keratinocytes
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Keratinocytes are the primary type of found in the , the outermost layer of the . In humans, they constitute 90% of epidermal skin cells. Basal cells in the of the skin are sometimes referred to as basal keratinocytes. Keratinocytes form a barrier against environmental damage by , , , , , , and . A number of structural , , , and contribute to maintain the important barrier function of the skin. Keratinocytes differentiate from epidermal in the lower part of the epidermis and migrate towards the surface, finally becoming and eventually be shed off, which happens every 40-56 days in humans.


Function

The primary function of keratinocytes is the formation of a barrier against environmental damage by , , , , , , and . Pathogens invading the upper layers of the epidermis can cause keratinocytes to produce mediators, particularly such as and (MCP-1) which attract s, s, , and s to the site of pathogen invasion.


Structure

A number of structural (, ), (), , and () contribute to maintain the important barrier function of the skin. Keratinization is part of the physical barrier formation (), in which the keratinocytes produce more and more keratin and undergo terminal differentiation. The fully cornified keratinocytes that form the outermost layer are constantly shed off and replaced by new cells.


Cell differentiation

Epidermal stem cells reside in the lower part of the epidermis (stratum basale) and are attached to the basement membrane through s. Epidermal stem cells divide in a random manner yielding either more stem cells or transit amplifying cells. Some of the transit amplifying cells continue to proliferate then commit to and migrate towards the surface of the epidermis. Those and their differentiated progeny are organized into columns named epidermal proliferation units. During this differentiation process, keratinocytes permanently withdraw from the , initiate expression of epidermal differentiation markers, and move suprabasally as they become part of the , , and eventually s in the . Corneocytes are keratinocytes that have completed their differentiation program and have lost their and ic . Corneocytes will eventually be shed off through as new ones come in. At each stage of differentiation, keratinocytes express specific , such as , , , and , but also other markers such as , , , filaggrin, and . In humans, it is estimated that keratinocytes from stem cells to desquamation every 40–56 days, whereas in the estimated is 8–10 days. Factors promoting keratinocyte differentiation are: * A gradient, with the lowest concentration in the stratum basale and increasing concentrations until the outer stratum granulosum, where it reaches its maximum. Calcium concentration in the stratum corneum is very high in part because those relatively dry cells are not able to dissolve the ions. Those elevations of calcium concentrations induces an increase in free calcium concentrations in keratinocytes. Part of that intracellular calcium increase comes from calcium released from intracellular stores and another part comes from transmembrane calcium influx, through both calcium-sensitive and voltage-independent cation channels permeable to calcium. Moreover, it has been suggested that an extracellular calcium-sensing (CaSR) also contributes to the rise in intracellular calcium concentration. * (cholecalciferol) regulates keratinocyte and differentiation mostly by modulating calcium concentrations and regulating the expression of involved in keratinocyte differentiation. Keratinocytes are the only cells in the body with the entire vitamin D metabolic pathway from vitamin D production to and expression. * . * TALE . * . Since keratinocyte differentiation inhibits keratinocyte proliferation, factors that promote keratinocyte proliferation should be considered as preventing differentiation. These factors include: * The p63, which prevents epidermal stem cells from differentiating into keratinocytes. * and its analogues. * . * . * .


Interaction with other cells

Within the epidermis keratinocytes are associated with other cell types such as and s. Keratinocytes form s with the nerves of the skin and hold the Langerhans cells and intra-dermal in position within the epidermis. Keratinocytes also modulate the : apart from the above-mentioned and they are also potent producers of anti-inflammatory mediators such as and . When activated, they can stimulate and Langerhans cell activation via and secretion. Keratinocytes contribute to protecting the body from radiation (UVR) by taking up s, vesicles containing the endogenous , from epidermal melanocytes. Each melanocyte in the epidermis has several s that stretch out to connect it with many keratinocytes. The melanin is then stored within keratinocytes and melanocytes in the perinuclear area as supranuclear “caps”, where it protects the from UVR-induced .


Role in wound healing

to the will be repaired in part by the migration of keratinocytes to fill in the gap created by the wound. The first set of keratinocytes to participate in that repair come from the bulge region of the and will only survive transiently. Within the healed epidermis they will be replaced by keratinocytes originating from the epidermis. At the opposite, epidermal keratinocytes, can contribute to ''de novo'' hair follicle formation during the healing of large wounds. Functional keratinocytes are needed for tympanic perforation healing.


Sunburn cells

A sunburn is a keratinocyte with a and that appears after exposure to or radiation or in the presence of s. It shows premature and abnormal , and has been described as an example of .


Aging

With age, tissue declines partly because fail to self-renew or . caused by exposure of stem/progenitor cells to (ROS) may play a key role in aging. Mitochondrial superoxide dismutase () ordinarily protects against ROS. Loss of SOD2 in mouse epidermal cells was observed to cause cellular that irreversibly arrested proliferation in a fraction of keratinocytes. In older mice, SOD2 deficiency delayed closure and reduced epidermal thickness.


Civatte body

A Civatte body (named after the French dermatologist Achille Civatte, 1877–1956) is a damaged basal keratinocyte that has undergone , and consist largely of keratin intermediate filaments, and are almost invariably covered with s, mainly IgM. Civatte bodies are characteristically found in skin lesions of various , particularly and . They may also be found in , s, inflammatory (such as lichenoid and -like keratosis), , , , , , , and lesions, , , , , and .


See also

* * * * * * *


References


External links

* {{Skin Layers and Appendages Human cells