Thyroid gland

The thyroid gland is unusual, in that the hormones are stored in cavities, surrounded by secretory cells, which make up a 'follicle'. To secrete the hormone, the hormone is re-absorbed from the cavity, and then released into the surrounding interstitial spaces. The stored hormone is bound to a glycoprotein, and this stored hormone is called 'colloid'.

This gland secretes iodine containing hormones called Tri-iodo thyronine (T3) and thyroxine (T4) of which T3 is more active. It regulates the basal metabolic rate, and it is regulated by the pituitary hormone TSH. It also secretes calcitonin - which regulates blood calcium levels. Secretion of calcitonin causes blood calcium levels to drop, and its secretion is directly dependent on blood calcium levels.

Colloid is an inactive precursor of T3 and T4. It is made up of a glycoprotein called thyroglobulin, made by the epithelial cells, which is bound to iodine. The iodine binds to the tyrosine residues of thyroglobulin.

Now have a look at this eMicroscope of a higher power image of the thyroid gland.

Can you identify the thyroid follicles, the simple cuboidal epithelium that lines the follicles, colloid, and can you find a 'clear cell'.

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The clear cells are parafollicular cells that are scattered among the follicular cells. As you might guess from their name, they have a pale cytoplasm. They are only found in the middle third or lateral lobes of the thyroid, and they have a different embryological origin to the follicular cells. These cells secrete calcitonin in response to increased levels of blood calcium.

In the condition known as hyperthyroidism, the thyroid becomes enlarged, and hyperactive, and the follicles look smaller.

A higher resolution version of this image (without labels) may also be viewed with the Zoomify viewer.

The parathyroid gland is embedded in the capsule of the thyroid gland. It contains two types of cells - chief or principle cells and oxyphil cells. The chief cells are small and pale eosinophilic staining. They secrete parathyroid hormone (PTH). Resting cells have paler cytoplasm than active cells. In adults about 80% of the cells are resting. In children, more cells are active.

PTH acts on osteoclasts, and the epithelial cells of the renal tubule, to increase plasma calcium by promoting bone resorption and increasing renal calcium resorption.