Keratinocyte
Encyclopedia
Keratinocytes are the predominant cell
type in the epidermis, the outermost layer of the human skin
, constituting 95% of the cells found there. Those keratinocytes found in the basal layer (Stratum germinativum
) of the skin are sometimes referred to as "basal cells" or "basal keratinocytes". The primary function of keratinocytes is the formation of a barrier against environmental damage such as pathogens (bacteria, fungi, parasites, viruses), heat, UV radiation and water loss. A number of structural proteins (filaggrin, keratin), enzymes (proteases), lipids and antimicrobial peptides (defensins) contribute to maintain the important barrier function of the skin. Once pathogens start to invade the upper layers of the epidermis, keratinocytes can react with the production of proinflammatory mediators and in particular chemokines such as CXCL10
, CCL2
which attract leukocytes to the site of pathogen invasion.
Keratinization is part of the physical barrier formation cornification, in which the keratinocytes produce more and more keratin and eventually undergo programmed cell death
. The fully cornified keratinocytes that form the outermost layer are constantly shed off and replaced by new cells. The average renewal / turnover time for the epidermis is 21 days.
Keratinocytes form tight junction
s with the nerves of the skin and hold the Langerhans cell
s and intra-dermal lymphocytes in position within the epidermis. Keratinocytes also modulate the immune system
: apart from the above mentioned antimicrobial peptides and chemokines they are also potent producers of anti-inflammatory mediators such as IL-10 and TGF-β. When activated, they can stimulate cutaneous inflammation
and Langerhans cell activation via TNFα and IL-1β
secretion.
Keratinocytes contribute to protecting the body from ultraviolet
radiation (UVR) by taking up melanosome
s, vesicles containing the endogenous photoprotectant
melanin
, from epidermal melanocyte
s. Each melanocyte in the epidermis has several dendrite
s that stretch out to connect it with many keratinocytes. The melanin is then stored in the keratinocytes' nuclei, where it protects the DNA
from UVR-induced damage.
Keratinocytes migrate with a rolling motion during the process of wound healing.
cytoplasm
that appears after exposure to UVC or UVB radiation or UVA in the presence of psoralen
s. It shows premature and abnormal keratinization, and has been described as an example of apoptosis
.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
type in the epidermis, the outermost layer of the human skin
Human skin
The human skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system. The skin has multiple layers of ectodermal tissue and guards the underlying muscles, bones, ligaments and internal organs. Human skin is similar to that of most other mammals,...
, constituting 95% of the cells found there. Those keratinocytes found in the basal layer (Stratum germinativum
Stratum germinativum
The stratum basale is the deepest layer of the five layers of the epidermis, which is the outer covering of skin in mammals. The stratum basale is a continuous layer of cells...
) of the skin are sometimes referred to as "basal cells" or "basal keratinocytes". The primary function of keratinocytes is the formation of a barrier against environmental damage such as pathogens (bacteria, fungi, parasites, viruses), heat, UV radiation and water loss. A number of structural proteins (filaggrin, keratin), enzymes (proteases), lipids and antimicrobial peptides (defensins) contribute to maintain the important barrier function of the skin. Once pathogens start to invade the upper layers of the epidermis, keratinocytes can react with the production of proinflammatory mediators and in particular chemokines such as CXCL10
CXCL10
C-X-C motif chemokine 10 also known as Interferon gamma-induced protein 10 or small-inducible cytokine B10 is an 8.7 kDa protein that in humans is encoded by the CXCL10 gene...
, CCL2
CCL2
For the ICAO airport code see Candle Lake Airpark, for the diradical compound see Dichlorocarbene.Chemokine ligand 2 also known as monocyte chemotactic protein-1 or small inducible cytokine A2 is a protein that in humans is encoded by the CCL2 gene. CCL2 is a small cytokine belonging to the CC...
which attract leukocytes to the site of pathogen invasion.
Keratinization is part of the physical barrier formation cornification, in which the keratinocytes produce more and more keratin and eventually undergo programmed cell death
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
. The fully cornified keratinocytes that form the outermost layer are constantly shed off and replaced by new cells. The average renewal / turnover time for the epidermis is 21 days.
Keratinocytes form tight junction
Tight junction
Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. It is a type of junctional complex present only in vertebrates...
s with the nerves of the skin and hold the Langerhans cell
Langerhans cell
Langerhans cells are dendritic cells of the skin and mucosa, and contain large granules called Birbeck granules. They are present in all layers of the epidermis, but are most prominant in the stratum spinosum. They also occur in the papillary dermis, particularly around blood vessels, as well as...
s and intra-dermal lymphocytes in position within the epidermis. Keratinocytes also modulate the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
: apart from the above mentioned antimicrobial peptides and chemokines they are also potent producers of anti-inflammatory mediators such as IL-10 and TGF-β. When activated, they can stimulate cutaneous inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
and Langerhans cell activation via TNFα and IL-1β
IL1B
Interleukin-1 beta also known as catabolin, is a cytokine protein that in humans is encoded by the IL1B gene. IL-1β precursor is cleaved by caspase 1 . Cytosolic thiol protease cleaves the product to form mature IL-1β.- Function :Interleukin 1 was discovered by Gery in 1972...
secretion.
Keratinocytes contribute to protecting the body from ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation (UVR) by taking up melanosome
Melanosome
In a biological cell, a melanosome is an organelle containing melanin, the most common light-absorbing pigment found in the animal kingdom.Cells that synthesize melanins are called melanocytes, and also the retinal pigment epithelium cells, whereas cells that have merely engulfed the melanosomes...
s, vesicles containing the endogenous photoprotectant
Photoprotection
Photoprotection is a group of mechanisms that nature has developed to minimize the damage that the human body suffers when exposed to UV radiation...
melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
, from epidermal melanocyte
Melanocyte
-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...
s. Each melanocyte in the epidermis has several dendrite
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
s that stretch out to connect it with many keratinocytes. The melanin is then stored in the keratinocytes' nuclei, where it protects the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
from UVR-induced damage.
Keratinocytes migrate with a rolling motion during the process of wound healing.
Sunburn cells
A sunburn cell is a keratinocyte with a pyknotic nucleus and eosinophilicEosinophilic
Eosinophilic refers to the staining of certain tissues, cells, or organelles after they have been washed with eosin, a dye.Eosin is an acidic dye; thus, the structure being stained is basic....
cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
that appears after exposure to UVC or UVB radiation or UVA in the presence of psoralen
Psoralen
Psoralen is the parent compound in a family of natural products known as furocoumarins. It is structurally related to coumarin by the addition of a fused furan ring, and may be considered as a derivative of umbelliferone...
s. It shows premature and abnormal keratinization, and has been described as an example of apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
.
External links
- http://www3.interscience.wiley.com/journal/123313996/abstract L. Tang1, J.J. Wu2, Q. Ma1, T. Cui1, F.M. Andreopoulos3, J. Gil1, J. Valdes1, S.C. Davis1, J. Li1,4 "Human lactoferrin stimulates skin keratinocyte function and wound re-epithelialization", British Journal of Dermatology Volume 163, Issue 1, pages 38–47 (July 2010). Article first published online: 6 MAR 2010. DOI: 10.1111/j.1365-2133.2010.09748.x]