cells form a squamous to cuboidal epithelium surrounding the follicle space
- Tends to be squamous when gland is relatively inactive and colloid is being stored
- Tends to be cuboidal when gland is active and releasing T3 and T4 into the circulation and colloid is being broken down
- Abundant RER in basal aspect making cell slightly basophilic
- RER cisternae often distended and irregular
- Well-developed golgi
- Mitochondria with shelf-like cristae
- Many small lysosomes present to hydrolyze the absorbed colloid into thyroid hormones
- Junctional complexes present at apical end of lateral cell membranes
6
Q
Colloid
A
- Consists mainly of thyroglobulin (inactive storage form of thyroid hormone)
- Follicles contain a 3-month supply
- Thyroglobulin broken down into T3 and T4
- Acidophilic in H&E because of high carb content
- PAS positive due to carbs
7
Q
Thyroid Blood Supply
A
- Extensively supplied by fenestrated capillaries.
- Forms dense vascular plexuses that surround follicles
- Uptake of amino acids and iodine from capillaries for production of thyroid hormones
- Release of hormones upon stimulation into capillaries
8
Q
Classes of thyroid hormones
A
- Thyroid hormone produced in a T4 to T3 ratio of 20:1
- T3 is the more potent form and most comes from peripheral conversion of T4 to T3 in the kidney, heart, and liver
- Calcitonin produced by parafollicular cells
9
Q
Thyroid Hormone Synthesis
A
- Synthesis of thyroglobulin
- Amino acids absorbed from blood to produce the protein portion of thyroglobulin
- Assembly of protein portion occurs in RER
- Protein glycosylated in RER and golgi
- Completed thyroglobulin secreted constitutively into follicular space by exocytosis
- Iodination of thyroglobulin
- Uptake of iodide from blood by follicular cells
- Able to concentrate 40x serum levels
- Iodide oxidized to iodine in the follicular lumen by thyroid peroxidase in the apical membrane of follicular cells
- Iodine released into follicular space
- Iodination of tyrosine residues on thyroglobulin occurs near microvilli of apical cell surface
- Uptake of iodide from blood by follicular cells
- Release of active hormones
- Stimulated by TSH from thyrotropes of anterior pituitary
- Colloid taken up by follicular cells by endocytosis
- Endosomes fuse with lysosomes
- Within lysosomes, hydrolysis of thyroglobulin results in the release of active thyroid hormones
- Production of T4 to T3 is 20:1
- Active hormones released into fenestrated capillaries
10
Q
Regulation of thyroid function
A
Functions through a feedback mechanism involving the hypothalamo-hypophyseal axis.
- Hypothalamus releases thyrotropin releasing hormone (TRH) in response to decreased blood levels of T3 and T4
- TRH stimulates thyrotropes in the anterior pituitary (adrenohypophysis) to produce and secrete thyroid stimulating hormone (TSH)
- TSH stimulates follicular resorption of colloid, hydrolysis of thyroglobulin, and release of thyroid hormones (T3 and T4)
11
Q
Parafollicular cells
Location & Function
A
- Derived from the neural crest cells of the ultimobranchial bodies
- Dispersed among the thyroid follicular cells
- Located within the basement membrane of the follicles
- Do not extend into the follicle lumen or contact colloid
- Produces calcitonin
- Therefore also known as C-cells or calcitonin cells
12
Q
Parafollicular cells
Microscopic Anatomy
A
- Large, round, pale-staining cells by H&E
- Have many small electron dense grandules usually concentrated in the basal aspect
- Contain stored calcitonin
- RER and golgi also abundant
13
Q
Calcitonin
A
- A non-essential calcium regulating hormone.
- Antagonist to parathyroid hormone
- High levels of Ca2+ in the blood leads to increased calcitonin production
- Decreases blood calcium levels
- by suppressing bone resorption
- by increasing calcification of osteoid and inhibiting the action of osteoclasts
- Helps prevent excessive bone loss during lactation
14
Q
Iodine Deficiency
A
- Produces under-iodinated and inactive forms of T3 and T4
- These forms do not exert negative feedback inhibition on the hypothalamic-hypophyseal axis
- Results in abnormally high levels of TSH
- Causes abnormal enlargement of the thyroid gland due to hypertrophy and hyperplasia of follicular cells and increase in stored colloid ⇒ goiter
15
Q
Cretinism
A
- Insufficient production of thyroid hormone in an infant (hypothyroidism)
- Characterized by:
- Lethargy
- Mental retardation
- Abnormal bone development
16
Q
Adult Hypothyroidism
A
- Results in myxedema
- Characterized by:
- Edema
- Low metabolic rate
- Lethargy
17
Q
Hashimoto’s thyroiditis
A
- Autoimmune destruction of follicular cells
- Leads to hypothyroidism
18
Q
Graves Disease
A
- Hyperthyroidism associated with hypersecretion of thyroid hormone
- Characterized by:
- Weight loss
- Increased metabolic activity
- Increased heart rate
- In some cases bulging of the eyes (exophtalmos)
H&E of thyroid below shows pale staining area around the periphery of follicular space due to excess colloid use.
19
Q
Parathyroid Glands
Location and Function
A
- Generally 2 pairs of parathyroid glands: superior and inferior
- Located on posterior surface of the thyroid gland
- Essential for life and produces parathyroid hormone (PTH)
- Removal of all four parathyroid glands results in death due to tetany.
20
Q
Parathyroid glands
Development
A
- Parenchyma of the parathyroid glands forms from:
-
Endoderm of the third (inferior pair) and fourth (superior pair) pharyngeal pouches
- Inferior pair pulled lower by the thymus during development
-
Endoderm of the third (inferior pair) and fourth (superior pair) pharyngeal pouches
- Ectopic parathyroid tissue may be found in the neck or chest cavity
21
Q
Parathyroid glands
Microscopic Anatomy
A
- Each gland is surrounded by a thin connective tissue capsule
- Adipocytes increases in number with age
- Parenchymal cells arranged in clusters or cords
- Two cell types:
-
Chief cells (principal cells)
- Small, pale staining, slightly acidophilic cells
- Contains granules at EM level
- Granules contain a storage form of PTH
- Can accumulate large amounts of glycogen
-
Oxyphil cells
- Larger than chief cells
- Contains large numbers of mitochondria making them more acidophilic than chief cells by LM
- Actual function unknown
- Increases in number with age
-
Chief cells (principal cells)
