Editing Vocal cords (section)

===In newborns===
Newborns have a uniform single layered lamina propria, which appears loose with no vocal ligament.<ref name="Sato, K. 2001">{{cite journal |vauthors=Sato K, Hirano M, Nakashima T |title=Fine structure of the human newborn and infant vocal fold mucosae |journal=Ann. Otol. Rhinol. Laryngol. |volume=110 |issue=5 Pt 1 |pages=417–24 |date=May 2001 |pmid=11372924 |doi=10.1177/000348940111000505 |s2cid=22257136 }}</ref> The monolayered lamina propria is composed of ground substances such as [[hyaluronic acid]] and [[fibronectin]], [[fibroblast]]s, elastic fibers, and collagenous fibers. While the fibrous components are sparse, making the lamina propria structure loose, the hyaluronic acid (HA) content is high.

HA is a bulky, negatively charged glycosaminoglycan, whose strong affinity with water procures hyaluronic acid its viscoelastic and shock absorbing properties essential to vocal biomechanics.<ref name=Ward02>{{cite journal |vauthors=Ward PD, Thibeault SL, Gray SD |title=Hyaluronic acid: its role in voice |journal=J Voice |volume=16 |issue=3 |pages=303–9 |date=September 2002 |pmid=12395982 |doi=10.1016/s0892-1997(02)00101-7}}</ref> Viscosity and elasticity are critical to voice production. Chan, Gray and Titze, quantified the effect of hyaluronic acid on both the viscosity and the elasticity of vocal folds by comparing the properties of tissues with and without HA.<ref name=Chan01>{{cite journal |vauthors=Chan RW, Gray SD, Titze IR |title=The importance of hyaluronic acid in vocal fold biomechanics |journal=Otolaryngol Head Neck Surg |volume=124 |issue=6 |pages=607–14 |date=June 2001 |pmid=11391249 |url=http://oto.sagepub.com/cgi/pmidlookup?view=long&pmid=11391249 |doi=10.1067/mhn.2001.115906}}</ref> The results showed that removal of hyaluronic acid decreased the stiffness of the vocal cords by an average of 35%, but increased their dynamic viscosity by an average of 70% at frequencies higher than 1&nbsp;Hz. Newborns have been shown to cry an average of 6.7 hours per day during the first 3 months, with a sustained pitch of 400–600&nbsp;Hz, and a mean duration per day of 2 hours.<ref name=Schweinfurth08>{{cite journal |vauthors=Schweinfurth JM, Thibeault SL |title=Does hyaluronic acid distribution in the larynx relate to the newborn's capacity for crying? |journal=Laryngoscope |volume=118 |issue=9 |pages=1692–9 |date=September 2008 |pmid=18596477 |doi=10.1097/MLG.0b013e3181782754 |s2cid=35188777 }}</ref> Similar treatment on adult vocal cords would quickly result in edema, and subsequently aphonia. Schweinfurth and al. presented the hypothesis that high [[hyaluronic acid]] content and distribution in newborn vocal cords is directly associated with newborn crying endurance.<ref name=Schweinfurth08/> These differences in newborn vocal fold composition would also be responsible for newborns inability to articulate sounds, besides the fact that their lamina propria is a uniform structure with no vocal ligament. The layered structure necessary for phonation will start to develop during the infancy and until the adolescence.<ref name="Sato, K. 2001"/>

The [[fibroblast]]s in the newborn [[Reinke's space]] are immature, showing an oval shape, and a large nucleus-cytoplasm ratio.<ref name="Sato, K. 2001"/> The rough endoplasmic reticulum and Golgi apparatus, as shown by electron micrographs, are not well developed, indicating that the cells are in a resting phase. The collagenous and reticular fibers in the newborn the vocal cords are fewer than in the adult one, adding to the immaturity of the vocal fold tissue.

In the infant, many fibrous components were seen to extend from the macula flava towards the Reinke's space. Fibronectin is very abundant in the Reinke's space of newborn and infant. Fibronectin is a glycoprotein that is believed to act as a template for the oriented deposition of the collagen fibers, stabilizing the collagen fibrils. Fibronectin also acts as a skeleton for the elastic tissue formation.<ref name="Sato, K. 2001"/> Reticular and collagenous fibers were seen to run along the edges of the vocal cords throughout the entire lamina propria.<ref name="Sato, K. 2001"/> Fibronectin in the Reinke's space appeared to guide those fibers and orient the fibril deposition. The elastic fibers remained sparse and immature during infancy, mostly made of microfibrils. The fibroblasts in the infant Reinke's space were still sparse but spindle-shaped. Their rough endoplasmic reticulum and Golgi apparatus were still not well developed, indicating that despite the change in shape, the fibroblasts still remained mostly in a resting phase. Few newly released materials were seen adjacent to the fibroblasts. The ground substance content in the infant [[Reinke's space]] seemed to decrease over time, as the fibrous component content increased, thus slowly changing the vocal fold structure.