Histology of thyroid gland and it's unique structure Thyroid follicle
Endocrine gland usually have a cord-like arrangement and their products to be secreted are kept within the individual cells. The thyroid gland is an exception to this rule.
It is encased by a thin connective tissue capsule that enters the substance of the lobes to further subdivide the gland into irregular lobular units. Each lobule contains a cluster of follicles, which are the structural and functional units of the thyroid gland.
Each thyroid follicle is surrounded by thin connective tissue stroma rich in fenestrated capillaries (along with the sympathetic nerves that innervate them) and lymphatics. Follicular epithelium is a simple epithelium consisting of low columnar, cuboidal or squamous cells depending on the level of activity of the follicle. When they are active, they appear cuboidal to low columnar, but when they are inactive the cells are squamous.
These follicular (principal) cells take up the necessary amino acid precursors and iodine at its basolateral surface and release the final product into the blood stream at its basal end. Follicular cells are responsible for producing thyroglobulin (an iodine rich, inactive form of the thyroid hormones), which is then stored as a semi-solid substance (colloid) in the lumen of the follicles.
The colloid stains pink with haematoxylin and eosin (H&E) staining, while the follicular cells have a purple appearance. The degree of activity of a follicle can also be assessed based on the amount and appearance of colloid it contains. Inactive follicular lumina are larger; colloid is abundant and appears solid. In contrast, active follicular lumina are smaller and there is little to no colloid present.
Another cell type that can be identified on histological preparations of thyroid tissue is parafollicular cells, also known as C (clear) cells. C-cells appear clear due to the fact that they are lightly stained on H&E preparation. They can be found within the basal lamina of the thyroid follicles without extending into the follicular lumen or between thyroid follicles in the interfollicular space, either singly or in the form of groups.
Parafollicular cells are a subtype of neuroendocrine cells (amine precursor uptake and decarboxylation – AUPD – system) that produce thyrocalcitonin (calcitonin). This hormone aids in the regulation of blood calcium levels by downregulating bone resorption (breakdown of bone and subsequent release of minerals into the blood) and limiting calcium reuptake in the kidneys.
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