Editing Vocal cords (section)

====Maturation====
Vocal fold structure in adults is quite different from that in newborns. Exactly how the vocal cord mature from an immature monolayer in newborns to a mature three layer tissue in adults is still unknown, however a few studies have investigated the subjects and brought some answers.

Hirano et al. previously found that the newborns did not have a true lamina propria, but instead had cellular regions called maculae flavae, located at the anterior and posterior ends of the loose vocal fold tissue.<ref name="Sato, K. 2001"/><ref>{{cite journal |vauthors=Sato K, Hirano M |title=Histologic investigation of the macula flava of the human newborn vocal fold |journal=Ann. Otol. Rhinol. Laryngol. |volume=104 |issue=7 |pages=556–62 |date=July 1995 |pmid=7598369 |doi=10.1177/000348949510400710|s2cid=32824702 }}</ref> Boseley and Hartnick examined at the development and maturation of pediatric human vocal fold lamina propria.<ref>{{cite journal |vauthors=Boseley ME, Hartnick CJ |title=Development of the human true vocal fold: depth of cell layers and quantifying cell types within the lamina propria |journal=Ann. Otol. Rhinol. Laryngol. |volume=115 |issue=10 |pages=784–8 |date=October 2006 |pmid=17076102 |doi=10.1177/000348940611501012|s2cid=21613826 }}</ref> Hartnick was the first one to define each layer by a change in their cellular concentration.<ref>{{cite journal |vauthors=Hartnick CJ, Rehbar R, Prasad V |title=Development and maturation of the pediatric human vocal fold lamina propria |journal=Laryngoscope |volume=115 |issue=1 |pages=4–15 |date=January 2005 |pmid=15630357 |doi=10.1097/01.mlg.0000150685.54893.e9 |s2cid=6024918 }}</ref> He also found that the lamina propria monolayer at birth and shortly thereafter was hypercellular, thus confirming Hirano's observations. By 2 months of age, the vocal fold started differentiating into a bilaminar structure of distinct cellular concentration, with the superficial layer being less densely populated than the deeper layer. By 11 months, a three-layered structure starts to be noted in some specimens, again with different cellular population densities. The superficial layer is still hypocellular, followed by an intermediate more hypercellular layer, and a deeper hypercellular layer, just above the vocalis muscle. Even though the vocal cords seem to start organizing, this is not representative of the trilaminar structure seen in adult tissues, where the layer are defined by their differential elastin and collagen fiber compositions. By 7 years of age, all specimens show a three-layered vocal fold structure, based on cellular population densities. At this point, the superficial layer was still hypocellular, the middle layer was the hypercellular one, with also a greater content of elastin and collagen fibers, and the deeper layer was less cellularly populated. Again, the distinction seen between the layers at this stage is not comparable to that seen in the adult tissue. The maturation of the vocal cords did not appear before 13 years of age, where the layers could be defined by their differential fiber composition rather than by their differential cellular population. The pattern now show a hypocellular superficial layer, followed by a middle layer composed predominantly of elastin fiber, and a deeper layer composed predominantly of collagen fibers. This pattern can be seen in older specimens up to 17 years of age, and above. While this study offers a nice way to see the evolution from immature to mature vocal cords, it still does not explain what is the mechanism behind it.