Nail bed and dermis

The nail bed is the sterile component of the nail unit and the primary site of nail keratinization.  The nail bed extends from the distal lunula to the point of origin of the hyponychium. It shares similar boundaries with the nail plate, with the lunula marking the proximal border of the nail bed, with the lateral folds marking the lateral border, and with the hyponychium marking the distal border.

The epidermis of the nail bed is relatively thin, with a dimensional thickness of only 1-2 cells. The epidermal surface of the bed has parallel longitudinal ridges that interlock with similar folds on the ventral surface of the nail plate.^[The ridges securely bind the nail plate to the nail bed, allowing a firm adhesion between the 2 structures. The area marking the point at which living cells become keratinized and continue to become dead horny cells of the nail plate is abrupt.

The nail bed is a highly vascular structure that lacks an underlying subcutaneous layer of adipose tissue. As a result, nothing separates the dermis from the periosteum of the distal phalanx. The lack of this layer becomes apparent when one is confronted with aggressive tumors of the nail because such tumors may easily extend to involve the underlying bone.

The connective tissue of the dermis, the dermal collagen, contains a balanced mixture of lymphatics, blood vessels, and elastic fibers. The dermis also has a wealth of glomus bodies and a rich capillary network longitudinally oriented with the parallel ridges. Splinter hemorrhages of the nail plate form when a small amount of blood leaks from the capillary bed into one of these folds and becomes trapped by the nail plate.^[2] Melanocytes are not found in the nail bed, although they are present in significant numbers in the matrix.