Editing Epidermis (section)

===Layers===
[[File:Skinlayers.png|thumb|Schematic image showing a section of epidermis, with epidermal layers labeled]]
The epidermis is composed of four or five layers, depending on the skin region.<ref>{{cite book|url=https://openstax.org/details/books/anatomy-and-physiology-2e |title=Anatomy and Physiology | edition = 2nd |publisher=OpenStax |page=164 | vauthors = Betts JG |display-authors=etal |year=2022 |isbn=978-1-711494-06-7}}</ref> Those layers from outermost to innermost are:<ref name="Lookingbill" />
; [[Keratin#Cornification|cornified layer]] (''[[stratum corneum]]''): [[File:Confocal image of the stratum corneum.jpg|thumb|Confocal image of the stratum corneum]]Composed of 10 to 30 layers of polyhedral, anucleated [[corneocyte]]s (final step of keratinocyte [[cell differentiation|differentiation]]), with the palms and soles having the most layers. Corneocytes contain a [[protein]] envelope (cornified envelope proteins) underneath the plasma membrane, are filled with water-retaining [[keratin]] proteins, attached together through [[desmosome|corneodesmosomes]] and surrounded in the [[extracellular]] space by stacked layers of [[lipids]].<ref name="bensouillah">{{cite web|title=Skin structure and function|url=http://www.radcliffe-oxford.com/books/samplechapter/7750/01_bensouillah-241a6c80rdz.pdf|url-status=dead|archive-url=https://web.archive.org/web/20101214062704/http://www.radcliffe-oxford.com/books/samplechapter/7750/01_bensouillah-241a6c80rdz.pdf|archive-date=2010-12-14|access-date=2015-01-07}}</ref> Most of the barrier functions of the epidermis localize to this layer.<ref name="elias">{{cite journal | vauthors = Elias PM | title = The skin barrier as an innate immune element | journal = Seminars in Immunopathology | volume = 29 | issue = 1 | pages = 3–14 | date = April 2007 | pmid = 17621950 | doi = 10.1007/s00281-007-0060-9 | s2cid = 20311780 }}</ref>
; 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]]''): [[File:Confocal image of the stratum granulosum.jpg|thumb|Confocal image of the stratum granulosum]]Keratinocytes lose their [[cell nucleus|nuclei]] and their [[cytoplasm]] appears granular. Lipids, contained into those keratinocytes within [[lamellar bodies]], are released into the extracellular space through [[exocytosis]] to form a lipid barrier that prevents water loss from the body as well as entry of foreign substances. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example [[glycosphingolipids]] become [[ceramides]] and [[phospholipids]] become [[free fatty acids]].<ref name="bensouillah" />
; spinous layer (''[[stratum spinosum]]''): [[File:Confocal image of the stratum spinosum with some basal cell clusters.jpg|thumb|Confocal image of the stratum spinosum already showing some clusters of basal cells]]Keratinocytes become connected through [[desmosomes]] and produce lamellar bodies, from within the [[golgi apparatus|Golgi]], enriched in polar lipids, [[glycosphingolipids]], free [[sterols]], [[phospholipids]] and catabolic enzymes.<ref name="Proksch" /> Langerhans cells, immunologically active cells, are located in the middle of this layer.<ref name="bensouillah" />
; basal/germinal layer (''[[stratum germinativum|stratum basale/germinativum]]''): [[File:Confocal image of the stratum basale showing some papillae.jpg|thumb|Confocal image of the stratum basale already showing some papillae]]Composed mainly of proliferating and non-proliferating keratinocytes, attached to the [[basement membrane]] by [[hemidesmosomes]]. [[Melanocyte]]s are present, connected to numerous keratinocytes in this and other strata through [[dendrites]]. [[Merkel cells]] are also found in the [[stratum basale]] with large numbers in touch-sensitive sites such as the [[fingertip]]s and [[lip]]s. They are closely associated with cutaneous [[nerves]] and seem to be involved in light touch sensation.<ref name="bensouillah" />
; [[Malpighian layer]] (''stratum malpighii''): This is usually defined as both the [[stratum basale]] and [[stratum spinosum]].<ref name="Rooks" />

The epidermis is separated from the dermis, its underlying [[tissue (biology)|tissue]], by a [[basement membrane]].