Editing Skin (section)

==Structure in fish, amphibians, birds, and reptiles==
===Fish===
{{Further|Fish scale}}
{{One source section|date=August 2021}}

The epidermis of [[fish]] and of most [[amphibian]]s consists entirely of live [[cell (biology)|cells]], with only minimal quantities of [[keratin]] in the cells of the superficial layer.<ref name="auto">{{Cite journal |last1=Varga |first1=Joseph F. A. |last2=Bui-Marinos |first2=Maxwell P. |last3=Katzenback |first3=Barbara A. |date=2019 |title=Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens |journal=Frontiers in Immunology |volume=9 |page=3128 |doi=10.3389/fimmu.2018.03128 |pmid=30692997 |pmc=6339944 |issn=1664-3224|doi-access=free }}</ref> It is generally permeable, and in the case of many [[amphibians]], may actually be a major respiratory organ.<ref>{{Cite journal |last1=Ferrie |first1=Gina M. |last2=Alford |first2=Vance C. |last3=Atkinson |first3=Jim |last4=Baitchman |first4=Eric |last5=Barber |first5=Diane |last6=Blaner |first6=William S. |last7=Crawshaw |first7=Graham |last8=Daneault |first8=Andy |last9=Dierenfeld |first9=Ellen |last10=Finke |first10=Mark |last11=Fleming |first11=Greg |date=2014 |title=Nutrition and Health in Amphibian Husbandry |journal=Zoo Biology |volume=33 |issue=6 |pages=485–501 |doi=10.1002/zoo.21180 |issn=0733-3188 |pmc=4685711 |pmid=25296396}}</ref> The [[dermis]] of [[bony fish]] typically contains relatively little of the [[connective tissue]] found in [[tetrapod]]s. <ref name="auto"/> Instead, in most species, it is largely replaced by solid, protective bony [[scale (zoology)|scales]].<ref>{{Cite web |last=Fisheries |first=NOAA |date=2022-05-03 |title=Fun Facts About Shocking Sharks {{!}} NOAA Fisheries |url=https://www.fisheries.noaa.gov/national/outreach-and-education/fun-facts-about-shocking-sharks |access-date=2022-05-11 |website=NOAA |language=en}}</ref> Apart from some particularly large dermal bones that form parts of the [[skull]], these [[scale (zoology)|scales]] are lost in [[tetrapods]], although many [[reptile]]s do have [[scale (zoology)|scales]] of a different kind, as do [[pangolin]]s.<ref>{{Cite web |title=Pangolin Fact Sheet {{!}} Blog {{!}} Nature {{!}} PBS |url=https://www.pbs.org/wnet/nature/blog/pangolin-fact-sheet/ |access-date=2022-05-11 |website=Nature |date=25 March 2020 |language=en-US}}</ref> [[Cartilaginous fish]] have numerous tooth-like [[Dermal denticle|denticles]] embedded in their skin, in place of true [[scale (zoology)|scales]].<ref>{{Cite journal |title=Shark and Ray Workbook 3-5 update 8-31 |url=https://www.floridaocean.org/sites/default/files/images/Shark%20and%20Ray%20Workbook%203-5%20update%208-31.pdf |journal=Florida Oceanographic Society}}</ref>

[[Sweat gland]]s and [[sebaceous gland]]s are both unique to [[mammals]], but other types of skin gland are found in other [[vertebrates]].{{fact|date=September 2024}} [[Fish]] typically have a numerous individual [[mucus]]-secreting skin [[cell (biology)|cells]] that aid in insulation and protection, but may also have [[poison]] [[glands]], [[photophore]]s, or [[cell (biology)|cells]] that produce a more watery, [[serous]] fluid. In [[amphibians]], the mucous cells are gathered together to form sac-like [[glands]]. Most living [[amphibians]] also possess ''granular [[glands]]'' in the skin, that secrete irritating or toxic compounds.<ref name=VB/>

Although [[melanin]] is found in the skin of many species, in the [[reptile]]s, the [[amphibians]], and [[fish]], the [[epidermis (skin)|epidermis]] is often relatively colorless. Instead, the color of the skin is largely due to [[chromatophore]]s in the [[dermis]], which, in addition to melanin, may contain [[guanine]] or [[carotenoid]] [[pigments]]. Many species, such as [[chameleon]]s and [[flounder]]s may be able to change the color of their skin by adjusting the relative size of their [[chromatophores]].<ref name=VB/>

=== Amphibians ===
{{Further|Amphibian#Skin}}

==== Overview ====
[[File:FrogSkin.png|thumb|616x616px|'''Frog gland anatomy–''' A: Mucous gland (alveolus), B: Chromophore, C: Granular gland (alveolus), D: Connective tissue, E: Stratum corneum, F: Transition zone (intercalary region), G: Epidermis (where the duct resides), H: Dermis]]

