Summary
Highlights
The integumentary system is the outermost system of the human body, comprising the skin and its derivatives like glands, hair, and nails. It acts as the first line of defense against the environment, maintaining internal conditions by keeping bacteria out and water and heat in.
The skin is composed of two main regions: the outer epidermis and the inner dermis. The epidermis is a thin, avascular layer made of keratinized stratified squamous epithelium, consisting of four different cell types and five layers. These layers, from deepest to most superficial, are the stratum basale (basal layer), stratum spinosum (prickly layer), stratum granulosum (granular layer), stratum lucidum (clear layer), and stratum corneum (horny layer). The stratum basale produces new cells, including keratinocytes, melanocytes, and tactile cells. Keratinization, where cells fill with keratin and die, begins in the stratum granulosum and culminates in the tough, protective stratum corneum.
Beneath the epidermis lies the dermis, a strong, flexible connective tissue layer rich in nerves and blood vessels, and where hair follicles originate. It has two sections: the papillary layer and the reticular layer. The thin papillary layer, made of areolar connective tissue, features dermal papillae that extend into the epidermis, containing tactile cells and forming fingerprints in areas of friction. The thicker reticular layer, composed of dense fibrous connective tissue, makes up the majority of the dermis. Below the dermis is the subcutaneous layer (hypodermis), primarily made of adipose tissue, which anchors the skin to underlying structures.
Skin color is determined by pigments such as melanin, produced by melanocytes in the stratum basale, which protects against UV radiation. Other contributing pigments include carotene (yellow-orange) and hemoglobin (red when oxygenated).
Hair, a component of the integumentary system, is largely made of dead, hard keratinized cells and serves protective functions. Each hair has a root (where keratinization occurs) and a shaft (where keratinization is complete), consisting of three layers: the medulla, cortex, and cuticle. Hair follicles, pockets extending into the dermis, include a hair bulb (with nerve endings for sensation), a hair papilla (supplying nutrients), and various sheath layers. The hair matrix within the follicle is responsible for hair growth. Arrector pili muscles attached to follicles cause goosebumps, aiding in heat retention and defense in some mammals.
Nails, modifications of the epidermis, are composed of hard keratin, providing tools for scratching and picking. Each nail has a free edge, a body, and a proximal root embedded in the skin. The nail sits on the nail bed, growing from the nail matrix. Skin folds (nail folds, eponychium, hyponychium) overlap the nail borders.
The integumentary system includes numerous glands. Sudoriferous (sweat) glands are abundant, with two main types: eccrine (merocrine) glands, which secrete watery sweat for cooling, and apocrine glands, found in specific areas, which secrete fat and protein components, leading to body odor. Ceruminous glands (earwax) and mammary glands (breast milk) are also types of apocrine glands. Sebaceous (oil) glands secrete sebum, an oily substance that softens and lubricates hair and skin, prevents water loss, and acts as an antibacterial agent.
The integumentary system, comprising skin, hair, nails, and glands, acts as a vital barrier, protecting the body from the external environment. It possesses remarkable abilities for self-repair, temperature regulation, and sensory perception.