NAILS

nThese plates of highly keratinized cells are analogous to, but harder than, the stratum corneum.
A. Nail Development: The formation of the nails is similar to that of hair, but involves produc ing plates rather than cylinders. At the end of the third month of embryonic development, a narrow plate of epidermis on the dorsal surface of the terminal phalanges invades the underlying dermis of each finger and toe. This invasion continues proximally, forming a furrow called the nail groove. Epithelial cells beneath the groove proliferate to form the nail matrix, whose composition and function are similar to those of the hair's germinal matrix. Proliferation in the nail matrix pushes the upper cells toward the surface. These cells differentiate, becoming highly keratinized to form the nail plate. The plate is gradually pushed out of the groove by further cell proliferation and differentiation in the nail matrix. The growing plate slides distally on the dorsal surface of the digit. The epidermis over which it slides becomes the nail bed.

B. Nail Complex Structure: The nail plate (or nail) consists of 2 parts: the nail body (the visible part of the nail) and the nail root--(the part hidden in the nail groove). The nail and its supporting structure are surrounded by papillary dermis. The nail matrix is a thickened region of epidermis containing proliferating cells in the layer that directly contacts the dermis, and keratinizing cells between this basal layer and the nail plate. The nail matrix surrounds the root and extends beyond the nail groove. The nail bed lies beneath the nail body, distal to the nail matrix. It consists of only the deeper epidermal strata, for which the nail serves as a stratum corneum. The eponychium (or cuticle) is a thick keratinized layer extending from the upper surface of the nail groove over the most proximal part of the nail body. The hyponychium is a local thickening of the stratum corneum underlying the free (distal) end of the tail. The lunula is the whitish, opaque, crescent-shaped region on the proximal nail body, adjacent to the nail groove. Its distal border corresponds roughly to the underlying nail matrix