Editing Skin (section)

==Structure in mammals==
{{anchor|Skin layers|Layers|Layer}} <!-- required for incoming redirect-->
{{Infobox anatomy
| Name        = Dermis
| Latin       =
| Image       = Gray942.png
| Caption     = The distribution of the blood vessels in the skin of the sole of the foot. (Corium – TA alternate term for dermis – is labeled at upper right.)
| Width       =
| Image2      = Gray940.png
| Caption2    = A diagrammatic sectional view of the skin (''click on image to magnify''). (Dermis labeled at center right.)
| Precursor   =
| System      =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}

Mammalian skin is composed of two primary layers:

* The ''[[epidermis]]'', which provides waterproofing and serves as a barrier to infection.
* The ''[[dermis]]'', which serves as a location for the [[appendages]] of skin.

===Epidermis===
{{main|Epidermis}}

The epidermis is composed of the outermost layers of the skin. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing [[pathogens]] from entering, and is a stratified squamous [[epithelium]],<ref name="Rooks">{{cite book
 | last1=McGrath |first1=J.A. |last2=Eady|first2=R.A. |last3=Pope|first3=F.M.
 | title=Rook's Textbook of Dermatology
 | year=2004 | edition=7th
 | publisher=Blackwell Publishing | isbn=978-0-632-06429-8
 | pages=3.1–3.6
}}</ref> composed of [[cell division|proliferating]] basal and [[cell differentiation|differentiated]] suprabasal [[keratinocyte]]s.

[[Keratinocyte]]s are the major [[cell (biology)|cells]], constituting 95% of the [[epidermis (skin)|epidermis]],<ref name="Rooks" /> while [[Merkel cell]]s,  [[melanocyte]]s and [[Langerhans cell]]s are also present. The [[epidermis (skin)|epidermis]] can be further subdivided into the following ''strata'' or layers (beginning with the outermost layer):<ref>{{cite book|url=https://openstax.org/details/books/anatomy-and-physiology-2e |title=Anatomy and Physiology 2e |publisher=OpenStax |page=164 |last1=Betts |first1=J. Gordon |display-authors=etal |year=2022 |isbn=978-1-711494-06-7}}</ref>
* [[Stratum corneum]]
* [[Stratum lucidum]] (only in [[Hand#Human anatomy|palms]] and [[sole (foot)|soles]])
* [[Stratum granulosum]]
* [[Stratum spinosum]]
* [[Stratum basale]] (also called the ''stratum germinativum'')
[[Keratinocyte]]s in the [[stratum basale]] proliferate through [[mitosis]] and the daughter [[cell (biology)|cells]] move up the strata changing shape and composition as they undergo multiple stages of [[cell differentiation]] to eventually become anucleated. During that process, [[keratinocyte]]s will become highly organized, forming [[cell junction|cellular junctions]] ([[desmosomes]]) between each other and secreting [[keratin]] [[proteins]] and [[lipids]] which contribute to the formation of an [[extracellular matrix]] and provide mechanical [[strength of materials|strength]] to the skin.<ref name="Pmid">{{cite journal | pmid=19333614 | year=2009 | last1=Breitkreutz | first1=D | last2=Mirancea | first2=N | last3=Nischt | first3=R | title=Basement membranes in skin: Unique matrix structures with diverse functions? | volume=132 | issue=1 | pages=1–10 | doi=10.1007/s00418-009-0586-0 | journal=Histochemistry and Cell Biology| s2cid=21633122 }}</ref> [[Keratinocyte]]s from the [[stratum corneum]] are eventually shed from the surface ([[desquamation]]).

The [[epidermis (skin)|epidermis]] contains no [[blood vessel]]s, and [[cell (biology)|cells]] in the deepest layers are nourished by diffusion from [[blood]] [[capillaries]] extending to the upper layers of the [[dermis]].

