Editing Spider silk (section)

===Silk gland===
{{further information|Spider anatomy}}
[[File:Schematic of the spiders spinning apparatus.svg|thumb|760x760px|Schematic of the spiders spinning apparatus and structural hierarchy in silk assembling related to assembly into fibers.<ref>{{Cite journal|last1=Zhao|first1=Yue|last2=Li|first2=Yanrong|last3=Hien|first3=K. T. T.|last4=Mizutani|first4=Goro|last5=Rutt|first5=Harvey N.|year=2019|title=Observation of Spider Silk by Femtosecond Pulse Laser Second Harmonic Generation Microscopy|journal=Surf. Interface Anal.|volume=51|issue=1|pages=50–56|arxiv=1812.10390|doi=10.1002/sia.6545|s2cid=104921418}}</ref><ref name="Rising 2015 309–15">{{Cite journal|last1=Rising|first1=A.|last2=Johansson|first2=J.|year=2015|title=Toward spinning artificial spider silk|journal=Nat. Chem. Biol.|volume=11|issue=5|pages=309–15|doi=10.1038/nchembio.1789|pmid=25885958}}</ref><ref name="Eisoldt 2012 355–61">{{Cite journal|last1=Eisoldt|first1=L.|last2=Thamm|first2=C.|last3=Scheibel|first3=T.|year=2012|title=The role of terminal domains during storage and assembly of spider silk proteins|journal=Biopolymers|volume=97|issue=6|pages=355–61|doi=10.1002/bip.22006|pmid=22057429|s2cid=46685716|doi-access=free}}</ref><ref>{{Cite journal|last1=Eisoldt|first1=L.|last2=Smith|first2=A.|last3=Scheibel|first3=T.|year=2011|title=Decoding the secrets of spider silk|journal=Mater. Today|volume=14|issue=3|pages=80–86|doi=10.1016/S1369-7021(11)70057-8|doi-access=free}}</ref><ref>{{Cite journal|last1=Tokareva|first1=O.|last2=Jacobsen|first2=M.|last3=Buehler|first3=M.|last4=Wong|first4=J.|last5=Kaplan|first5=D. L.|year=2014|title=Structure–function–property–design interplay in biopolymers: Spider silk|journal=Acta Biomater.|volume=10|issue=4|pages=1612–26|doi=10.1016/j.actbio.2013.08.020|pmid=23962644|pmc=3926901}}</ref> In the process of dragline production, the primary structure protein is secreted first from secretory granules in the tail.<ref name="Vollrath 2001 541–48">{{Cite journal|last1=Vollrath|first1=F.|last2=Knight|first2=D. P.|year=2001|title=Liquid crystalline spinning of spider silk|journal=Nature|volume=410|issue=6828|pages=541–48|doi=10.1038/35069000|pmid=11279484|bibcode=2001Natur.410..541V|s2cid=205015549}}</ref> In the ampullate (neutral environment, pH = 7), the proteins form a soft micelle of several tens of nanometers by self-organization because the hydrophilic terminals are excluded.<ref name="Kluge 2008 244–51">{{Cite journal|last1=Kluge|first1=J. A.|last2=Rabotyagova|first2=O.|last3=Leisk|first3=G. G.|last4=Kaplan|first4=D. L.|year=2008|title=Spider silks and their applications|journal=Trends Biotechnol.|volume=26|issue=5|pages=244–51|doi=10.1016/j.tibtech.2008.02.006|pmid=18367277}}</ref> In ampullate, the concentration of the protein is high.<ref>{{Cite journal|last1=Hijirida|first1=D. H.|last2=Do|first2=K. G.|last3=Michal|first3=C.|last4=Wong|first4=S.|last5=Zax|first5=D.|last6=Jelinski|first6=L. W.|year=1996|title=13C NMR of Nephila clavipes major ampullate silk gland|journal=Biophys. J.