Epidermis
Epidermis | |
---|---|
Details | |
Part of | Skin |
System | Integumentary system |
Identifiers | |
Latin | epidermis |
MeSH | D004817 |
TA98 | A16.0.00.009 |
TA2 | 7046 |
TH | H3.12.00.1.01001 |
FMA | 70596 |
Anatomical terms of microanatomy |
The epidermis is the outermost of the three layers that comprise the
The epidermis is composed of multiple layers of flattened cells[4] that overlie a base layer (stratum basale) composed of columnar cells arranged perpendicularly. The layers of cells develop from stem cells in the basal layer. The thickness of the epidermis varies from 31.2μm for the penis to 596.6μm for the sole of the foot with most being roughly 90μm. Thickness does not vary between the sexes but becomes thinner with age.[5] The human epidermis is an example of epithelium, particularly a stratified squamous epithelium.
The word epidermis is derived through Latin from
Structure
Cellular components
The epidermis primarily consists of
. The epidermis itself has no blood supply and is nourished almost exclusively by diffused oxygen from the surrounding air.[8] Cellular mechanisms for regulating water and sodium levels (ENaCs) are found in all layers of the epidermis.[9]Cell junctions
Epidermal cells are tightly interconnected to serve as a tight barrier against the exterior environment. The junctions between the epidermal cells are of the
Layers
The epidermis is composed of 4 or 5 layers, depending on the region of skin being considered.[10] Those layers from outermost to innermost are:[2]
- cornified layer (stratum corneum)
- clear/translucent layer (stratum lucidum, only in palms and soles)
- This narrow layer is found only on the palms and soles. The epidermis of these two areas is known as "thick skin" because with this extra layer, the skin has 5 epidermal layers instead of 4.
- granular layer (stratum granulosum)
- spinous layer (stratum spinosum)
- basal/germinal layer (stratum basale/germinativum)
- Malpighian layer (stratum malpighii)
- This is usually defined as both the stratum basale and stratum spinosum.[4]
The epidermis is separated from the dermis, its underlying tissue, by a basement membrane.
Cellular kinetics
Cell division
As a
Calcium concentration
Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum. Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions. This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers.[3]
Elevation of extracellular calcium concentrations induces an increase in
Development
Epidermal
This inner layer is a
The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the
Epidermal development is a product of several growth factors, two of which are:[20]
- autocrine growth factor by which basal cells stimulate their own division.
- of basal cells is regulated.
Function
Barrier
The epidermis serves as a barrier to protect the body against
- Characteristics
- Physical barrier: Epidermal keratinocytes are tightly linked by cytoskeletal proteins, giving the epidermis its mechanical strength.[3]
- Chemical barrier: Highly organized lipids, acids, hydrolytic enzymes, and antimicrobial peptides[3]inhibit passage of external chemicals and pathogens into the body.
- Immunologically active barrier: The cellular constituents of the immune system[3]found in the epidermis actively combat infection.
- Water content of the stratum corneum drops towards the surface, creating hostile conditions for pathogenic microorganism growth.[12]
- An acidic pH (around 5.0) and low amounts of water make the epidermis hostile to many microorganic pathogens.[12]
- Non-pathogenic microorganisms on the surface of the epidermis help defend against pathogens by competing for secretions that inhibit the growth of pathogenic microbiota.[12]
- Permeability
- glucocorticoids, compromises the stratum corneum and thus the barrier function.[22]
- Sudden and large shifts in humidity alter stratum corneum hydration in a way that could allow entry of pathogenic microorganisms.[23]
Skin hydration
The ability of the skin to hold water is primarily due to the
Skin color
The amount and distribution of
Touch
The skin contains specialized epidermal touch receptor cells called
Clinical significance
Laboratory culture of keratinocytes to form a 3D structure (artificial skin) recapitulating most of the properties of the epidermis is routinely used as a tool for drug development and testing.
Hyperplasia
Epidermal hyperplasia (thickening resulting from cell proliferation) has various forms:
- Acanthosis is diffuse epidermal hyperplasia (thickening of the skin, and not to be confused with acanthocytes).[30] It implies increased thickness of the Malpighian layer (stratum basale and stratum spinosum).[31] Acanthosis nigricans is a black, poorly defined, velvety hyperpigmentedacanthosis, usually observed in the back of neck, axilla, and other folded regions of the skin.
- Focal epithelial hyperplasia (Heck's disease) is an asymptomatic, benign neoplastic condition characterized by multiple white to pinkish papules that occur diffusely in the oral cavity.[32][6]: 411
- Pseudoepitheliomatous hyperplasia (PEH) is a benign condition characterized by hyperplasia of the epidermis and epithelium of skin appendages,[33] with irregular squamous strands extending down into the dermis,[34] and closely simulating squamous cell carcinoma (SCC).[33]
-
Pseudoepitheliomatous hyperplasia (PEH), low magnification, with acanthotic squamous epithelium with irregular thick finger-like downgrowths into the underlying dermis.
-
PEH, high magnification, with reactive-appearing squamous downgrowths with no significant cytologic atypia.
In contrast, hyperkeratosis is a thickening of the stratum corneum, and is not necessarily due to hyperplasia.
Additional images
-
Epidermis and dermis of human skin
-
Cross-section of all skin layers
-
Illustration of epidermal layers
-
Optical coherence tomography of fingertip
See also
References
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- ^ a b c d "Skin structure and function" (PDF). Archived from the original (PDF) on 2010-12-14. Retrieved 2015-01-07.
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- ^ Stone MS, Ray TL (September 1995). "Acanthosis". DermPathTutor. Department of Dermatology, University of Iowa. Archived from the original on 29 May 2012. Retrieved 17 May 2012.
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