Thursday, September 04, 2014
Finger and toe nails: The factoids
Human nails grow at a rate which varies with many factors: age, sex and the finger or toe in question as well as nutrition. However, typically in healthy populations fingernails grow at about 0.1mm/day and toenails at about 0.05mm/day. Primates appear to have evolved long fingers for grasping and claws flattened into nails to protect the distal small bones that lay beneath. Fingernails grow about three-four times faster than toenails. Scientists remain baffled as to the biological mechanism behind the different growth rates. But, they do have theories based on more than 100 years of finger and toenail observations. Nails begin in the nail matrix behind and below the nail plate underneath the epidermis. This specialized epithelial tissue produces modified keratin, a tough, fibrous protein that gives structure to the nail plate. The nail plate is attached to the skin below (nail bed) by a series of grooves. Some people believe fingernails and toenails grow fastest during the summer, when circulation is best. In the case of the latter we are more likely to see our feet during the summer months and therefore more aware of toenails than when safe housed in winter boots.
The little we know about nails comes from scientists and others doing controlled experiments on themselves. For much of the 20th century, Dr William B Bean (1909 - 1989) was fascinated with finger nails and undertook a linear (longitudinal) self study of fingernail growth. His research was published in a series of papers in the Archives of Internal Medicine. The works were entitled Nail Growth: Twenty-Five Years' Observation in 1968. Further works, Nail Growth: 30 Years of Observation appeared in 1974. From the age of 32 he scratched a line on his nail from where it emerged at the cuticle on the first day of every month. Then he measured how far these lines travelled. Later to aid accuracy he had a small dot tattooed just above his cuticle and used this as a baseline. After his first twenty years of observations, he discovered his rate of nail growth had slowed by more than a month. This led him to believe that blood flow and metabolism were linked to the rate of growth. He continued his observations and published, Nail Growth: Thirty-Five Years of Observation in 1980. After 35-years of observation of the growth of his nails he was able to conclude normal nail growth slows with age. The average daily growth of the left thumbnail, for instance, has varied from 0.123mm a day during the first part of the study when I was 32 years of age to 0.095mm a day at the age of 67.
Joseph Honoré Simon Beau (1806–1865) was French physician fascinated by signs and symptoms of circulatory changes in the tissues. He believed fingernails grow faster because they are closer to your heart, and therefore receive more blood. He also observed when toenails were damaged by trauma the replacement nail was thicker caused by increased circulation to the matrix. When the area got better the epithelial growth slowed down causing the thicker nail plate to thin out. Beau's lines describe deep grooved lines that run from side to side on the fingernail or the toenail. They may look like indentations or ridges in the nail plate. As the nail grows out, the ridge visibly moves upwards toward the nail edge. When the ridge reaches the nail edge the fingertips can become quite sore due to the misshapen nail pressing into the flesh deeper than usual. Beau’s lines may also be a sign of systemic disease.
Dermatologist, Dr Rodney Dawber did his own experiment after his left ring finger was jammed during a rugby match. Based on some early research he had read, Dawber believed the growth rate of the nail depended upon “terminal trauma,” i.e. how often a fingertip is used. A splinted finger, he reasoned, would get a lot less fingertip use. Accordingly, he hypothesized that the nail on his splinted finger would lag behind the rest of his fingernails. For the three months it was splinted, Dawber’s left ring fingernail grew 25 percent slower than the three months after he took the splint off. Dawber acknowledged the injury itself might have affected the growth, but he noted that the damage was limited to his tendon, not the blood vessels or bone. He also noted the fingernails on his dominant right hand grew faster than his left, while his toenails on both feet grew at the same speed. Dawber concluded fingernails grew in response to how much their corresponding fingertip was used. Frequent fingertip use signals to the nail matrix, the nail is probably being worn down faster, so it calibrates by increasing the rate of growth. It slows down with less use, so the nail does not grow too long and get in the way.