Article by: Kelly Dobos
Fingernails and toenails are made of compact layers of keratinized epithelial cells and cover the dorsal surface of fingertips and toes. The nail plate is comprised of about 25 layers of flat keratinized cells and is typically between 0.5 and 1.0 mm in thickness. Keratins contain a high amount of the amino acid cysteine. Cysteine’s thiol group (C-SH) participates in disulfide bonding between keratin filaments giving the nail its strength.
Nail production occurs at the root, which lies just underneath the eponychium, or cuticle. As new cells are laid down, older cells are pushed forward lengthening the nail. While many other factors are involved, the growth rate of nails is related to the length of the terminal phalanges, the outermost bone of the fingers or toes. Fingernails can grow at a rate of 4 times faster than toenails.
The nail covers the nail bed and the free edge extends past the hyponychium, an area of thickened stratum corneum. The nail is mostly transulecent, but at the base of the nail, actively dividing cells in the nail bed are thicker and obscure the vasculature below. This is why you see a white, half-moon shaped feature called the lunula (from the Latin word for moon, luna). The lunula is most obvious in the thumb and may not be visible in the pinky finger.
Due to it’s highly keratinized nature; the nail is hard to penetrate for treatment of infection or disease. Understanding the structure nails allows for improved design of treatments and also, decorative nail polishes.