Overview: Skin
- Skin consists of two layers: epidermis and dermis.
- Skin is the body's heaviest organ, and it forms a protective, sensory, temperature-regulating surface over the entire body.
- Organ means a body structure made of multiple tissues working together, so skin is more than a simple covering.
- Body weight contribution is high because skin accounts for about sixteen percent of total body weight.
- The epidermis is the outer skin layer that forms the main protective epithelial surface.
- The dermis is the deeper skin layer made mostly of connective tissue that gives skin strength, elasticity, blood supply, and sensory support.
- Interdigitate means two surfaces fit together like interlocking fingers, and the epidermis and dermis do this at their shared boundary.
- Irregular interface between epidermis and dermis increases contact area and helps resist shearing forces.
- Hypodermis is the superficial fascial layer beneath the skin that attaches skin loosely to deeper tissue.
- Fascia is connective tissue that helps bind, separate, or support body structures.
- Loose connective tissue in the hypodermis allows skin to move over underlying muscles and organs.
- Underlying tissue means the structures below the skin, such as fascia, muscle, or deeper connective tissue.
- Epidermal derivatives are structures that develop from epidermis but specialize for functions such as secretion, hair growth, or protection.
- Sweat glands are epidermal derivatives that help regulate temperature and excrete small amounts of waste.
- Hair follicles are epidermal invaginations that produce hair shafts.
- Sebaceous glands are epidermal derivatives that secrete oily sebum into hair follicles.
- Nails are hard keratinized epidermal plates on fingers and toes.
- Mammary glands are modified skin glands specialized for milk production.
- Integument means skin plus its derivatives, including hair, nails, and cutaneous glands.
- Protection from injury includes resisting friction, minor trauma, chemicals, and microbial entry.
- Desiccation means drying out, and skin prevents excessive water loss from the body.
- Infection prevention depends on the physical barrier, immune cells, antimicrobial secretions, and dry surface conditions.
- Body temperature regulation depends on sweating, blood-vessel changes, and insulation from skin-associated tissues.
- Ultraviolet radiation is light energy from the sun that can help vitamin D synthesis but can also injure cells.
- Vitamin D synthesis begins when ultraviolet radiation changes precursor molecules in skin.
- Touch receptors help detect pressure, texture, vibration, and movement at the skin surface.
- Temperature receptors help detect warmth and cold.
- Pain receptors warn about tissue injury or potentially harmful stimuli.
- Excretory function means skin can eliminate small amounts of substances through gland secretions.
- Sebaceous secretion contributes oils and lipids to the skin surface.
- Sweat secretion helps remove water, salts, urea, and lactic acid.
- Apocrine glands produce thicker secretions that become odorous after bacterial action.
- Barrier function is the central theme of skin histology because most skin structures either strengthen, sense, maintain, or protect the surface.
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