{{Chembox new
| ImageFile = 7-Dehydrochlolesterol.png
| ImageSize = 
| IUPACName = 
| OtherNames = 
| Section1 = {{Chembox Identifiers
|   CASNo = 434-16-2
|   PubChem = 172
|   SMILES = 
|   MeSHName = 7-dehydrocholesterol
  }}
| Section2 = {{Chembox Properties
|   Formula = C<sub>27</sub>H<sub>44</sub>O
|   MolarMass = 384.638
|   Appearance = 
|   Density = 
|   MeltingPt = 
|   BoilingPt = 
  }}
| Section3 = {{Chembox Hazards
|   Solubility = 
|   MainHazards = 
|   FlashPt = 
|   Autoignition = 
  }}
}}
thumb|250px|right|The epidermal strata of the skin. 
'''7-Dehydrocholesterol''' is a serum as a vitamin D<sub>3</sub> in the vitamin D<sub>3</sub> from ultra-violet rays in the sun light, via an intermediate isomer pre-vitamin D<sub>3</sub>. It is also found in the milk of several mammalian species.<ref>"7-dehydrocholesterol." The American Heritage Stedman's Medical Dictionary. Houghton Mifflin Company, 2002. 21 Jan. 2007. [http://www.answers.com/topic/7-dehydrocholesterol Answers.com].</ref> It was discovered by Nobel-laureate organic chemist Adolf Windaus.

==Location ==
The skin consists of two primary layers: the inner layer called the epidermis. The thickness of the epidermis ranges from 0.08mm to more than 0.6mm (0.003 to 0.024 inches).<ref> Frediric Martini et al. (2006). Human Anatomy. Pearson/Benjamin-Cummings Publishers, p. 89. ISBN:0-8053-7211-3.</ref> The epidermis consists of five ''strata''; from outer to inner they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The highest concentrations of 7-dehydrocholesterol are found in the epidermal layer of skin, specifically in the stratum basale and stratum spinosum.<ref name= Norman> Norman, Anthony W. (1998) [http://www.ajcn.org/cgi/reprint/67/6/1108.pdf Sunlight, season, skin pigmentation, vitamin D, and 25-hydroxyvitamin D:integral components of the vitamin D endocrine system.] Am J Clin Nutr;67:1108–10.</ref> The production of pre-vitamin D<sub>3</sub> is therefore greatest in these two layers, whereas production in the other layers is reduced. 

==Radiation==
Synthesis of pre-vitamin D<sub>3</sub> in the skin involves nm and thus the production of vitamin D<sub>3</sub> will only occur at those wavelengths. The two most important factors that govern the generation of pre-vitamin D<sub>3</sub> are the quantity (intensity) and quality (appropriate wavelength) of the UVB irradiation reaching the 7-dehydrocholesterol deep in the stratum basale and stratum spinosum.<ref name= Norman/> Another important consideration is the quantity of 7-dehydrocholesterol present in the skin. Under normal circumstances, ample quantities of 7-dehydrocholesterol (about 25&ndash;50 mg/cm<sup>2</sup> of skin) are available in the stratum spinosum and stratum basale of human skin to meet the body's vitamin D requirements.

{{Infobox Chemikalie
 | Strukturformel      = 200px
 | Andere Namen        = 
*(3''S'',10''R'',13''R'',17''R'')-10,13-Dimethyl- 17-(6-methylheptan-2-yl)- 2,3,4,9,11,12,14,15,16,17-decahydro- 1''H''-cyclopenta[a]phenanthren-3-ol
*Cholesta-5,7-dien-3β-ol
*Provitamin D<sub>3</sub>
 | Summenformel        = C<sub>27</sub>H<sub>44</sub>O
 | CAS                 = 434-16-2
 | Beschreibung        = weißer Feststoff<ref name=sigma/>
 | Molare Masse        = 384,34 g·mol<sup>-1</sup>
 | Aggregat            = fest
 | Dichte              = 
 | Schmelzpunkt        = 148–152 °C<ref name=sigma/>
 | Siedepunkt          = 
 | Dampfdruck          = 
 | Löslichkeit         = fettlöslich
 | Quelle GefStKz      = <ref name=sigma>[http://www.sigmaaldrich.com/catalog/search/ProductDetail/SIGMA/D4429 Sicherheitsdatenblatt Sigma-Aldrich]</ref>
 | Gefahrensymbole     = 
 | R                   = 
 | S                   = 
 | MAK                 = 
}}
'''7-Dehydrocholesterol''' ist ein Sterin, welches der Vorläufer von Adolf Windaus entdeckt. 

== Funktion ==
=== Bildung von Cholesterin ===
Durch das Enzym 7-Dehydrocholesterol Reduktase wird 7-Dehydrocholesterol zu Cholesterin reduziert. 

Dieses Enzym ist beim Smith-Lemli-Opitz-Syndrom durch verschiedene Mutationen in seiner Aktivität unterschiedlich eingeschränkt. Dies führt dazu, dass bei diesem Syndrom der 7-Dehydrocholesterolspiegel im Blut zu hoch und der Cholesterinspiegel zu niedrig ist. Cholesterin wiederum dient als Ausgangssubstanz für die Steroidhormone und Gallensäuren.

=== Bildung von Vitamin D ===
Durch Vitamin D<sub>3</sub>.
7-Dehydrocholesterol findet sich vor allem im Stratum spinosum und Stratum basale der Haut. Wird diese mit UVB-Licht der Wellenlängen 270-315 nm bestrahlt, wird aus dem 7-Dehydrocholesterol unter anderem Cholecalciferol.