Thyroid (Exam II) Flashcards
Why does the presence of numerous mitochondria make the cytoplasm of a cell eosinophilic? What endocrine gland cell contains extremely large numbers of mitochondria, yet has no known function?
An abundance of mitochondria makes the cytoplasm of a cell eosinophilic because the mitochondrial membranes contain unusually large amounts of protein (electron transport chain proteins, ATP synthase complexes, etc.), and proteins are generally eosinophilic in fixed tissues. It has nothing to do with the fact that hydrogen ion gradients play a role in the functioning of living mitochondria. Most of those hydrogen ions are long gone in fixed tissue. The oxyphils of the parathyroid gland are characterized by extraordinary numbers of mitochondria that essentially fill their cytoplasm. Yet these cells, which increase in number with age, have no known functions.
Explain why thyroid tissue can sometimes be found within the tongue.
The thyroid develops from a downgrowth of the endoderm in the floor of the pharynx. Its point of origin is indicated in an adult by the foramen cecum on the dorsal surface of the tongue. During development, the thyroid normally migrates inferiorly through the neck until it comes to a halt anterior to the upper end of the trachea. Occasionally the thyroid fails to migrate out of the tongue or leaves some remnants behind, resulting in a lingual thyroid, which can contain functional thyroid tissue.
The thyroid follicles can sometimes be confused with the lactating mammary gland. How can you distinguish between the two by LM?
The thyroid is an endocrine gland and hence lacks ducts, but the mammary gland is an exocrine gland and has an extensive, branching duct system. So you should look for ducts. Since thyroid follicles tend to be spherical structures that are separate from one another, any elongated tubular structures that you find (that aren’t blood vessels) will probably be longitudinal sections through a duct. Look within lobules for intralobular ducts and also between lobules for the larger interlobular ducts. Remember that ducts should be lined by a cuboidal to columnar epithelium, whereas blood vessels are lined by a squamous endothelium.
What membrane junctions are found between thyroid follicular cells? Why are they important to the functioning of the organ?
Tight junctions are found between thyroid follicular cells. They are important because they inhibit the leakage of thyroglobulin (which is biologically inactive) out of the follicle and thus conserve the raw material that can eventually be hydrolyzed to release active T3 & T4.
Active thyroid follicular cells and aging plasma cells tend to develop irregular, distended cisternae of RER. If you had an EM of a cell that showed these swollen RER cisternae, how would you decide if it were a follicular cell or a plasma cell?
Follicular cells are morphologically polarized cells that are part of an epithelium. They have an apical end that abuts on a lumen, a basal end that sits on abasement membrane, and lateral surfaces that are joined to neighboring follicular cells by tight junctions. Plasma cells are spherical cells with no apical or basal end. They are found as individual cells scattered about in connective tissue or in lymphoid tissue. They are not joined to any neighboring cells by tight junctions and are not part of an epithelium.
Thyroid Gland
Contains two endocrine systems:
- Follicular cells that produce T3 & T4
- Parafollicular cells that produce calcitonin
Thyroid Follicle
- Is the structural & functional unit of the thyroid Is a spherical structure with a single layer of follicular cells surrounding a lumen filled with colloid (Colloid is the term used for any PAS+ material that accumulates extracellularly in endocrine glands)
- Main component of colloid in the thyroid is thyroglobulin (TG). TG is the inactive storage form of the thyroid hormones
- Thyroid is unique among endocrine glands because it stores most of its secretory product extracellularly instead of in vacuoles in the cytoplasm
- Extensive network of fenestrated capillaries surrounds each follicle Follicular cell height increases from squamous to columnar as TSH stimulation increases
Thyroid Follicular Cells
- Are morphologically polarized cells with obvious basal & apical ends
- Abundant RER in basal cytoplasm
- Is the site of TG synthesis RER cisternae of follicular cells are characteristically dilated & irregular in shape
- Cells are joined together by tight junctions
- Apical plasma membrane has short sparse microvilli
