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IMS: Microanatomy > Thyroid and Parathyroid > Flashcards

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
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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
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3
Q

Thyroid Capsule

A
  • Dense fibrous connective tissue envelops the gland
  • CT also divides the gland into lobules
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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
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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
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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
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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
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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
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9
Q

Thyroid Hormone Synthesis

A
  1. 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
  2. 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
  3. 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
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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)
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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
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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
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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
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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​
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15
Q

Cretinism

A
  • Insufficient production of thyroid hormone in an infant (hypothyroidism)
  • Characterized by:
    • Lethargy
    • Mental retardation
    • Abnormal bone development
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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
  • 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:
    1. 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
    2. 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
22
Q

Parathyroid Hormone

(PTH)

A
  • Raises blood calcium levels by:
    • Increasing the breakdown of bone tissue through the activity of osteoclasts
    • Stimulating the development of more osteoclasts
    • Promoting reuptake of Ca2+ from the glomerular filtrate by kidney tubules
    • Promoting uptake of Ca2+ from the GI tract
  • Essential for life

IMS: Microanatomy (16 decks)

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