Classification of endocrinocytes
Glandular endocrinocytes are located in the walls of follicles and within the extrafollicular epithelium. Three main cell types are distinguished:
- Follicular cells (thyrocytes) — make up the vast majority of the entire population.
- Oxyphilic Askanazy cells (also known as Hürthle cells) — their number increases with age, although their precise origin and functions remain unclear.
- Parafollicular cells (referred to as C cells or calcitoninocytes) — a rare population accounting for only about 0.1%.
Morphology of follicular hyperactivation
During enhanced organ activity, colloid resorption processes begin to prevail over the rate of its formation. Due to this, the total volume of colloid and the sizes of the follicles themselves noticeably decrease. Large resorption vacuoles form within the colloid, and the mitotic activity of thyrocytes increases. The main process determining such morphology is the rate of colloid resorption, which serves as the primary target for the sympathetic nervous system and thyroid-stimulating hormone (TSH).
Structure and origin of parafollicular cells
Embryologically, parafollicular cells develop from the neural crest. They can lie in groups between follicles or be part of the follicular epithelium. If a cell is located in the follicular wall, it has a triangular shape. Such cells are larger than thyrocytes, and their apical surface does not reach the follicular lumen.
Mitochondria, the Golgi apparatus, and the rough endoplasmic reticulum are well developed in the cytoplasm. Depending on the cell type, they contain specific granules detected by silver nitrate or osmium:
Effects and regulation of thyrocalcitonin
Thyrocalcitonin is synthesized by parafollicular C cells and actively participates in calcium metabolism. cAMP acts as a secondary messenger in this process.
- Lowers blood calcium levels.
- Inhibits calcium excretion from bones, promoting its accumulation in bone tissue.
- Prevents the reabsorption of calcium and phosphates from the renal tubules, accelerating their excretion in the urine.
- Decreases the concentration of calcium inside the cell cytoplasm due to the activation of the plasmalemma calcium pump and stimulation of ion uptake by mitochondria.
The hormone concentration increases during pregnancy, breastfeeding, and periods of bone recovery after fractures.
C cells = Strongest calcium-lowering agent (Lowers blood calcium, Arise from the neural crest, Make up hundredths of a percent).
Class and exam questions
Can parafollicular cells be called parathyrocytes?
No, you cannot. The term "parathyrocytes" is reserved exclusively for the cells of the parathyroid glands, despite the outer similarity of the names.
What types of parafollicular cells are distinguished by granule composition?
The first type contains small, well-stained osmium granules and produces calcitonin. The second type contains large, poorly stained osmium granules and synthesizes growth hormone. They can also secrete norepinephrine and serotonin.
What happens to follicle sizes during gland hyperfunction?
Follicle sizes decrease because enhanced colloid resorption begins to exceed the rate of its production. Large resorption vacuoles appear in the colloid itself.
What else to study on this topic
- Origin and age-related dynamics of oxyphilic Askanazy cells
- Ultrastructural features of the rough endoplasmic reticulum and Golgi apparatus in C cells
- The role of the sympathetic nervous system in the regulation of colloid resorption
- Mechanisms of action of the secondary messenger cAMP in calcitoninocytes
- Comparative characteristics of secretory granules of the first and second types of parafollicular cells
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