- Small, clear secretory vesicles in apical cytoplasm which contain newly synthesized TG about to be released into follicular lumen
- Larger PAS-positive phagocytic vacuoles called colloid droplets. These contain colloid recently taken up from the lumen TSH stimulation causes: - increase in surface projections of apical membrane - formation of increased number of colloid droplets - extensive phagocytosis can cause “scalloping” of colloid
- Many lysosomes and well-developed Golgi
Production of Thyroglobulin
- Synthesis of TG on RER, packaging in Golgi
- Constitutive release into follicle lumen by merocrine secretion
- Uptake and oxidation of iodide Actively transported across basal plasma membrane Oxidized by thyroid peroxidase (TPO), an enzyme on membrane of microvilli
- Iodination of thyroglobulin Thyroid peroxidase iodinates tyrosine residues of TG to form monoiodotyrosine (MIT) or di-iodotyrosine (DIT) 5. Coupling reaction Thyroid peroxidase catalyzes transfer of a ring from MIT or DIT to another MIT or DIT to form T3 (MIT + DIT) & T4 (DIT + DIT) At this point T3 & T4 are still part of the primary amino acid sequence of TG
- Uptake of colloid into colloid droplets
- Fusion of lysosomes with colloid droplets Lysosomal enzymes break down TG & thus release T3 & T4 from TG T3 & T4 cross basal plasma membrane and enter fenestrated capillaries
Parafollicular Cells (C Cells)
- Rare in human thyroid
- Cytoplasm typically appears pale or clear (hence C cells)
- Derived from neural crest cells that form the ultimobranchial body associated with the 5th pharyngeal pouch of the fetus
- Most are part of the follicular epithelium
- Rest on basement membrane but do not reach follicular lumen
- By EM, have small secretory granules in basal cytoplasm
- Secrete calcitonin in response to high blood calcium
Parathyroid Gland
Embedded in capsule of thyroid gland In young parathyroid, cells are arranged like beads on a string Contains:
- Principal cells (chief cells)
- Oxyphils (rare in young parathyroid)
- Adipocytes (rare in young parathyroid)
Chief Cells
- Most numerous cell type
- Small polygonal cells with round centrally placed nucleus
- Cells sometimes contain large amounts of glycogen Extraction of glycogen makes cytoplasm pale-staining
- EM: Has small secretory granules that contain parathyroid hormone (PTH)
Oxyphils
- Larger and rounder than chief cells
- Very eosinophilic due to many mitochondria
- Nuclei often smaller & darker staining by LM than those of chief cells
- Oxyphils often occur in clusters Number increases with age, but they never outnumber chief cells
- Function unknown
How to distinguish parathyroid from pars distalis of hypophysis
- The eosinophilic cell is the major cell type in the pars distalis (acidophils), & the minor cell type in the parathyroid (oxyphils)
- Adipocytes are rare in pars distalis at any age, but common in older parathyroids
- Oxyphils often occur in clusters, while acidophils are somewhat more evenly distributed among the basophils & chromophobes
Ectopic thyroid tissue
Due to the migration of the thyroid from the floor of the oral cavity, thyroid tissue can be located any where along the migratory pathway.
What type of cells are these?
Chief cells found in the parathyroid.
Small cells with round nuclei
Cytoplasm can have glycogen
What are on the apical ends (the arrows–not the pointers)
Colloid droplets in the thyroid.
What is this an EM image of?
Thyroid follicular cell. Note the lysosomes and weird rER.
What is the difference between these two images?
Left: low TSH stimulation with squamous cells
Right: high TSH stimulation with columnar cells
What is this an EM of? What is so particular about the oxyphils?
Oxyphils and Chief cell. Note the high mitochondria content in the oxyphils.
In the parathyroid, what two cell types are there? What determines why one is more eosinophilic than the other?
Chief cells (release PTH) which are darker and Oxyphils (??) which have a ton of mitochondria and are eosinophilic.
What is this an EM of?
Parafollicular Cell
What is the major cell type of the parathyroid?
Chief cells.
What are these two structures and how do you differentiate?
Left: pars distalis of adenohypophysis
Right: parathyroid (less eosiniophilic cells more adipocytes)
What is this an image of?
Thyroid Follicle.
What is the significance of this image?
Vascular cast of thyroid. Fenestrated capillaries near basement membrane
How can you tell the age of this structure?
Parathyroid. Little oxyphil and arrangement in linear swirls
