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Epidermis (skin): Wikis

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Epidermis

The epidermis is the outer layer of the skin,[1] composed of terminally differentiated stratified squamous epithelium,[2] acting as the body's major barrier against an inhospitable environment.[3] It is the thinnest on the eyelids at .05 mm (0.0020 in) and the thickest on the palms and soles at 1.5 mm (0.059 in).[4] It is ectodermal in origin.

Contents

Structure

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Cellular components

The epidermis is avascular, nourished by diffusion from the dermis, and composed of four types of cells, i.e. keratinocytes, melanocytes, Langerhans cells, and the Merkel cells.[1] Keratinocytes are the major constituent, constituting 95% of the epidermis.[2]

Layers

Section of epidermis

The epidermis is composed of 4-5 layers depending on the region of skin being considered. Those layers in descending order are the cornified layer (stratum corneum), clear/translucent layer (stratum lucidum), granular layer (stratum granulosum), spinous layer (stratum spinosum), and basal/germinal layer (stratum basale/germinativum).[3] The term Malpighian layer (stratum malpighi) refers to both the basal and spinosum layers.[2]

Cellular kinetics

The stratified squamous epithelium is maintained by cell division within the basal layer. Differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where anucleate corneal cells are continually shed from the surface (desquamation). In normal skin, the rate of production equals the rate of loss,[2] taking about two weeks for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional four weeks to cross the stratum corneum.[3]

Organogenesis

Epidermal organogenesis, the formation of the epidermis, begins in the cells covering the embryo after neurulation, the formation of the central nervous system. In most vertebrates, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer or stratum germinativum has formed. [5]

This inner layer is a germinal epithelium that give rise to all epidermal cells. It divides to form the outer 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. [5]

The cells in the granular layer do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally migrate to form 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 granular layer and throughout life they are shed at a rate of 1.5 g (0.053 oz) per day. [5]

Epidermal development is a product of several growth factors, two of which are:[5]

Skin color

The amount and distribution of melanin pigment in the epidermis is the main reason for variation in skin color in modern Homo sapiens. Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melansomes varies between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and oriental skin the melansomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melansomes in the keratinocytes increases with UV radiation exposure, whilst their distribution remain largely unaffected. [6]

Additional images

Epidermis and dermis of human skin  
Cross-section of all skin layers  
Vector version of image to the left  

References

  1. ^ a b James, William; Berger, Timothy; Elston, Dirk (2005) Andrews' Diseases of the Skin: Clinical Dermatology (10th ed.). Saunders. Page 2-3. ISBN 0721629210.
  2. ^ a b c d McGrath, J.A.; Eady, R.A.; Pope, F.M. (2004). Rook's Textbook of Dermatology (7th ed.). Blackwell Publishing. ISBN 9780632064298. 
  3. ^ a b c Marks, James G; Miller, Jeffery (2006). Lookingbill and Marks' Principles of Dermatology (4th ed.). Elsevier. pp. 1-7. ISBN 1-4160-3185-5. 
  4. ^ Brannon, Heather (2007). "Dermatology - Epidermis". About.com. http://dermatology.about.com/cs/skinanatomy/g/epidermis.htm. 
  5. ^ a b c d Gilbert, Scott F (2003). "The Epidermis and the Origin of Cutaneous Structures". Developmental Biology. Sinauer Associates. http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2929. 
  6. ^ Montagna, William; Prota, Giuseppe; Kenney, John A. (1993). Black skin: structure and function. Gulf Professional Publishing. p. 69. ISBN 012505260X. http://books.google.com/books?id=BpL8V2YJkQ4C&pg=PA69. 

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