Non-Keratinocytes (Note & Video)
The oral epithelium, apart from keratinocytes, also harbors a small population of cells called non-keratinocytes, that make up 10% of the population.
Unlike the keratinocytes, non-keratinocytes lack tonofilaments and desmosomes, do not undergo maturation like a keratinocyte nor do they desquamate.
During histologic preparations for routine light microscopy, the cytoplasm of non-keratinocytes shrink around their nuclei.
This causes a halo around their nuclei making the cytoplasm look clear.
Non-keratinocytes are hence also referred to as “clear cells“.
TYPES OF NON-KERATINOCYTES
Non keratinocytes include 4 different types
Melanocytes
Langerhans cells
Merkel cells
Inflammatory cells (primarily lymphocytes)
💡KNOW THY FACTS
Do not confuse between what non-keratinocytes are and what a non-keratinized epithelium is. The non-keratinized epithelium lines the lining mucosa and is so named because it lacks the keratinized or corneal layer. The non-keratinized layer is made of epithelial cells/keratinocytes.MELANOCYTES
Melanocytes are melanin (pigment) producing cells located in the basal layer.
Melanocytes are derived from neural crest cells.
They are dendritic cells, with their dendritic processes passing through several layers of the epithelium in between keratinocytes.
Melanocytes associate with keratinocytes by forming a epidermal melanin unit.
A melanocyte through its long dendritic processes can establish contact with about 30-40 keratinocytes.
💡 FUNCTION OF MELANOCYTES
- The main function of a melanocyte is to form melanin pigment.
- Melanin is packed and stored in intracytoplasmic granules called melanosomes.
- These melanosomes are transferred to cytoplasm of adjacent keratinocytes.
- Pigmentation can be usually seen in the gingiva, hard palate, tongue and buccal mucosa.Under light microscopy, melanin pigment can sometimes be seen dispersed in focal areas in the connective tissue.
These are actually macrophages that have engulfed dispersed melanosomes (hence making them pigmented).
Such macrophages are termed melanophages.
Under light microscopy, melanin pigment can sometimes be seen dispersed in focal areas in the connective tissue.
These are actually macrophages that have engulfed dispersed melanosomes (hence making them pigmented).
Such macrophages are termed melanophages.
💡 KNOW THY FACTS
The difference in pigmentation seen in dark and light skinned individuals is not due to the difference in the number of melanocytes. The number is more or less the same; the difference lies in the activity of the melanocyte i.e in the amount of melanin produced in the individual.LANGERHANS CELLS
Langerhans cells are immune cells that are hematopoietic in origin.
They are dendritic and are also called “Antigen Presenting Cells (APC)“.
Langerhans cells enter the connective tissue through blood vessels and then penetrate the epithelium.
Langerhans cells are usually found above the basal layers.
Ultra-structurally, Langerhans cells have characteristic rod like granules called “Birbeck granules“, in their cytoplasm.
💡 FUNCTION OF LANGERHANS CELLS
- Langerhans cells are also called dendritic cells and play an important role in immunity.
- Their main function is to ingest external antigens that penetrate the epithelium, break them to peptides and present them to T-cells (lymphocyte). - Hence the name "Antigen presenting cell".
- They either present it to T-cells in the connective tissue or migrate to the nearest lymph node to present the antigens.MERKEL CELLS
Merkel cells are located in the basal layer of the oral epithelium.
They are more common in the masticatory epithelia like the gingiva.
Unlike melanocytes and Langerhans cells, Merkel cells do not possess dendritic processes.
Merkel cells also differ from other non-keratinocytes in that they may possess tonofilaments and desmosomes. Hence their origin is suspected to be epithelial. In fact, Merkel cells can be identified by immuno-histochemical staining. They stain for CK 8/18 and 20.
💡 FUNCTION OF MERKEL CELLS
Merkel cells are thought to have a sensory function.
Merkel cells are usually associated with an axon terminal (nerve tissue) in the connective tissue.
Ultra-structurally, they posses numerous granules, concentrated near the axon terminal.
These granules are thought to release a transmitter like substance at this junction (between the Merkel cell and the axon terminal) to trigger an impulse.INFLAMMATORY CELLS
The oral epithelium may occasionally show few inflammatory cells in various layers.
These are mostly lymphocytes.
These cells migrate from the connective tissue and infiltrate the epithelium.
Inflammatory cells have an immunologic function and participate in innate and adaptive immune responses.
HIGHLIGHTS - VIVA & ENTRANCE EXAM PERSPECTIVE
The oral epithelium, apart from keratinocytes, also harbors a small population of cells called non-keratinocytes, that make up 10% of the population.
Non-keratinocytes lack tonofilaments and desmosomes.
Non-keratinocytes are also referred to as “clear cells“.
Non keratinocytes include 4 different types -> a) Melanocytes, b) Langerhans cells, c) Merkel cells and d) Inflammatory cells (primarily lymphocytes).
Melanocytes are melanin (pigment) producing cells located in the basal layer.
Melanocytes associate with keratinocytes by forming a epidermal melanin unit.
A melanocyte through its long dendritic processes can establish contact with about 30-40 keratinocytes.
The main function of a melanocyte is to form melanin pigment.
Melanin is packed and stored in intracytoplasmic granules called melanosomes.
Langerhans cells are dendritic and are also called “Antigen Presenting Cells (APC)“.
Ultra-structurally, Langerhans cells have characteristic rod like granules called “Birbeck granules“, in their cytoplasm.
The main function of a Langerhans cell is to ingest any external antigen that penetrates the epithelium, break them to peptides and present it to T-cells (lymphocyte).
Merkel cells are located in the basal layer of the oral epithelium.
Merkel cells are thought to have a sensory function.
The oral epithelium may occasionally show few inflammatory cells in various layers. These are mostly lymphocytes.
REFERENCES AND FURTHER READING
Berkovitz BKB, Hollan GR, Moxham BJ. Oral Anatomy, Histology and Embryology. 4th ed. Mosby Elsevier; 2009.
Nanci A. Tencate’s Oral Histology. Development, Structure and Function. 8th ed. Elsevier; 2013.
Kumar GS. Orban’s Oral Histology and Embryology.13th ed. Elsevier; 2011.
Avery JK. Oral development and Histology. 3rd ed. Thieme Medical Publishers; 2002.
Feller L, Masilana A, Khammissa RA, Altini M, Jadwat Y, Lemmer J. Melanin: the biophysiology of oral melanocytes and physiological oral pigmentation. Head & Face Medicine. 2014;10:8.
Righi A, Betts CM, Marchetti C, Marucci G, Montebugnoli L, Prati C, Eusebi LH, Muzzi L, Ragazzini T, Foschini MP. Merkel cells in the oral mucosa. Int J Surg Pathol. 2006 Jul;14(3):206-11.