The thyroid gland contains two main types of cells: follicular cells that produce thyroxin and C cells that produce calcitonin. A thin fibrous capsule with blood vessels, lymphatics, and nerves, most prominent at the poles, encloses the thyroid. The histologic appearance of thyroid follicles and colloid varies greatly as a reflection of secretory activity. Variation in follicle size is common, with the larger follicles tending to be at the periphery of the gland (Figure 1). Follicular epithelial lining ranges from flattened to cuboidal (Figure 2 , Figure 3 , Figure 4 , and Figure 5). Active follicles are typically lined by more cuboidal epithelium (Figure 4 and Figure 5) and may have resorption vacuoles at the interface of the epithelia and the colloid (Figure 3). As mice age, their follicles become less active, more distended, and lined by flattened epithelium. Tinctorial variations in colloid are commonly seen.
C cells occur singly or in small clusters and increase in number and prominence with age. C cells are readily identified in rats but not prominent in routine H&E sections of mouse thyroid.
As a highly reactive organ, the thyroid may be a direct or indirect target of insult by many physical, chemical, or environmental agents. Since effects on the thyroid may be subtle or age related, it is important to assess changes with respect to concurrent age-matched controls. For common age-related changes, a diagnosis may not be necessary unless there is a potential treatment-related effect.
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