THE NAILS
PART 3.4
THE NAILS
Author
Dr. Lawrence Silverberg
Vice President of Technology & Clinical Director
NailPure
ABSTRACT
This chapter discusses the structure of nails and describes in detail various abnormalities that may occur. Since nails and their appearance are a significant part of beauty/cosmetic care, it is important to understand the various issues that make nails unsightly and what can be done about them. The discussion also focuses on the potential effects of various chemicals used in nail beautification and provides critical thinking directions for improving nail appearance.
3.4.1 Introduction -Toenails and Fingernails
3.4.2 Histology, Ultrastructure, and Composition
a. Absence of Nails (Anonychia)
b. Nail Shedding (Onychomadesis)
c. Nail Separation from the Nailbed (Onycholysis)
f. Spoon-Shaped Nails (Koilonychia)
3.4.1 INTRODUCTION -TOENAILS AND FINGERNAILS
The primary function of toenails and fingernails is to protect the distal aspect of the digits. They protect the bones and surrounding soft tissues from injuries. They enhance tactile sensation to the digits. Nails are also used as a tool to increase precision gripping of small objects by human beings.
Originally, polishing of the nails dates back to 3000 bc in China and later in Egypt. The color of the nail polish symbolized social classes [1]. The modern history of polishing the fingernails and toenails as a fashion statement dates back to the beginning of the 19th century. Oils, powders, and creams were applied to the nails and the nails were buffed, leaving them shiny [2]. Today, most nail polishes are derivatives of pigments in solvents.
The nails are located at the distal ends of both the fingers and the toes on the dorsal aspect. Their maintained function is to protect the digits from trauma [3]. The curvature of the nail helps to define the shape of the distal end of the digits. Although there is a complete absence of nerve endings inside the nail, nails enhance the tactile sensation of the digits with pressure on the surrounding soft tissues and their nerve endings. This property, along with their useful ability to grip small objects, allows humans to have a more precise use of the fingers.
The “Nail Unit” consists of the following parts:
- ● Nail plate (Corpus Unguis)
- ● Nailbed
- ● Medial nail groove
- ● Lateral nail groove
- ● Distal nail groove (Hyponychium)
- ● Epionychium
- ● Posterior nail groove (Cuticle)
- ● Nail root (lunula, Matrix, Matrix Unguis)
The nail plate is the actual hard part of the nail. It is made of the tough protein keratin. Unlike the keratin of skin, the nails do not peel off in the form of scales. The keratin of nails also contains a much lower lipid content than skin, which allows much less water flux across the nail.
The nailbed in the skin below the nail plate contains living dermal tissue, which includes capillaries and glands. This vascular area gives the nail plate a pink appearance where it is adhered. At the distal end, the nail plate appears whiter in color where it is separated from the vascular nailbed.
The medial and lateral nail grooves form the indentation on the lateral borders of the nail plate. They are sometimes called the nail folds.
The distal nail groove, called the hyponychium, is at the end of the digit where the epithelium of the skin meets the nail plate. It forms a seal that protects the nailbed from infection and trauma.
The epionychium is a thin band of epithelium that connects the nail plate to the proximal skin.
The posterior nail groove, called the cuticle, is a thin layer of epithelium that extends from the skin over the epionychium and the proximal end of the nail plate.
The nail root, also call the nail matrix, is the germinal tissue that is responsible for producing cells that become the hard nail plate. The nail root extends 5–10 mm proximal to the posterior nail groove. The visible part of the nail root beyond the skin is called the lunula and appears as a half-moon-shaped, lighter-colored region at the proximal part of the nail.
The earliest signs of fingernail development appear in the ninth week in utero. Fingernails and toenails develop by the same process; however, toenail development is slightly slower and about four weeks behind that of fingernails.
Skin folds and grooves begin to form at the distal end of the fingers and will eventually define the structure of the nail unit. The first structure to appear is the matrix at week 11. By week 20, matrix cells exhibit adult keratinization.
At the week 32 a hard nail plate of the finger is formed. At birth, a long thin nail plate is present, which overhangs and curls over the distal digit.
In the discussion of nail formation, one must look at each component of the nail unit and what it contributes. The proximal nail fold is the wedge-shaped structure on the proximal edge of the nail unit. Its dorsal side differentiates to normal epidermis, while its ventral side differentiates to form the cuticle, which adheres to the dorsal surface of the nail plate.
The hyponychium is similar in that the distal portion contributes to the normal epidermis of the volar skin, while the proximal portion helps in adhering to the nail plate.
The matrix differentiates to form the nail plate as we know it. It occurs by specialized tissue kinetics, as described by Zaias [4], who showed this in monkeys and rats by introducing a radio-labeled marker; this was later confirmed by Norton [5] in humans. These studies materially advanced our understanding of matrix kinetics.
The nailbed epithelium differentiates both in an upward and a lateral motion. The upward growth produces specialized epithelium, which adheres in a special interconnecting manner to the developing nail plate, thus aiding in the adherence of the nail plate to the nailbed.
The lateral growth of the nailbed epithelium is toward the distal edge and moves at the same rate as the nail plate formation. Other authors have suggested different theories of nail plate formation, included here primarily for historical reasons [6, 7].
The matrix lies flat in the proximal nail unit. It produces a sheet of onychocytes (corneocytes), which grow diagonally along its entire length and move distally in a contiguous fashion. This development of multiple sheets of onychocytes eventually forms the nail and produces the dorsal surface of the nail plate; the midportion of the matrix produces the midnail plate; and the distal portion produces the ventral surface of the nail plate. Based on this anatomical arrangement, it can be concluded that the thickness of the nail plate is directly related to the length of the matrix.
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