Editing Gap junction (section)

==Occurrence and distribution==
Gap junctions have been observed in various animal organs and tissues where cells contact each other. From the 1950s to 1970s they were detected in:

* Human islet of Langerhans,<ref>{{cite journal |vauthors=Orci L, Malaisse-Lagae F, Amherdt M, etal |date=November 1975 |title=Cell contacts in human islets of Langerhans |url=http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=1102552 |archive-url=https://archive.today/20130414164326/http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=1102552 |url-status=dead |archive-date=April 14, 2013 |journal=J. Clin. Endocrinol. Metab. |volume=41 |issue=5 |pages=841–4 |doi=10.1210/jcem-41-5-841 |pmid=1102552|url-access=subscription }}</ref> myometrium,<ref>{{cite journal |last1=Garfield |first1=RE |last2=Sims |first2=SM |last3=Kannan |first3=MS |last4=Daniel |first4=EE |date=November 1978 |title=Possible role of gap junctions in activation of myometrium during parturition |journal=Am. J. Physiol. |volume=235 |issue=5 |pages=C168–79 |doi=10.1152/ajpcell.1978.235.5.C168 |pmid=727239 |s2cid=31610495}}</ref> and eye lens<ref>{{cite journal |last1=Goodenough |first1=DA |date=November 1979 |title=Lens gap junctions: a structural hypothesis for nonregulated low-resistance intercellular pathways |journal=Invest. Ophthalmol. Vis. Sci. |volume=18 |issue=11 |pages=1104–22 |pmid=511455}}</ref>
* Rat pancreas, liver, adrenal cortex, epididymis, duodenum, muscle,<ref>{{cite journal |vauthors=Friend DS, Gilula NB |date=June 1972 |title=Variations in tight and gap junctions in mammalian tissues |journal=J. Cell Biol. |volume=53 |issue=3 |pages=758–76 |doi=10.1083/jcb.53.3.758 |pmc=2108762 |pmid=4337577}}</ref> and seminiferous tubules<ref>{{cite journal |vauthors=McGinley D, Posalaky Z, Provaznik M |date=October 1977 |title=Intercellular junctional complexes of the rat seminiferous tubules: a freeze-fracture study |journal=Anat. Rec. |volume=189 |issue=2 |pages=211–31 |doi=10.1002/ar.1091890208 |pmid=911045 |s2cid=19611753}}</ref>
* Rabbit cornea,<ref>{{cite journal |author=Kreutziger GO |date=September 1976 |title=Lateral membrane morphology and gap junction structure in rabbit corneal endothelium |journal=Exp. Eye Res. |volume=23 |issue=3 |pages=285–93 |doi=10.1016/0014-4835(76)90129-9 |pmid=976372}}</ref> ovary,<ref>{{cite journal |last1=Albertini |first1=DF |last2=Anderson |first2=E. |date=Oct 1974 |title=The appearance and structure of intercellular connections during the ontogeny of the rabbit ovarian follicle with particular reference to gap junctions |journal=J Cell Biol |volume=63 |issue=1 |pages=234–50 |doi=10.1083/jcb.63.1.234 |pmc=2109337 |pmid=4417791}}</ref> and skin<ref>{{cite journal |author=Prutkin L |date=February 1975 |title=Mucous metaplasia and gap junctions in the vitamin A acid-treated skin tumor, keratoacanthoma |url=http://cancerres.aacrjournals.org/cgi/pmidlookup?view=long&pmid=1109802 |journal=Cancer Res. |volume=35 |issue=2 |pages=364–9 |pmid=1109802}}</ref>
* Monkey retina<ref>{{cite journal |last1=Raviola |first1=E |last2=Gilula |first2=NB |date=Jun 1973 |title=Gap junctions between photoreceptor cells in the vertebrate retina |journal=Proc Natl Acad Sci U S A |volume=70 |issue=6 |pages=1677–81 |bibcode=1973PNAS...70.1677R |doi=10.1073/pnas.70.6.1677 |pmc=433571 |pmid=4198274 |doi-access=free}}</ref>
* Chick embryos<ref>{{cite journal |last1=Bellairs |first1=R |last2=Breathnach |first2=AS |last3=Gross |first3=M |date=Sep 1975 |title=Freeze-fracture replication of junctional complexes in unincubated and incubated chick embryos |journal=Cell Tissue Res. |volume=162 |issue=2 |pages=235–52 |doi=10.1007/BF00209209 |pmid=1237352 |s2cid=38441429}}</ref>
