Can you identify the thyroid follicles, colloid and septa in this
low power image of the thyroid?
Follicular cells - are almost columnar in appearance in some regions,
whilst elsewhere they have a low cuboidal appearance.
This is because in active glands, the follicles are smaller, and
have reduced colloid - the cuboidal lining cells are relatively
tall because they are actively making and secreting hormones - so
packed full of ER and golgi.
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'.
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.