|volume=71|issue=6|pages=3442–47|doi=10.1016/S0006-3495(96)79539-5|pmid=8968613|pmc=1233831|bibcode=1996BpJ....71.3442H}}</ref><ref>{{Cite journal|last1=Lefvre|first1=T.|last2=Boudreault|first2=S.|last3=Cloutier|first3=C.|last4=Pezolet|first4=M.|year=2008|title=Conformational and orientational transformation of silk proteins in the major ampullate gland of Nephila clavipes spiders|journal=Biomacromolecules|volume=9|issue=9|pages=2399–407|doi=10.1021/bm800390j|pmid=18702545}}</ref> Then, the micelles are squeezed into the duct. The long axis direction of the molecules is aligned parallel to the duct by a mechanical frictional force and partially oriented.<ref name="Vollrath 2001 541–48"/><ref name="Kluge 2008 244–51"/><ref>{{Cite journal|last=Lewis|first=R. V.|year=2006|title=Spider silk: Ancient ideas for new biomaterials|journal=Chem. Rev.|volume=106|issue=9|pages=3762–74|doi=10.1021/cr010194g|pmid=16967919}}</ref> The continuous lowering of pH from 7.5 to 8.0 in the tail to presumably close to 5.0 occurs at the end of the duct.<ref name="Rising 2015 309–15"/><ref>{{Cite journal|last=Andersson|first=M. |display-authors=etal |year=2014|title=Carbonic anhydrase generates CO2 and H+ that drive spider silk formation via opposite effects on the terminal domains|journal=PLOS Biol.|volume=12|issue=8 |pages=e1001921|doi=10.1371/journal.pbio.1001921 |pmid=25093327 |pmc=4122339 |doi-access=free }}</ref><ref>{{Cite journal|last=Kronqvist|first=N. |display-authors=etal |year=2014|title=Sequential pH-driven dimerization and stabilization of the N-terminal domain enables rapid spider silk formation|journal=Nat. Commun.|volume=5|pages=3254|doi=10.1038/ncomms4254 |pmid=24510122 |bibcode=2014NatCo...5.3254K |doi-access=free}}</ref> Ion exchange, acidification, and water removal all happen in the duct.<ref name="Eisoldt 2012 355–61"/> The shear and elongational forces lead to phase separation.<ref name="Eisoldt 2012 355–61"/> In the acidic bath of the duct, the molecules attain a high concentration liquid crystal state.<ref>{{Cite journal|last1=Knight|first1=D. P.|last2=Vollrath|first2=F.|year=1999|title=Liquid crystals and flow elongation in a spider's silk production line|journal=Proc. R. Soc. B|volume=266|issue=1418|pages=519–23|doi=10.1098/rspb.1999.0667|pmc=1689793}}</ref> Finally, the silk is spun from the taper exterior. The molecules become more stable helixes and β-sheets from the liquid crystal.]]
The gland's visible, or external, part is termed the [[spinneret (spider)|spinneret]]. Depending on the complexity of the species, spiders have two to eight spinnerets, usually in pairs. Species have varying specialised glands, ranging from a sac with an opening at one end, to the complex, multiple-section ampullate glands of the [[golden silk orb-weaver]]s.<ref>{{cite journal |author= Dicko, C.|author2= Porter, D.|author3= Bond, J.|author4= Kenney, J. M.|author5= Vollratht, F.|name-list-style= amp |date= 2008 |title= Structural disorder in silk proteins reveals the emergence of elastomericity |journal= Biomacromolecules |volume= 9 |pages= 216–21 |doi= 10.1021/bm701069y |pmid= 18078324 |issue= 1}}</ref>

