Ian Smith, of Mosgiel, asks :-
Why do fingernails grow much faster than toenails?
Brian Hyland, a physiologist at the University of Otago, responded.
Nails are made of dead cells. In the nail bed, living cells divide and the top layer gets pushed forward as more and more division happens behind. As they get pushed forward, the cells get flattened and squashed together and gradually lose the machinery necessary to be considered living, and instead become completely filled with a many copies of a tough, string-like protein called keratin, which makes the nail hard. Keratin is also found in hair and skin, teeth, feathers hooves and claws. Differences in hardness and flexibility between these different structures depend on the exact type of keratin present, how much is in the cell, how it is arranged, and the presence of other substances.
Determining the rates of growth for specific nails is actually quite important for health research. As the body makes the hard nail, substances that happen in the body at that time get included and "locked in", so the nail holds a record of the amount present in the blood at the time. So nails can be used as a record of diet, and of exposure to toxins such as lead, arsenic and tobacco smoke over long periods of time. It is necessary to know how fast they grow to know how much time is represented by a particular nail clipping.
The exact mechanism for why fingernails grow faster than toenails has not been established. The favourite current theory is that the cells at the base of the nail that produce it are sensitive to the small movements created when the nail is tapped or moved during normal use, and this stimulates accelerated growth. Thus, the fingers, which are much more actively used than the toes in daily life, get more of this stimulus and so grow faster. Consistent with this, nails on the right hand are said to grow faster than the left in right-handed people, and fingers that are strapped together during treatment for a fracture in one of the fingers show lower growth rates than free-moving fingers. But research actually proving this mechanism is in fact how it works is still waiting to be done - this is true for a surprising number of small but interesting facts about the body.