What is the Difference Between Follicular and Parafollicular Cells

The main difference between follicular and parafollicular cells is that follicular cells secrete thyroxine (T4) and triiodothyronine (T3), which are responsible for regulating the metabolism of the body. Meanwhile, parafollicular cells secrete the hormone calcitonin, which is responsible for the regulation of calcium levels of the body. Furthermore, follicular cells are the major type of cells, while parafollicular cells are the minor type of cells. 

In brief, follicular and parafollicular cells are the two types of cells that occur in the thyroid tissue. The thyroid tissue belongs to the thyroid gland, an endocrine gland located in front of the neck. 

Key Areas Covered 

1. What are Follicular Cells
     – Definition, Anatomy, Function
2. What are Parafollicular Cells
     – Definition, Anatomy, Function
3. What are the Similarities Between Follicular and Parafollicular Cells
     – Outline of Common Features
4. What is the Difference Between Follicular and Parafollicular Cells
     – Comparison of Key Differences

Key Terms 

Calcitonin, Follicular Cells, Parafollicular Cells, Thyroid Gland, Thyroxine, Triiodothyronine

What are Follicular Cells 

Follicular cells (thyrocytes or thyroid epithelial cells) are the major cell types of the thyroid gland. They make up simple cuboidal epithelium arranged in spherical follicles surrounding an interior space known as follicular lumen. Here, the follicular lumen is a fluid-filled space known as the colloid. Besides, the basolateral membrane of follicular cells contains thyrotropin receptors that bind to thyroid-stimulating hormone (TSH) found circulating in the blood. 

Figure 1: Thyroid Hormone Synthesis

Furthermore, follicular cells take up iodide and amino acids in order to synthesize thyroglobulin and thyroperoxidase, which are then secreted into the thyroid follicles along with iodide. After undergoing, oxidation, iodination, and conjugation, thyroglobulin enters into the follicular cells that extract thyroid hormones and release them into the blood. Of these, the two types of thyroid hormones releasing are thyroxine (T4) and triiodothyronine (T3). Besides, a normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 is about four times as potent as T4. Also, the main function of the thyroid hormones is to regulate the metabolism of the body;  the conversion of oxygen and carbohydrates to energy. 

What are Parafollicular Cells 

Parafollicular cells or C cells are the neuroendocrine cells in the thyroid gland. Also, they are the minor type of cells interspersed between follicular cells. In exact terms, they occur adjacent to the thyroid follicles, residing in the connective tissue. In addition to that, parafollicular cells are larger than the follicular cells and their cytoplasm is strained in pale color with respect to the cytoplasm of the follicular cells. Therefore, they do not have direct contact with the follicular lumen.

Figure 2: Follicular and Parafollicular Cells

Moreover, the main function of parafollicular cells is to secrete calcitonin, which regulates calcium metabolism of the body. Specifically, calcitonin lowers blood calcium levels by inhibiting the resorption of bone by osteoclasts, proportionally to the blood calcium concentration. In addition to that, parafollicular cells secrete several neuroendocrine peptides such as serotonin, somatostatin or CGRP in small quantities. 

Similarities Between Follicular and Parafollicular Cells 

• Follicular and parafollicular cells are the two types of cells that occur in the thyroid gland. Here, the thyroid gland is an endocrine gland located in the front of the neck. Also, it consists of two connected lobes. 
• Also, the thyroid gland consists of spherical thyroid follicles, lined with follicular cells (thyrocytes), and occasional parafollicular cells that surround a lumen containing colloid.  
• Furthermore, both types of cells produce and secrete hormones responsible for regulating the metabolism of the body. 

Difference Between Follicular and Parafollicular Cells 

Definition 

Follicular cells refer to the major types of cells in the thyroid gland, producing and secreting thyroid hormones thyroxine (T4) and triiodothyronine (T3). Meanwhile, parafollicular cells refer to the neuroendocrine cells in the thyroid, primarily responsible for secreting calcitonin. 

Abundance 

Follicular cells are the major type of cells in the thyroid gland, while parafollicular cells are the minor type of cells. 

Occurrence 

Also, follicular cells are arranged in spherical follicles, surrounding the follicular lumen, while parafollicular cells occur adjacent to the thyroid follicles, residing in the connective tissue.  

Contact to the Follicular Lumen 

Furthermore, follicular cells have direct contact with the follicular lumen, while parafollicular cells do not have direct contact with the follicular lumen. 

Identification 

Moreover, the cytoplasm of the follicular cells is stained heavily using H&E, while the cytoplasm of the parafollicular cells is stained lighter using H&E. 

Secreting Hormones 

Besides, follicular cells secrete thyroxine (T4) and triiodothyronine (T3), while parafollicular cells secrete calcitonin. 

Function 

Functionally, thyroxine and triiodothyronine regulate the metabolism of the body, while calcitonin regulates calcium metabolism. 

Conclusion 

In summary, follicular cells are the major types of cells in the thyroid gland. Also, they occur around the thyroid follicle. In addition to that, follicular cells secrete two thyroid hormones. Basically, they are the thyroxine (T4) and triiodothyronine (T3), which are mainly responsible for regulating the metabolism of the body. In contrast, parafollicular cells are the minor types of cells in the thyroid gland, occurring occasionally with follicular cells. However, parafollicular cells secrete calcitonin, which regulates calcium metabolism. Therefore, the main difference between follicular and parafollicular cells is the type of hormones secreting. 

References:

1. “The Thyroid Gland.” Johns Hopkins Medicine, The Johns Hopkins University, Available Here.

Image Courtesy:

1. “Thyroid hormone synthesis” By Mikael Häggström. (Häggström, Mikael (2014). “Medical gallery of Mikael Häggström 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. (CC0) via Commons Wikimedia  
2. “Tiroides 03 100X” By Andrea Mazza – Own work (CC BY-SA 3.0) via Commons Wikimedia