Abstract: The nail is a specialized keratinous skin appendage that grows approximately 2 to 3 mm per 8 month, with complete replacement achieved in 6 to 9 months. Although this structure can be easily 9 overlooked, nail disorders comprise approximately 10% of all dermatologic conditions. This 10 contribution first provides an overview on the basic anatomy of the nail that will delineate between 11 the nail unit (eg, hyponychium, nail bed, proximal nail fold, and matrix) and anatomic components not 12 part of the nail unit (eg, lateral nail folds, nail plate, and eponychium). The function of each nail 13 structure will also be presented. The chemical profile of the normal nail plate is reviewed with a 14 discussion of its keratin content (hair type keratin vs epithelial type keratin), sulfur content, and mineral 15 composition, including magnesium, calcium, iron, zinc, sodium, and copper. The remainder will focus 16 on nail manifestations seen in states of malnutrition. Virtually every nutritional deficiency can affect the 17 growth of the nail in some manner. Finally, the discussion will include anecdotal use of nutritional and 18 dietary supplements in the setting of brittle nail syndrome as well as a brief overview of biotin and its 19 promising utility in the treatment of nail disorders.

Introduction: 22 Although skin might encompass a large portion of the 23 field, dermatology also includes the study of its appendages, 24 namely the nails and hair. These two skin appendages are 25 often discussed together, or are at least compared with one 26 another, in the literature. The dynamics of hair and nail 27 growth control are related in several ways: both are skin 28 appendages serving protective functions, and both are 29 differentiated epithelial products, which consist of tightly 30 bound cells made rigid by special intermediate filament 31 proteins, the hard keratins.

Conclusions: 431 Nails as a skin appendage are considered ancillary and 432 may be neglected by the nondermatologist in an examination; 433 however, there are a myriad of recognizable patterns that can 434 alter each individual part of the nail apparatus. Because 435 systemic illness can manifest through subtle changes in the 436 nail, clinicians may need to be reminded of these physical 437 findings in determining the cause of nail complaints.