Table 1.

Biological Markers of Skin Quality

Attribute	Marker	Definition	Rationale for Study
Elasticity	Collagen density10,21,22	Concentration of collagen, a structural protein in the skin and other connective tissues	Collagen strengthens the skin, helping it to resist mechanical deformation
	Elastin6,23,24	Protein in the skin and connective tissue that maintains elasticity and flexibility	Elastin constitutes about 90% of the elastic fibers in the skin
	Fibrillin-16	Glycoprotein that supports the integrity of the dermal elastic fiber network and is expressed in dermal–epidermal junctions	Fibrillin-1 is an essential component of the network of fibers that imparts elasticity to the skin
Hydration	AQP325,26	Water- and glycerol-transporting membrane proteins expressed in the epidermis	AQP3 is a major protein with a critical role in maintaining hydration in the skin
	GAGs3,7	Linear polysaccharide molecules, including hyaluronic acid, that bind water and help regulate the hydration of the dermis	GAGs hold and maintain water, and skin aging is associated with reduced GAG expression
	Hyaluronic acid (HA)7,15,16	Linear polysaccharide molecule that maintains the hydration and structural integrity of skin and other connective tissues	HA has an immense capacity to retain water; a primary function in the skin is to regulate moisture homeostasis
	Epidermal capacitance27,28	A measure of the humidity level of the outermost cutaneous layers of the stratum corneum	Epidermal capacitance is a well-established measure of skin hydration in dermatological studies

AQP3, aquaporin-3; GAGs, glycosaminoglicans.