STRUCTURE AND PERMEABILITY OF HUMAN NAIL 365 Nail Wall Cuticle Hyponychium Nail Plate Lunula Matrik (B) Eponychium Cuticle Nail Plate I ::: :i :i ?.::::•. •..i:i :. i i.•:-:7:7•.'7 .":':i:.-:•: :• :.: ! • !: •: Matrix I Hyponychium '" (c) Dorsal Nail Plate Intermediate Nail Plate Ventral Nail Plate Figure 1. Schematic diagram of the human nailß A: top view. B: longitudinal viewß C: longitudinal section of the nail plate. Adapted from references 5, 9.

366 JOURNAL OF COSMETIC SCIENCE and accounts for three fourths of the whole nail thickness. The ventral nail is one to two cell layers thick and is comprised of soft, hyponychial keratin. The ventral nail serves to connect the nail plate securely to the substratum (9). BLOOD SUPPLY TO THE NAIL The nail matrix and nail bed have a rich blood supply that originates from two main arterial arches lying below the nail plate. In addition, there is an extensive capillary blood supply to the tissues around the nail in particular, a capillary loop system supplies blood to the whole nail fold. Furthermore, the nail bed has a rich supply of lymphatic vessels and glomus bodies that are thought to be involved in regulating blood supply to the extremities in cold weather. Despite this extensive vascular network, defective peripheral circulation may be one of the main causes of nail deformities (10). GROWTH OF THE NAIL The nail is formed continually from the matrix, unlike hair, which is formed cyclically. The growth rate depends on matrix cell turnover. Toenails grow 30-50% more slowly than fingernails. Typically, the fingernail grows at an average rate of 3 mm per month, and nails of the dominant hand (i.e., right vs. left) exhibit a faster rate of growth. Growth rate of the nail is usually independent of nail thickness. However, growth occurs in a definite order in relation to the length of the fingers in general, the longer the finger, the faster the growth. Various pathological conditions can cause either an increase or decrease in growth rate. The regeneration time of the nail is dependent on growth rate. The fingernails usually have a total regeneration time of about 160 days, while toenails take approximately a year to regenerate. Due to long regeneration times, treatment of nail diseases requires several months (9). CHEMICAL COMPOSITION OF THE NAIL The main chemical constituent of the human nail is keratin, a scleroprotein containing large amounts of sulfur. Both epidermal- and hair-type keratins are present, although the latter make up more than 90% of the protein content. Recent studies using double-label immunofluorescence techniques have shown that the nail matrix contains three popu- lations of cells: cells expressing either skin or hair type keratins and cells expressing both keratins (11-12). The o•-keratin filaments (helical) interact with the cystine-rich non- helical matrix keratins, and are oriented perpendicular to the direction of nail growth and parallel to the free edge of the nail. This alignment of keratin filaments and the strong adherence of nail cells to each other are thought to contribute to nail hardness (8-9). The nail contains various amounts of amino acids that condense together to form the polypeptide chains present in keratin. The amino acid composition of human nails is shown in Table I. For comparison, the amino acids present in human hair and stratum corneum, as well as those in sheep horn, hoof, and wool are included. As seen from Table I, the content of glycine and half-cystine in human nails is similar to that of human hair and sheep wool, rather than human stratum corneum. However, the amino acid com-