ANATOMY, SKIN

Anatomic model of the skin (frontal section).
The integumentary system includes the skin and its appendages (hairs, glands, muscles and nerves). The skin is constituted by two tissue layers, the epidermis and the dermis, covering the subcutaneous tissue.
The epidermis, the outermost layer of the skin, is a stratified pavimentous epithelium. The basal layer (in deep blue), the deepest one, is constituted by keratinocytes, cells with a high proliferation rate and producing keratin, and melanocytes synthesizing melanin (responsible for the skin color). As they renew, the keratinocytes are pushed upward to form successively the spinous and granular layers (in blue) and then the horny layer of the epidermis (in beige). The latter is composed of dead keratinocytes that will desquamate. Pores open on the epidermis surface, at the end of the hair shafts (in black) and of the sweat ducts (in white).
The dermis (in pinkish beige), the inner layer, is a fibrous connective tissue conferring skin resistance and elasticity. The superficial area of the dermis, the papillary dermis, draws ridges, the papillae, that increase its surface. They include loops of blood capillaries (venous in blue and arterial in red) and nerve endings (in yellow), that are free and not specialized, or end with sensory touch receptors (Meissner's corpuscles, in peach). The deeper reticular dermis includes hair follicles, blood vessels, nerves (in yellow), sebaceous glands (in mauve) and eccrine sweat glands (in white, responsible for the perspiration).
The hair is constituted by severeal separate areas: the outer shaft, the root, implanted in the dermis, and the bulb, which includes the papilla (white bulge that contains the blood supply and feeds the hair). An hair is composed of several concentric cylindric tissue layers: the medulla (in the center, in white), the cortex (in black) and then the cuticle (in yellow) forming the hair root. The latter is surrounded by the hair follicle, a cover including an epithelial root sheath (with an inner, in azure, and a outer root sheath, in gray) and a connective tissue layer (in orange), outwardly.
Each hair is associated with a sebaceous gland (in mauve), producing sebum to lubricate it, and an erector pili muscle (rust-coloured) allowing its erection (giving goose bump).
The underlying subcutaneous tissu includes interstitial tissue and fat cells (in sunset yellow) organized in lobules. It also comprises the Pacinian corpuscles (in orange), sensory touch receptors for deep pressure, made of a capsule (in khaki) covering concentric lamellae of the outer bulb (in orange) wrapping the inner bulb (in yellow).