Behind each spinneret on the surface of the spider lies a gland, a generalised form of which is shown in the figure.

{{Clear}}
;Gland characteristics

[[File:Spider silk duct.svg|right|thumb|280px|Schematic of a generalised gland of a [[Golden silk orb-weaver]]. Each differently coloured section highlights a discrete section of the gland.<ref>{{cite journal |author= Lefèvre, T.|author2= Boudreault, S.|author3= Cloutier, C.|author4= Pézolet, M.|name-list-style= amp |date= 2008 |title= Conformational and orientational transformation of silk proteins in the major ampullate gland of Nephila clavipes spiders |journal= Biomacromolecules |volume= 9 |pages= 2399–407 |doi= 10.1021/bm800390j |issue= 9 |pmid= 18702545}}</ref><ref>{{cite journal |author= Heim, M.|author2= Keerl, D.|author3= Scheibel, T.|name-list-style= amp |date= 2009 |title= Spider Silk: From Soluble Protein to Extraordinary Fiber |journal= Angewandte Chemie International Edition |volume= 48 |pages= 3584–96 |doi= 10.1002/anie.200803341 |pmid= 19212993|issue= 20}}</ref>]]

# The leftmost section is the secretory or tail section. The walls of this section are lined with cells that secrete proteins Spidroin I and Spidroin II, the main components of this spider's dragline. These proteins are found in the form of droplets that gradually elongate to form long channels along the length of the final fibre, hypothesised to assist in preventing crack formation or self-healing.<ref>{{cite journal |author= Heinhorst, S.|author2= Cannon, G. |date= 2002 |title= Nature: Self-Healing Polymers and Other Improved Materials |journal=J. Chem. Educ. |volume= 79 |issue= 1 |page= 10 |doi=10.1021/ed079p10|bibcode = 2002JChEd..79...10H }}</ref>
# The ampulla (storage sac) is next. This stores and maintains the gel-like unspun silk dope. In addition, it secretes proteins that coat the surface of the final fibre.<ref name="Vollrath 410">{{cite journal |author=Vollrath, F.|author2=Knight, D. P.|name-list-style=amp |date=2001 |title=Liquid crystalline spinning of spider silk |journal= Nature |volume= 410 |pages= 541–48 |doi=10.1038/35069000 |issue=6828 |pmid=11279484 |bibcode=2001Natur.410..541V|s2cid=205015549}}</ref>
# The funnel rapidly reduces the large diameter of the storage sac to the small diameter of the tapering duct.
# The final length is the tapering duct, the site of most of the fibre formation. This consists of a tapering tube with several tight sharp turns, a valve near the end includes a spigot from which the solid silk fibre emerges. The tube tapers hyperbolically, therefore the unspun silk is under constant elongational [[shear stress]], an important factor in fibre formation. This section is lined with cells that exchange ions, reduce the dope pH from neutral to acidic, and remove water from the fibre.<ref name=":0">{{Cite journal|last1=Knight|first1=D. P.|last2=Vollrath|first2=F.|date=2001-04-01|title=Changes in element composition along the spinning duct in a Nephila spider|journal=Die Naturwissenschaften|volume=88|issue=4|pages=179–82|issn=0028-1042|pmid=11480706|doi=10.1007/s001140100220|bibcode=2001NW.....88..179K|s2cid=26097179}}</ref> Collectively, the shear stress and the ion and pH changes induce the liquid silk dope to undergo a phase transition and condense into a solid protein fibre with high molecular organisation. The spigot at the end has lips that clamp around the fibre, controlling fibre diameter and further retaining water.
# Almost at the end is a valve. Though discovered some time ago, its precise purpose is still under discussion. It is believed to assist in restarting and rejoining broken fibres,<ref name="Vollrath 98">{{cite journal |author= Vollrath, F.|author2= Knight, D. P.|name-list-style= amp |date= 1998 |title= Structure and function of the silk production pathway in spider Nephila edulis |journal= Int J Biol Macromol |volume= 24 |pages= 243–49 |doi=10.1016/S0141-8130(98)00095-6 |issue= 2–3 |pmid= 10342771}}</ref> acting much in the way of a [[helical pump]], regulating the thickness of the fibre,<ref name="Wilson69"/> and/or clamping the fibre as a spider falls upon it.<ref name="Vollrath 98"/><ref>{{cite journal |author= Wilson, R. S. |date= 1962 |title= The Control of Dragline Spinning in the Garden Spider |journal= Quarterly Journal of Microscopical Science |volume= 103 |pages= 557–71 }}</ref> The similarity of the silk worm's silk press and the roles each of these valves play in the silk production in these two organisms are under discussion.

Throughout the process the silk appears to have a nematic texture,<ref>{{cite journal |author= Magoshi, J.|author2= Magoshi, Y.|author3= Nakamura, S.|name-list-style= amp |date= 1985 |title= Physical properties and structure of silk: 9. Liquid crystal formation of silk fibroin |journal= Polym. Commun. |volume= 26 |pages= 60–61 }}</ref> in a manner similar to a [[Liquid crystal#Nematic phase|liquid crystal]], arising in part due to the high protein concentration of silk dope (around 30% in terms of weight per volume).<ref>{{Cite journal|last1=Chen|first1=Xin|last2=Knight|first2=David P.|last3=Vollrath|first3=Fritz|date=2002-07-01|title=Rheological characterization of nephila spidroin solution|journal=Biomacromolecules|volume=3|issue=4|pages=644–48|issn=1525-7797|pmid=12099805|doi=10.1021/bm0156126}}</ref> This allows the silk to flow through the duct as a liquid while maintaining molecular order.

As an example of a complex spinning field, the spinneret apparatus of an adult ''[[Araneus diadematus]]'' (garden cross spider) consists of many glands shown below.<ref name="Heimer"/> A similar gland architecture appears in the black widow spider.<ref>{{cite journal|pmc=3341101|year=2011|last1=Jeffery|first1=F|title=Microdissection of Black Widow Spider Silk-producing Glands|journal=Journal of Visualized Experiments|issue=47|pages=2382|last2=La Mattina|first2=C|last3=Tuton-Blasingame|first3=T|last4=Hsia|first4=Y|last5=Gnesa|first5=E|last6=Zhao|first6=L|last7=Franz|first7=A|last8=Vierra|first8=C|doi=10.3791/2382|pmid=21248709}}</ref>
* 500 pyriform glands for attachment points
* 4 ampullate glands for the web frame
* 300 aciniform glands for the outer lining of egg sacs, and for ensnaring prey
* 4 tubuliform glands for egg sac silk
* 4 aggregate glands for adhesive functions
* 2 coronate glands for the thread of adhesion lines