Chapter 75. Development of The Nails - Review of Medical Embryology Book

Nails are modifications of the epidermis and correspond to the claws and hoofs of lower animals

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

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
Development at the tips of the fingers precedes the development of the toenails

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

During month 5, specialized keratin fibrils differentiate in the matrix layer, without having passed through a keratohyalin or eleidin stage (ordinary method of cornification)
The keratinized cells flatten and consolidate into the compact tissue of which the nail plate is composed
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)
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
The dermis, beneath the nail, is thrown into parallel longitudinal folds to produce the characteristic ridges and grooves

THE STRATUM CORNEUM AND PERIDERM of the epidermis, for a time, cover completely the free nail and are jointly referred to as the eponychium

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)

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

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

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
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
The horny zone of the nail is attached to the underlying nail bed
The matrix, or proximal part of the nail bed, produces hard keratin
The fingernails reach the fingertips by week 32, and the toenails reach the toe tops by week 36
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

CONGENITAL ALOPECIA (atrichia congenita): fetal loss or absence of hair may occur by itself or with other skin derivative abnormalities
HYPERTRICHOSIS: excessive hairiness due to the development of supernumerary follicles or persistence of fetal hair that normally disappears
ANONYCHIA: partial or complete absence of the nails due to a failure of the matrix to form or give origin to the nails
MISSHAPEN NAILS are a common occurrence