skin

Skin is the body's largest organ, forming a protective barrier between the internal environment and the external world. It serves multiple roles, including sensation, thermoregulation, metabolic functions, and immune defense, and it participates in the production of vitamin D.

The skin is organized into three major layers. The epidermis is the outermost layer, composed mainly of keratinocytes and including melanocytes that determine skin color. It contains several sublayers, such as the stratum basale, spinosum, granulosum, and, in thicker skin, lucidum, culminating in the tough outer stratum corneum. The dermis lies beneath the epidermis and consists of connective tissue rich in collagen and elastin, with blood vessels, nerves, hair follicles, and sweat and sebaceous glands. The hypodermis, or subcutaneous tissue, lies below and is primarily adipose tissue and loose connective tissue that provide insulation and cushioning.

Functions of the skin include forming a barrier against microorganisms, chemicals, and mechanical injury, preventing excessive water loss, and enabling temperature regulation through sweat production and blood flow. It houses sensory receptors for touch, pressure, temperature, and pain, and participates in immune defense via resident immune cells. The epidermis contains cells that contribute to skin pigmentation and protection from UV radiation, while keratin and collagen provide strength and structure. The skin also plays a role in metabolic processes and vitamin D synthesis when exposed to sunlight.

Developmentally, the epidermis arises from ectoderm, while the dermis and hypodermis derive from mesoderm. Wound healing proceeds through inflammation, proliferation, and remodeling. Aging can lead to thinning of the epidermis and reduced elasticity due to changes in collagen and sebum production.

Common topics related to skin include pigmentation, microbiome, and diseases such as dermatitis, psoriasis, and skin cancer. Protective sun exposure habits and routine skin checks are widely recommended for health maintenance.