The nail has many soft tissue structures that help support and form the hard-outer nail, known as the nail plate. The attached figure depicts the gross structures described below.
Nail Folds
The nail folds are soft tissue structures that protect the lateral and proximal edges of the nail plate. The proximal nail fold protects most of the nail matrix from trauma and ultraviolet rays.
Mantle
The mantle is the skin covering the matrix and base of the nail plate.
Cuticle
The cuticle (also known as the eponychium) grows from the proximal nail bed and adheres to the nail plate. Together, the proximal nail fold and cuticle form a protective seal against any irritants that may disrupt the matrix underneath.
Nail Matrix
The nail matrix is located deep to the proximal nail fold and nail plate. The proximal nail matrix begins about halfway between the distal interphalangeal joint and the proximal nail fold. The distal nail matrix is visible through the nail plate as a white half-moon structure called the lunula. The nail matrix is responsible for the formation of the hard nail plate and is the only part of the nail unit that contains melanocytes. The nail cells, termed onychocytes, are pushed superficially and distally to form the nail plate. Different parts of the nail matrix form different sections of the nail plate. Generally, the dorsal aspect of the nail plate is formed from the proximal nail matrix, and the ventral nail plate is formed from the distal nail matrix. However, 80% of the nail plate is made from the proximal nail matrix. Therefore, a biopsy or surgery of the distal nail matrix will produce minimal damage to the nail plate.
Nail Plate
The nail plate is the hard, keratinized structure made from compact onychocytes, organized in a lamellar pattern. The dorsal surface of the nail plate is smooth with longitudinal ridges. Below the nail plate are the nail bed and part of the nail matrix. The nail bed and nail folds act as strong attachment points that help the free edge of the nail function as a tool without loosening the nail plate or causing pain.
Nail Bed
The nail bed is attached to the ventral surface of the nail plate and begins distal to the lunula and terminates at the hyponychium. The nail plate is attached to the nail bed through longitudinal epidermal ridges. These ridges on the ventral nail plate surface are complementary to the ridges on the nail bed, which function to increase the surface area of the nail plate’s attachment to the underlying nail bed, thus augmenting the adhesion between these two surfaces. The nail bed does not produce a stratum corneum since the keratins necessary for the formation of this layer of the epidermis are not present. However, if onycholysis or loss of the nail plate occurs, the nail bed loses the longitudinal ridges and begins to express the keratins necessary to produce the stratum corneum. Below the nail bed is a thin layer of the collagenous dermis which adheres to the periosteum of the underlying distal phalanx. The lack of subcutaneous fat can increase the risk of osteomyelitis of the distal phalanx in the setting of a nail infection.
Hyponychium
The hyponychium is the area distal to the nail bed and beneath the free edge of the nail plate.
Onychodermal Band
The onychodermal band is part of the distal nail bed that is grossly represented in a contrasting color. It functions as the first barrier of protection on the free edge of the nail and is analogous to the cuticle. Changes in the color can correspond to different diseases or variations in vascular supply.
