75. Development of The Nails

  1. Nails are modifications of the epidermis and correspond to the claws and hoofs of lower animals
    1. THE FIRST INDICATION of a nail is foreshadowed at week 10 by a thickened area of epidermis, the nail field, seen on the dorsum of each digit
      1. The adjoining area, on each side and at the base of the field, tends to overgrow the field, giving rise to shallow lateral nailfolds which continue into a much deeper proximal nailfold that extends nearly to the proximal end of the terminal phalanx
      2. Development at the tips of the fingers precedes the development of the toenails
    2. THE MATERIAL of the true nail is developed within the underlayer of the proximal nailfold (although the primitive nail field undergoes some local cornification and forms a so-called false nail). This layer is named the matrix
      1. During month 5, specialized keratin fibrils differentiate in the matrix layer, without having passed through a keratohyalin or eleidin stage (ordinary method of cornification)
      2. The keratinized cells flatten and consolidate into the compact tissue of which the nail plate is composed
      3. Thus, the nail substance differentiates in the proximal nailfold as far distal as the outer edge of the lunula (the whitish crescent at the base of the exposed nail)
      4. Beyond the lunula, the nail plate merely shifts progressively over the nail bed and reaches the tip of the finger about 1 month before birth
      5. The dermis, beneath the nail, is thrown into parallel longitudinal folds to produce the characteristic ridges and grooves
    3. THE STRATUM CORNEUM AND PERIDERM of the epidermis, for a time, cover completely the free nail and are jointly referred to as the eponychium
      1. This layer, in late fetuses, is lost except for horny portions that continue to adhere to the nail plate along the curved rim of the nailfold (the cuticle)
    4. UNDERNEATH THE FREE END of the nail, the epidermal cells also accumulate to form a piled-up epidermal mass, the hyponychium, or substance beneath the nail
  2. Nail anatomy
    1. THE HORNY ZONE of the nail is composed of hard keratin and has a distal, exposed part or body, and a proximal, hidden portion, the root
      1. The root is covered by a prolongation of the stratum corneum of the skin which is composed of soft keratin and is called the eponychium
      2. The lunula or "half-moon" lies distal to the eponychium and is a part of the horny zone which is opaque to the underlying capillaries
      3. The horny zone of the nail is attached to the underlying nail bed
      4. The matrix, or proximal part of the nail bed, produces hard keratin
      5. The fingernails reach the fingertips by week 32, and the toenails reach the toe tops by week 36
      6. On the average, after birth, the nail grows about 0.5 mm a week. They grow faster in the summer than in the winter; growth is also age dependent
  3. Malformations of the hair and nails
    1. CONGENITAL ALOPECIA (atrichia congenita): fetal loss or absence of hair may occur by itself or with other skin derivative abnormalities
    2. HYPERTRICHOSIS: excessive hairiness due to the development of supernumerary follicles or persistence of fetal hair that normally disappears
    3. ANONYCHIA: partial or complete absence of the nails due to a failure of the matrix to form or give origin to the nails
    4. MISSHAPEN NAILS are a common occurrence

development of the nails: image #1