* Frog embryos<ref name="ReferenceA">J. Cell Biol. 1974 Jul;62(1) 32-47.Assembly of gap junctions during amphibian neurulation. Decker RS, Friend DS.</ref>
* Fish [[blastoderm]]<ref>{{cite journal |vauthors=Lentz TL, Trinkaus JP |date=March 1971 |title=Differentiation of the junctional complex of surface cells in the developing Fundulus blastoderm |journal=J. Cell Biol. |volume=48 |issue=3 |pages=455–72 |doi=10.1083/jcb.48.3.455 |pmc=2108114 |pmid=5545331}}</ref>
* [[Crayfish]] nerves<ref name="robertson">{{cite journal |last1=Robertson |first1=JD |date=February 1953 |title=Ultrastructure of two invertebrate synapses |journal=Proceedings of the Society for Experimental Biology and Medicine |volume=82 |issue=2 |pages=219–23 |doi=10.3181/00379727-82-20071 |pmid=13037850 |s2cid=39294652}}</ref>
* [[Lamprey]] and [[tunicate]] heart<ref>{{cite journal |last1=Shibata |first1=Y |last2=Yamamoto |first2=T |date=March 1977 |title=Gap junctions in the cardiac muscle cells of the lamprey |journal=Cell Tissue Res. |volume=178 |issue=4 |pages=477–82 |doi=10.1007/BF00219569 |pmid=870202 |s2cid=21426059}}</ref><ref>{{cite journal |last1=Lorber |first1=V |last2=Rayns |first2=DG |date=April 1977 |title=Fine structure of the gap junction in the tunicate heart |journal=Cell Tissue Res. |volume=179 |issue=2 |pages=169–75 |doi=10.1007/BF00219794 |pmid=858161 |s2cid=21604678}}</ref>
* Goldfish and hamster pressure-sensing acoustico-vestibular receptors<ref>{{cite journal |vauthors=Hama K, Saito K |date=February 1977 |title=Gap junctions between the supporting cells in some acoustico-vestibular receptors |journal=J. Neurocytol. |volume=6 |issue=1 |pages=1–12 |doi=10.1007/BF01175410 |pmid=839246 |s2cid=30090247}}</ref>
* ''[[Daphnia]]'' [[hepatic caecum]]<ref>{{cite journal |last1=Hudspeth |first1=AJ |last2=Revel |first2=JP |date=Jul 1971 |title=Coexistence of gap and septate junctions in an invertebrate epithelium |journal=J. Cell Biol. |volume=50 |issue=1 |pages=92–101 |doi=10.1083/jcb.50.1.92 |pmc=2108432 |pmid=5563454}}</ref>
* [[Cephalopod]] digestive epithelium<ref>{{cite journal |last1=Boucaud-Camou |first1=Eve |year=1980 |title=Junctional structures in digestive epithelia of a cephalopod |journal=Tissue Cell |volume=12 |issue=2 |pages=395–404 |doi=10.1016/0040-8166(80)90013-0 |pmid=7414602}}</ref>
* [[Hydra (genus)|''Hydra'']] muscle<ref name="Hand72" />
* Cockroach hemocyte capsules<ref>{{cite journal |author=Baerwald RJ |year=1975 |title=Inverted gap and other cell junctions in cockroach hemocyte capsules: a thin section and freeze-fracture study |journal=Tissue Cell |volume=7 |issue=3 |pages=575–85 |doi=10.1016/0040-8166(75)90027-0 |pmid=1179417}}</ref>
* Reaggregated cells<ref>{{cite journal |vauthors=Johnson R, Hammer M, Sheridan J, Revel JP |date=November 1974 |title=Gap junction formation between reaggregated Novikoff hepatoma cells |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=71 |issue=11 |pages=4536–40 |bibcode=1974PNAS...71.4536J |doi=10.1073/pnas.71.11.4536 |pmc=433922 |pmid=4373716 |doi-access=free}}</ref><ref>{{cite journal |last1=Knudsen |first1=KA |last2=Horwitz |first2=AF |year=1978 |title=Toward a mechanism of myoblast fusion |journal=Prog Clin Biol Res |volume=23 |pages=563–8 |pmid=96453}}</ref>

