Editing Integumentary system (section)

====Epidermis====
{{Main|Epidermis}}
[[File:Normal Epidermis and Dermis with Intradermal Nevus 10x.JPG|thumb|left|Epidermis and dermis of human skin]]
The [[epidermis]] is the strong, superficial layer that serves as the first line of protection against the outer environment. The human epidermis is composed of [[Stratified squamous epithelium|stratified squamous epithelial cells]], which further break down into four to five layers: the [[stratum corneum]], [[stratum granulosum]], [[stratum spinosum]] and [[stratum basale]]. Where the skin is thicker, such as in the palms and soles, there is an extra layer of skin between the stratum corneum and the stratum granulosum, called the [[stratum lucidum]]. The epidermis is regenerated from the stem cells found in the basal layer that develop into the corneum. The epidermis itself is devoid of blood supply and draws its nutrition from its underlying dermis.<ref name="statpearls2"/>

Its main functions are protection, absorption of nutrients, and [[homeostasis]]. In structure, it consists of a keratinized stratified [[squamous epithelium]]; four types of cells: [[keratinocytes]], [[melanocytes]], [[Merkel cells]], and [[Langerhans cells]].

The predominant cell [[keratinocyte]], which produces [[keratin]], a fibrous protein that aids in skin protection, is responsible for the formation of the epidermal water barrier by making and secreting [[lipid]]s.<ref name="statpearls">{{cite book |last1=Yousef |first1=Hani |last2=Alhajj |first2=Mandy |last3=Sharma |first3=Sandeep |title=StatPearls |publisher=StatPearls Publishing |url=https://www.ncbi.nlm.nih.gov/books/NBK470464/ |chapter=Anatomy, Skin (Integument), Epidermis|year=2022 |pmid=29262154 }}</ref> <!--An overwhelming amount of keratin can cause disease by giving rise to eruptions from the skin that will protrude outwards and lead to infection.{{Citation needed|date=March 2017}}--> The majority of the skin on the human body is keratinized, with the exception of the lining of [[mucous membrane]]s, such as the inside of the mouth. Non-keratinized cells allow water to "stay" atop the structure.

The protein keratin stiffens epidermal tissue to form [[fingernail]]s. Nails grow from a thin area called the [[Matrix (nail)|nail matrix]] at an average of 1&nbsp;mm per week. The [[lunula (anatomy)|lunula]] is the crescent-shape area at the base of the nail, lighter in color as it mixes with matrix cells. Only [[primate]]s have nails. In other vertebrates, the keratinizing system at the terminus of each digit produces claws or hooves.<ref name="Kardong2019"/>

The epidermis of vertebrates is surrounded by two kinds of coverings, which are produced by the epidermis itself. In [[fish]] and aquatic [[amphibian]]s, it is a thin mucus layer that is constantly being replaced. In terrestrial vertebrates, it is the [[stratum corneum]] (dead keratinized cells). The epidermis is, to some degree, glandular in all vertebrates, but more so in [[fish]] and [[amphibian]]s. Multicellular epidermal glands penetrate the dermis, where they are surrounded by blood capillaries that provide nutrients and, in the case of endocrine glands, transport their products.<ref>{{cite journal |last1=Quay |first1=Wilbur B. |title=Integument and the Environment Glandular Composition, Function, and Evolution |journal=Integrative and Comparative Biology |date=1 February 1972 |volume=12 |issue=1 |pages=95–108 |url=https://academic.oup.com/icb/article/12/1/95/2107657}}</ref>
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