Histology of the Thyroid Gland A Introduction The thyroid hormone is derived The thyroid gland is responsible for the secretion of the from the amino acid tyrosine thyroid hormone that controls the basal metabolic rate in (iodinated tyrosine residue). our body. Structurally, thyroid gland secretory cells are arranged in circular unites, which are called thyroid follicles. These follicles are lined by a single layer of secretory cells, and these secretory cells secrete thyroid hormones. In the center of each follicle, there is a fluid called colloid. One of the unique features of the thyroid gland is that it stores its secretions before the release to the blood stream. The follicular cells will synthesize the thyroid hormone, but instead of releasing it to the blood stream directly, they will store it inside the follicles (the colloid), once it is needed, the follicular cells will uptake the thyroid hormones form the colloid by endocytosis and release them to the blood capillaries that are present between the follicles (the whitish places in the pic). The follicular cells are epithelial in origin and have an apical surface and basal surface, the apical surface faces the colloid while the basal surface faces the capillaries. The thyroid hormones are : The number is designated to the T3 (3 iodine molecules on tyrosine residue). number of iodine molecules T4 (4 iodine molecules on tyrosine residue). attached to the tyrosine residue. Thyroid hormones are not stored as T3 or T4 alone inside the colloid, however, inside the colloid there is a very large protein called thyroglobulin, this protein contains very large amount of iodinated tyrosine residues, once the thyroid hormones is needed the follicular cells take the thyroglobulin and cleave it into T3 and T4 then it will be secreted into the blood stream. 1
; The shape of the thyroid follicular cells is different according to the activity of the cell; Squamous or flattened cells: this indicates an inactive or hypothyroidism status (don t produce a lot of thyroid hormones). Columnar cells: this indicates that the cells are in overactive or hyperactive status. The thyroid gland has another sit of cells which are called the Parafollicular cells. These cells are located (as the name indicated) between the follicles, but it is misnamed because they can be found normally in the wall of the follicle itself, but mostly they are found between the follicles. They are responsible for the production of calcitonin hormone. Calcitonin is a hormone that is related to the calcium metabolism, it decreases the Ca+2 concentration in the blood (so the Parafollicular cells can be named as C cells or clear cells) The Parafollicular cell in the wall of the follicle, it could be bi-nucleated The Parafollicular cell is between the follicles The follicular cells More numerous but smaller in size. Darker staining. The Parafollicular cells Smaller in number but larger in size. The cytoplasm is paler in comparison with the follicular cells. 2
A The embryology of the thyroid gland Most of the endocrine glands have a dual embryological origin. The follicular cells are derived from the endodermal layer of the pharyngeal pouches (at the base of the tongue), while the Parafollicular cells are derived from the neural crest cells which are found in the last pharyngeal pouch (also known as ultimobranchial body). Some Anatomists consider the last pharyngeal pouch as the 5 th pouch, but others consider it as a part of the 4 th pouch. These pictures show you the histology of the thyroid gland: C: clear cells or Parafollicular cells. F: follicular cells. Here we can see electron microscopic pictures for the Parafollicular cell: We can see the bi-nucleation of the cell. The calcitonin hormones are stored in a very small vesicles so the cell will have a vesicular cytoplasm. 3
A The parathyroid gland We have usually 4 parathyroid glands (2 superior and 2 inferior), but sometimes maybe less or maybe more. Usually located in the posterior surface of the thyroid gland, but sometimes they can be found embedded in the thyroid gland. The superior parathyroid glands are derived from the 4 th pharyngeal pouch, whereas the inferior parathyroid glands are derived from the 3 rd pharyngeal pouch. The thymus gland is originated form the 3 rd pouch, and it will migrate all the way down to the mediastinum behind the sternum, but during its migration, it will pull the inferior parathyroid glands with it, so sometimes you can find the inferior parathyroid glands in variable locations due to the migration. The parathyroid gland secretes parathyroid hormones or parathormons. The parathyroid hormone is important to increase the Ca+2 concentration in the blood. Under the microscope, the secretory part of the parathyroid glands is divided into two groups: The chief cells: ( P the last pic) - A group of cells with small nuclei aggregated to each other. - Small cells and pale. - Contain a lot of secretory granules which contain the parathyroid hormones. The Oxyphilic cells: - Small in number and they are away from each other. - Larger in size in comparison with chief cells. - They stain heavily acidophilic, because they have abnormally shaped mitochondria. - They synthesize parathyroid hormones but in a very low amount. 4
A The pancreas It is a mixed gland having exocrine glands and endocrine glands. Pancreatic Islets The endocrine part is islet of endocrine cells scattered between the exocrine parts (called islet of Langerhans). They are compact spherical masses all around the pancreas. Each islet of Langerhans is surrounded by a thin connective tissue layer to separate it from the remaining acinus cells. The tail of the pancreas is the most common place that contains islets of Langerhans. In the pancreas, there is about one million islets (only 1-2% of the weight of the pancreas). In the pancreas, there is no dual origin in the embryological development, so the exocrine part and the endocrine part are derived from endoderm of the GIT. The type of cells in the islet: 1. cells: - They are about 80% of the cells of the islet of Langerhans. - Concentrated in the center of the islet. - They secrete insulin. 2. cells: - Constitute about 15% of the islet. - They are usually peripheral in location. - Secrete glucagon. 3. cells: - Constitute about 5-10%. - They are scattered. - Secrete somatostatin, which is an inhibitory hormone that will inhibit the release of growth hormone, it also inhibits cells and cells, so they control the secretion of the islet itself. 5
4. PP (F) cells: - Constitute less than 1%. - Scattered. - They secrete pancreatic polypeptides, which inhibit the exocrine secretion of the pancreas. 5. DNES cells (diffuse neuroendocrine cells): - The old name for them is APUD cells. - Contain G cells which secrete gastrin and also contain entero-chromaffin cells which secrete serotonin. Under the light microscope you can t differentiate between these types of cells, because all of these cells are lightly stained, and they are also peptide hormone secreting cells so all of them will have RER, well developed Golgi and secretory granules, but to differentiate between them, you need specific immunohistochemistry techniques. A Pineal Gland It is located in the cranial cavity in the brain and it is related to the central nervous system. It is named because of its shape. The circadian rhythm means It is one of the glands that controls the circadian the difference between the rhythm of the human body. day and night rhythmic The pineal gland is connected to the retina, so the activity and the metabolic pineal gland can know the day and night. activity of the human body. At the night time the pineal gland will secrete the hormone melatonin. Melatonin is derived from the During day time the gland will secrete serotonin, which amino acid tryptophan which is a stimulatory neurotransmitter. is found mainly in milk. The secretory cells of the pineal gland are called pinealocytes, which secrete melatonin and serotonin (amino acid producing cells), so they contain RER, Golgi apparatus and Euchromatic nucleus. 6
These cells are branched, they have cytoplasmic processes at their end, they are also dilated and these dilatation contain the secretory granules. In this figure you can see the pinealocytes (Pi), and also there is another small population of cells called interstitial cells. The interstitial cells are smaller in number, they are located between the pinealocytes and they are supporting cells. Any supporting cell in the CNS is called a glial cell, the interstitial cells are considered as glial supporting cells that support the pinealocytes (they resemble glial cell in the brain which is called astrocyte). Also, you can see a large dark masses that contain concretions of calcium and magnesium crystals, we call these masses brain sands (BS) or corpora arenacea. Transformers Team A 7