Gap junctions have continued to be found in nearly all healthy animal cells that touch each other. Techniques such as [[confocal microscopy]] allow more rapid surveys of large areas of tissue. Tissues that were traditionally considered to have isolated cells such as in bone were shown to have cells that were still connected with gap junctions, however tenuously.<ref>{{cite journal |vauthors=Jones SJ, Gray C, Sakamaki H, etal |title=The incidence and size of gap junctions between the bone cells in rat calvaria |journal=Anat. Embryol. |volume=187 |issue=4 |pages=343–52 |date=April 1993 |pmid=8390141 |doi=10.1007/BF00185892 |s2cid=33191311 }}</ref> Exceptions to this are cells not normally in contact with neighboring cells, such as blood cells suspended in blood plasma. Adult skeletal muscle is a possible exception to the rule though their large size makes it difficult to be certain of this. An argument used against skeletal muscle gap junctions is that if they were present gap junctions may propagate contractions in an arbitrary way through cells making up the muscle. However, other muscle types do have gap junctions which do not cause arbitrary contractions.<ref>{{cite journal | pmid = 15679888 | doi=10.1186/1475-925X-4-7 | pmc=549032 | volume=4 | issue=1 | title=Gap-junction channels inhibit transverse propagation in cardiac muscle | year=2005 | journal=Biomed Eng Online | pages=7 | last1 = Sperelakis | first1 = Nicholas | last2 = Ramasamy | first2 = Lakshminarayanan | doi-access=free }}</ref> Sometimes the number of gap junctions are reduced or absent in diseased tissues such as cancers<ref>{{cite journal |vauthors=Larsen WJ, Azarnia R, Loewenstein WR |title=Intercellular communication and tissue growth: IX. Junctional membrane structure of hybrids between communication-competent and communication-incompetent cells |journal=J. Membr. Biol. |volume=34 |issue=1 |pages=39–54 |date=June 1977 |pmid=561191 |doi=10.1007/BF01870292 |s2cid=2831462 }}</ref><ref>{{cite journal |vauthors=Corsaro CM, Migeon BR|title=Comparison of contact-mediated communication in normal and transformed human cells in culture |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=74 |issue=10 |pages=4476–80 |date=October 1977 |pmid=270694 |pmc=431966 |bibcode=1977PNAS...74.4476C |doi=10.1073/pnas.74.10.4476 |doi-access=free }}</ref><ref>{{cite journal | pmid = 11145599 | volume=142 | issue=1 | title=Developmental exposure to estrogens alters epithelial cell adhesion and gap junction proteins in the adult rat prostate |date=January 2001 | journal=Endocrinology | pages=359–69 | doi = 10.1210/endo.142.1.7893 | last1 = Habermann | first1 = H | last2 = Chang | first2 = WY | last3 = Birch | first3 = L | last4 = Mehta | first4 = P | last5 = Prins | first5 = GS| doi-access = free }}</ref> or the aging process.<ref>{{cite journal | pmid = 761600 | volume=119 | issue=1 | title=Development of the aging cell surface. Reduction of gap junction-mediated metabolic cooperation with progressive subcultivation of human embryo fibroblasts (IMR-90) |date=March 1979 | journal=Exp. Cell Res. | pages=127–43 | doi=10.1016/0014-4827(79)90342-2 | last1 = Kelley | first1 = Robert O. | last2 = Vogel | first2 = Kathryn G. | last3 = Crissman | first3 = Harry A. | last4 = Lujan | first4 = Christopher J. | last5 = Skipper | first5 = Betty E.}}</ref>

