Epidermis

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Epidermal hyperplasia
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Epidermis
Microscopic image of the epidermis, which constitutes the outer layer of skin, shown here by the white bar
Microscopic image showing the layers of the epidermis. The stratum corneum appears more compact in this image than above because of different sample preparation.
Details
Part ofSkin
SystemIntegumentary system
Identifiers
Latinepidermis
MeSHD004817
TA98A16.0.00.009
TA27046
THH3.12.00.1.01001
FMA70596
Anatomical terms of microanatomy

The epidermis is the outermost of the three layers that comprise the

pathogens[2] and regulates the amount of water released from the body into the atmosphere through transepidermal water loss.[3]

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

Ancient Greek
derma 'skin'. Something related to or part of the epidermis is termed epidermal.

Structure

Cellular components

The epidermis primarily consists of

capillaries are found beneath the epidermis, and are linked to an arteriole and a venule
. 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

cadherins. Inside the cell, the cadherins are linked to actin filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells,[9] although the actin filaments are actually located inside the cell and run parallel to the cell membrane. Because of the proximity of the neighboring cells and tightness of the junctions, the actin immunofluorescence appears as a border between cells.[9]

Layers

Schematic image showing a section of epidermis, with epidermal layers labeled

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)
lipids.[11] Most of the barrier functions of the epidermis localize to this layer.[12]
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)
free fatty acids.[11]
spinous layer (stratum spinosum)
phospholipids and catabolic enzymes.[3] Langerhans cells, immunologically active cells, are located in the middle of this layer.[11]
basal/germinal layer (
stratum basale/germinativum
)
nerves and seem to be involved in light touch sensation.[11]
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

Differentiating cells delaminate from the basement membrane and are displaced outward through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface (desquamation). Differentiated keratinocytes secrete keratin proteins, which contribute to the formation of an extracellular matrix that is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss,[4] taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum.[2] The entire epidermis is replaced by new cell growth over a period of about 48 days.[13]

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

chloride channels[17] and voltage-independent cation channels permeable to calcium.[18] Moreover, it has been suggested that an extracellular calcium-sensing receptor (CaSR) also contributes to the rise in intracellular calcium concentration.[19]

Development

Epidermal

This inner layer is a

spinous layer (stratum spinosum). The cells of these two layers, together called the Malpighian layer(s) after Marcello Malpighi, divide to form the superficial granular layer (Stratum granulosum) of the epidermis.[20]

The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the

cornified layer (stratum corneum), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell. After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 30 - 90 milligrams of skin flakes every hour, or 0.720 - 2.16 grams per day.[21]

Epidermal development is a product of several growth factors, two of which are:[20]

Function

Barrier

The epidermis serves as a barrier to protect the body against

chemical compounds, and provides mechanical resistance to minor injury. Most of this barrier role is played by the stratum corneum.[12]

Characteristics
Permeability

Skin hydration

The ability of the skin to hold water is primarily due to the

healthy skin.[24] Skin hydration is quantified using corneometry.[25] Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss.[26][27]

Skin color

The amount and distribution of

UV radiation exposure, while their distribution remain largely unaffected.[28]

Touch

The skin contains specialized epidermal touch receptor cells called

Merkel cells. Historically, the role of Merkel cells in sensing touch has been thought to be indirect, due their close association with nerve endings. However, recent work in mice and other model organisms demonstrates that Merkel cells intrinsically transform touch into electrical signals that are transmitted to the nervous system.[29]

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 nigricans
  • Heck's disease
  • Pseudoepitheliomatous hyperplasia (PEH), low magnification, with acanthotic squamous epithelium with irregular thick finger-like downgrowths into the underlying dermis.
    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.
    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

See also

References

  1. .
  2. ^ .
  3. ^ .
  4. ^ .
  5. .
  6. ^ .
  7. ^ TheFreeDictionary > rete ridge Citing: The American Heritage Medical Dictionary Copyright 2007, 2004
  8. PMID 11826181
    .
  9. ^ .
  10. .
  11. ^ a b c d "Skin structure and function" (PDF). Archived from the original (PDF) on 2010-12-14. Retrieved 2015-01-07.
  12. ^
    S2CID 20311780
    .
  13. .
  14. .
  15. .
  16. .
  17. .
  18. .
  19. .
  20. ^ .
  21. .
  22. .
  23. .
  24. .
  25. PMID 2459872.{{cite journal}}: CS1 maint: DOI inactive as of January 2024 (link
    )
  26. PMID 2950180.{{cite journal}}: CS1 maint: DOI inactive as of February 2024 (link
    )
  27. .
  28. .
  29. .
  30. .
  31. ^ 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.
  32. PMID 24353818
    .
  33. ^ .
  34. .