What are Parafollicular Cells?
Parafollicular cells, also known as
C cells, are a type of neuroendocrine cell found in the thyroid gland. They are located within the connective tissue surrounding the thyroid follicles but are not part of the follicular epithelium. These cells are responsible for the secretion of the hormone
calcitonin, which plays a crucial role in calcium homeostasis.
What is the Role of Calcitonin?
Calcitonin is a hormone produced by parafollicular cells that helps regulate calcium levels in the blood. It is particularly important in lowering blood calcium levels by inhibiting osteoclast activity in bones, reducing calcium absorption in the intestines, and increasing calcium excretion in the kidneys. This hormone works in opposition to
parathyroid hormone (PTH), which increases blood calcium levels.
Where are Parafollicular Cells Located?
Parafollicular cells are dispersed throughout the thyroid gland, primarily located in the interstitial spaces between thyroid follicles. They originate from the neural crest cells and migrate to the thyroid during embryonic development. These cells can be found individually or in small clusters and are usually more abundant in the central regions of the thyroid lobes.
How Do Parafollicular Cells Differ from Follicular Cells?
Parafollicular cells differ from
follicular cells in several ways. While follicular cells line the thyroid follicles and are involved in the production of thyroid hormones (T3 and T4), parafollicular cells are located between the follicles and secrete calcitonin. Additionally, parafollicular cells are derived from neural crest cells, whereas follicular cells originate from endodermal tissue.
What is the Developmental Origin of Parafollicular Cells?
The developmental origin of parafollicular cells is the neural crest, a group of cells that emerge from the embryonic ectoderm and migrate to various parts of the body. During embryogenesis, neural crest cells migrate to the ultimobranchial bodies, which later fuse with the developing thyroid gland. This fusion integrates the parafollicular cells into the thyroid tissue.
What are the Histological Features of Parafollicular Cells?
Histologically, parafollicular cells have several distinguishing features. They are polygonal or oval in shape with a large, centrally located nucleus. The cytoplasm of these cells is typically clear or pale due to the presence of secretory granules containing calcitonin. Electron microscopy reveals numerous dense-core secretory granules, which are indicative of their neuroendocrine nature.
Can Parafollicular Cells be Found in Other Organs?
While parafollicular cells are primarily found in the thyroid gland, similar neuroendocrine cells that secrete calcitonin can be found in other organs as well. For example, calcitonin-producing cells have been identified in the lungs and gastrointestinal tract. However, their primary role and highest concentration are within the thyroid gland.
Conclusion
Parafollicular cells play a vital role in maintaining calcium homeostasis through the secretion of calcitonin. Their unique histological characteristics, developmental origin, and clinical significance, particularly in the context of medullary thyroid carcinoma, make them an important subject of study in histology and medicine. Identifying and understanding these cells contribute significantly to our knowledge of thyroid function and endocrine pathology.