[[File:Cell pannexin tree.svg|thumb|Cell pannexin tree with white squares having communication proteins yet to be discovered]]Since the discovery of innexins, pannexins and unnexins, gaps in our knowledge of [[intercellular communication]] are becoming more defined. Innexins look and behave similarly to connexins and can be seen to fill a similar role to connexins in invertebrates. Pannexins also look individually similar to connexins though they do not appear to easily form gap junctions. Of the over 20 metazoan groups connexins have been found only in [[vertebrata]] and [[tunicata]]. Innexins and pannexins are far more widespread including innexin homologues in vertebrates.<ref>{{cite journal |last1=Phelan |first1=Pauline |title=Innexins: members of an evolutionarily conserved family of gap-junction proteins |journal=Biochimica et Biophysica Acta (BBA) - Biomembranes |date=June 2005 |volume=1711 |issue=2 |pages=225–245 |doi=10.1016/j.bbamem.2004.10.004|pmid=15921654 |doi-access=free }}</ref><ref>{{cite journal |last1=Hervé |first1=Jean-Claude |last2=Phelan |first2=Pauline |last3=Bruzzone |first3=Roberto |last4=White |first4=Thomas W. |title=Connexins, innexins and pannexins: Bridging the communication gap |journal=Biochimica et Biophysica Acta (BBA) - Biomembranes |date=December 2005 |volume=1719 |issue=1–2 |pages=3–5 |doi=10.1016/j.bbamem.2005.11.013|doi-access=free |pmid=16359939 }}</ref> The unicellular [[Trypanosomatida]]e parasites presumably have unnexin genes to aid in their infection of animals including humans.<ref>{{cite journal |last1=Güiza |first1=Juan |last2=García |first2=Aníbal |last3=Arriagada |first3=Javiera |last4=Gutiérrez |first4=Camila |last5=González |first5=Jorge |last6=Márquez-Miranda |first6=Valeria |last7=Alegría-Arcos |first7=Melissa |last8=Duarte |first8=Yorley |last9=Rojas |first9=Maximiliano |last10=González-Nilo |first10=Fernando |last11=Sáez |first11=Juan C. |last12=Vega |first12=José L. |title=Unnexins: Homologs of innexin proteins in Trypanosomatidae parasites |journal=Journal of Cellular Physiology |date=February 2022 |volume=237 |issue=2 |pages=1547–1560 |doi=10.1002/jcp.30626|pmid=34779505 |s2cid=244116450 }}</ref> The even smaller [[adenovirus]] has its own vinnexin,<ref>{{cite journal |last1=Turnbull |first1=Matthew W. |last2=Volkoff |first2=Anne-Nathalie |last3=Webb |first3=Bruce A. |last4=Phelan |first4=Pauline |title=Functional gap junction genes are encoded by insect viruses |journal=Current Biology |date=July 2005 |volume=15 |issue=13 |pages=R491–R492 |doi=10.1016/j.cub.2005.06.052|doi-access=free |pmid=16005277 |bibcode=2005CBio...15.R491T }}</ref> apparently derived from an innexin, to aid its transmission between the virus's insect hosts.

The term ''gap junction'' cannot be defined by a single protein or family of proteins with a specific function. For example, gap junction structures are found in [[porifera|sponges]], despite the absence of pannexins. While we are still at the early stages of understanding the nervous system of a sponge<ref>{{cite journal |last1=Moroz |first1=Leonid L. |last2=Romanova |first2=Daria Y. |title=Alternative neural systems: What is a neuron? (Ctenophores, sponges and placozoans) |journal=Frontiers in Cell and Developmental Biology |date=23 December 2022 |volume=10 |pages=1071961 |doi=10.3389/fcell.2022.1071961|pmid=36619868 |pmc=9816575 |doi-access=free }}</ref> the ''gap junctions'' of sponges may as yet indicate intercellular communications pathways.<ref>{{cite journal |last1=Bergquist |first1=P.R. |last2=Green |first2=C.R. |title=An ultrastructural study of settlement and metamorphosis in sponge larvae |journal=Cah. Biol. Mar. |date=1977 |volume=18 |pages=289–302}}</ref><ref>{{cite journal |last1=Green |first1=C.R. |last2=Bergquist |first2=P.R. |title=Cell membrane specializations in the Porifera |journal=Coll Int Cent Natn Res Scient |date=1979 |volume=291 |pages=153–158}}</ref>