Amphibians possess two types of [[gland]]s, [[Mucous gland|mucous]] and granular (serous). Both of these glands are part of the [[integument]] and thus considered [[cutaneous]]. Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. The three individual parts of the gland are the duct, the intercalary region, and lastly the alveolar gland (sac). Structurally, the duct is derived via [[keratinocyte]]s and passes through to the surface of the [[Epidermis|epidermal]] or outer skin layer thus allowing external secretions of the body. The gland alveolus is a sac-shaped structure that is found on the bottom or base region of the granular gland. The cells in this sac specialize in secretion. Between the alveolar gland and the duct is the intercalary system which can be summed up as a transitional region connecting the duct to the grand alveolar beneath the epidermal skin layer. In general, granular glands are larger in size than the mucous glands, which are greater in number.<ref name=":0">{{cite journal |last1=Toledo |first1=R.C. |last2=Jared |first2=C. |title=Cutaneous granular glands and amphibian venoms |journal=Comparative Biochemistry and Physiology Part A: Physiology |date=May 1995 |volume=111 |issue=1 |pages=1–29 |doi=10.1016/0300-9629(95)98515-I }}</ref>

==== Granular glands ====
Granular glands can be identified as [[venom]]ous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within the amphibians. They are located in clusters differing in concentration depending on [[amphibian]] taxa. The toxins can be fatal to most vertebrates or have no effect against others. These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors.<ref name=":0" />

Structurally, the ducts of the granular gland initially maintain a cylindrical shape. When the ducts mature and fill with fluid, the base of the ducts become swollen due to the pressure from the inside. This causes the epidermal layer to form a pit like opening on the surface of the duct in which the inner fluid will be secreted in an upwards fashion.<ref name=":1">{{cite journal |last1=Dawson |first1=A. B. |title=The integument of necturus maculosus |journal=Journal of Morphology |date=December 1920 |volume=34 |issue=3 |pages=486–589 |doi=10.1002/jmor.1050340303 |s2cid=83534922 |url=https://books.google.com/books?id=UklQAQAAMAAJ&pg=PA487}}</ref>

The intercalary region of granular glands is more developed and mature in comparison with mucous glands. This region resides as a ring of cells surrounding the basal portion of the duct which are argued to have an [[ectoderm]]al muscular nature due to their influence over the lumen (space inside the tube) of the duct with dilation and constriction functions during secretions. The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body.<ref name=":1" />

The gland alveolus is a sac that is divided into three specific regions/layers. The outer layer or tunica fibrosa is composed of densely packed connective-tissue which connects with fibers from the spongy intermediate layer where elastic fibers, as well as nerves, reside. The nerves send signals to the muscles as well as the epithelial layers. Lastly, the epithelium or tunica propria encloses the gland.<ref name=":1" />

==== Mucous glands ====
[[Mucous gland]]s are non-venomous and offer a different functionality for amphibians than granular. Mucous glands cover the entire surface area of the amphibian body and specialize in keeping the body lubricated. There are many other functions of the mucous glands such as controlling the pH, thermoregulation, adhesive properties to the environment, anti-predator behaviors (slimy to the grasp), chemical communication, even anti-bacterial/viral properties for protection against pathogens.<ref name=":0" />

The ducts of the mucous gland appear as cylindrical vertical tubes that break through the epidermal layer to the surface of the skin. The cells lining the inside of the ducts are oriented with their longitudinal axis forming 90-degree angles surrounding the duct in a helical fashion.<ref name=":1" />

Intercalary cells react identically to those of granular glands but on a smaller scale. Among the amphibians, there are taxa which contain a modified intercalary region (depending on the function of the glands), yet the majority share the same structure.<ref name=":1" />

The alveolar or mucous glands are much more simple and only consist of an epithelium layer as well as connective tissue which forms a cover over the gland. This gland lacks a tunica propria and appears to have delicate and intricate fibers which pass over the gland's muscle and epithelial layers.<ref name=":1" />

===Birds and reptiles===
{{Main|Reptile scales}}
{{One source section|date=August 2021}}

The [[epidermis (skin)|epidermis]] of [[bird]]s and [[reptile]]s is closer to that of [[mammals]], with a layer of dead keratin-filled [[cell (biology)|cells]] at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial [[amphibians]] such as [[toad]]s. In these animals, there is no clear [[cell differentiation|differentiation]] of the [[epidermis (skin)|epidermis]] into distinct layers, as occurs in [[human]]s, with the change in [[cell (biology)|cell]] type being relatively gradual. The mammalian [[epidermis (skin)|epidermis]] always possesses at least a [[stratum germinativum]] and [[stratum corneum]], but the other intermediate layers found in [[human]]s are not always distinguishable.
[[Hair]] is a distinctive feature of mammalian skin, while [[feather]]s are (at least among living species) similarly unique to [[bird]]s.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia|pages= 129–145|isbn= 978-0-03-910284-5}}</ref>

[[Bird]]s and [[reptile]]s have relatively few skin [[gland]]s, although there may be a few structures for specific purposes, such as [[pheromone]]-secreting [[cell (biology)|cells]] in some [[reptile]]s, or the [[uropygial gland]] of most birds.<ref name=VB/>