Ed's Histology Notes: Thyroid and Parathyroid


Thyroxine (T4, "thyroid hormone") is a special amino acid which contains four iodine atoms. Tri-iodo-thyronine (T3) is the locally active form with one less iodine atom. Thyroid hormone serves a variety of useful purposes around the body. It is synthesized by modifying tyrosine moieties in a special glycoprotein (thyroglobulin) which is then stored in the thyroid gland and is available for hydrolysis as the hormone is needed.

The gland originates from the epithelium on the back of the tongue, and travels in the unborn child down into the front of the neck. Often the track survives as the "pyramidal lobe". You know the gross anatomy (two lobes, isthmus, thin fibrous capsule).

Because the thyroid is an endocrine gland, the connections to the surface (which would become the ducts of an endocrine gland) undergo apoptosis and the endocrine cells secrete their product into the local capillaries. (Every endocrine cell abuts a capillary, and the capillaries of endocrine glands are fenestrated.) The stroma (i.e., connective tissue framework) of endocrine glands is just substantial enough to carry the blood vessels and (sometimes) nerves to the cells.

Thyroid cells form follicles, spheres of epithelial cells (always single-layered in health, usually more-or-less cuboidal, variably tall or short). As you'd expect, the normal thyroid follicle cell is full of rough endoplasmic reticulum to synthesize protein, a Golgi apparatus to glycosylate the thyroglobulin, secretory vacuoles full of thyroglobulin, lamellipodia ("layer-feet") on the apical surfaces to engulf thyroglobulin for resorption, and lysosomes and phagosomes.

Iodine is pumped into the cell actively ("the iodine pump" in the basal portion of the follicle cell. The tyrosine units of thyroglobulin are modified into thyroid hormone while the thyroglobulin is in storage in the colloid.

TSH (thyrotropin, from the pituitary gland) makes thyroid follicle cells larger, causes them to resorb colloid and release thyroxine, causes them to soak up iodine better, and causes them to divide. The nervous system control of the thyroid, described by Junquira, isn't worth knowing about even it if it really happens.

A goiter just means a large thyroid. You may have excess thyroid cells (i.e., iodine deficiency, TSH-excess, or an antibody mimicking TSH), excess stroma, excess lymphocytes, excess macrophages, or excess colloid. You'll learn about each of these.

Thyroid hormone is necessary for brain development and growth in children (deficient kids are "cretins"), and maintaining mitochondrial function at any age. It's a terrible reflection on governments in the today's goiter belts that 5,000,000 children per year are doomed to mental retardation just from lack of iodine in the diet.

The C-cells (parafollicular cells) of the thyroid are visible between the follicles; you need a special stain for calcitonin to be certain about a cell's identity. These cells produce the vestigial calcium-lowering hormone calcitonin when serum ionized calcium gets high. This hormone is completely overshadowed by parathyroid hormone.


Most people have around four parathyroid glands, generally more-or-less where they're drawn in the anatomy books, but sometimes in surprising places; Junquira mentions only the common case of a parathyroid hiding inside the thyroid gland itself. Each is the size, shape, and color of a lentil.

For now, the function of parathyroid hormone is to raise the serum ionized calcium as needed. There is a continuous low-level secretion of the hormone, which will become much greater if something brings the ionized calcium levels down.

The parathyroids have a thin fibrous capsule, from which arises a very delicate connective tissue stroma, and in teens and adults, there's often some fat. The parenchymal cells are polygonal. The two best-known types are (1) chief cells, which are typical endocrine cells, and (2) oxyphils cells, which are packed with mitochondria, making them appear red and granular. Junquira does not mention the third type of cell, easily distinguished: (3) Water-clear cells (waterhole, etc.), packed with glycogen. Contrary to Junquira, any of these cells may or may not be producing parathyroid hormone at any time.


Secretory granules in the salivary glands


Be sure you know these words:

Parenchyma ("important stuff"): the business cells of any epithelially-based organ.

Stroma ("mattress"): the connective tissue of any epithelially-based organ.

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