===Basement membrane===
{{main|Basement membrane}}

The [[epidermis]] and [[dermis]] are separated by a thin sheet of [[fibers]] called the [[basement membrane]], which is made through the action of both [[tissue (biology)|tissues]].
The basement membrane controls the traffic of the [[cell (biology)|cells]] and [[molecules]] between the dermis and epidermis but also serves, through the binding of a variety of [[cytokine]]s and [[growth factors]], as a reservoir for their controlled release during [[physiological]] remodeling or repair processes.<ref>{{cite journal |last1=Iozzo |first1=Renato V. |title=Basement membrane proteoglycans: from cellar to ceiling |journal=Nature Reviews Molecular Cell Biology |date=August 2005 |volume=6 |issue=8 |pages=646–656 |doi=10.1038/nrm1702 |pmid=16064139 |s2cid=22151754 }}</ref>

===Dermis===
{{main|Dermis}}

The '''dermis''' is the layer of skin beneath the [[epidermis (skin)|epidermis]] that consists of [[connective tissue]] and cushions the body from [[stress (biology)|stress]] and strain. The [[dermis]] provides tensile [[strength of materials|strength]] and [[elasticity (physics)|elasticity]] to the skin through an [[extracellular matrix]] composed of [[collagen fibrils]], [[microfibrils]], and [[elastic fibers]], embedded in [[hyaluronan]] and [[proteoglycan]]s.<ref name="Pmid" /> Skin proteoglycans  are varied and have very specific locations.<ref>{{cite journal |last1=Smith |first1=Margaret Mary |last2=Melrose |first2=James |title=Proteoglycans in Normal and Healing Skin |journal=Advances in Wound Care |date=March 2015 |volume=4 |issue=3 |pages=152–173 |doi=10.1089/wound.2013.0464 |pmid=25785238 |pmc=4352701 }}</ref> For example, [[hyaluronan]], [[versican]] and [[decorin]] are present throughout the dermis and [[epidermis]] [[extracellular matrix]], whereas [[biglycan]] and [[perlecan]] are only found in the epidermis.

It harbors many [[mechanoreceptor]]s (nerve endings) that provide the sense of [[touch]] and heat through [[nociceptor]]s and [[thermoreceptor]]s. It also contains the [[hair follicle]]s, [[sweat gland]]s, [[sebaceous gland]]s, [[apocrine glands]], [[lymphatic vessel]]s and [[blood vessel]]s. The [[blood vessel]]s in the [[dermis]] provide nourishment and waste removal from its own [[cell (biology)|cells]] as well as for the [[epidermis (skin)|epidermis]].

Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals.<ref name=":2" />

The [[dermis]] is tightly connected to the [[epidermis (skin)|epidermis]] through a [[basement membrane]] and is structurally divided into two areas: a superficial area adjacent to the epidermis, called the ''papillary region'', and a deep thicker area known as the ''reticular region''.

====Papillary region====
The papillary region is composed of loose [[areolar connective tissue]]. This is named for its fingerlike projections called ''papillae'' that extend toward the [[epidermis (skin)|epidermis]]. The papillae provide the [[dermis]] with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.

====Reticular region====
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular [[connective tissue]] and receives its name from the dense concentration of [[collagen]]ous, [[Elastic fibers|elastic]], and [[Reticular fiber|reticular]] fibers that weave throughout it. These [[protein]] fibers give the [[dermis]] its properties of [[strength of materials|strength]], [[extensibility]], and [[elasticity (physics)|elasticity]].
Also located within the reticular region are the [[roots of the hair]], [[sweat gland]]s, [[sebaceous gland]]s, [[Cutaneous receptor|receptors]], [[nail (anatomy)|nails]], and [[blood vessel]]s.

===Subcutaneous tissue===
{{main|Subcutaneous tissue}}

The [[subcutaneous tissue]] (also hypodermis)  is not part of the skin, and lies below the [[dermis]]. Its purpose is to attach the skin to underlying [[bone]] and [[muscle]] as well as supplying it with [[blood vessel]]s and [[nerve]]s. It consists of loose [[connective tissue]] and [[elastin]]. The main [[cell (biology)|cell]] types are [[fibroblast]]s, [[macrophage]]s and [[adipocyte]]s (the subcutaneous tissue contains 50% of [[body fat]]). [[Fat]] serves as padding and insulation for the body.

[[Microorganisms]] like ''[[Staphylococcus epidermidis]]'' colonize the skin surface. The density of [[skin flora]] depends on region of the skin. The disinfected skin surface gets recolonized from [[bacteria]] residing in the deeper areas of the [[hair follicle]], [[gastrointestinal tract|gut]] and [[urogenital]] openings.

===Detailed cross section===
{{wide image|Skin_layers.svg|800px|Skin layers, of both the hairy and hairless skin}}