The skin is the largest organ of the human body. It forms the outer protective covering and is part of the integumentary system, which also includes hair, nails, sweat glands, and sebaceous (oil) glands.
The skin has three main layers:
Epidermis – the outermost layer made mostly of epithelial cells. It provides a waterproof barrier and creates our skin tone.
Dermis – the middle layer containing connective tissue, blood vessels, nerve endings, sweat glands, and hair follicles.
Hypodermis (subcutaneous layer) – the deepest layer made mainly of fat and connective tissue that insulates the body and cushions underlying structures.
Protection – Acts as a barrier against pathogens, UV radiation, and physical injury.
Regulation of body temperature – Through sweating and control of blood flow to the skin surface.
Sensation – Contains sensory receptors for touch, pain, pressure, and temperature.
Excretion – Removes waste products such as urea and salts through sweat.
Vitamin D synthesis – Produces vitamin D when exposed to sunlight, which is important for calcium absorption.
Water resistance – Prevents excessive loss or absorption of water.
Immune defense – Contains immune cells (like Langerhans cells) that help detect and fight infections.
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Human skin comprises two types. Thick skin, limited to the palms and soles, lacks hair and has abundant sweat glands. Thin skin has hairs and covers the rest of the body.
Thick or thin, the skin consists of two distinct but tightly attached layers—the epidermis and dermis—which are underlain by the hypodermis.
02. Dermal–epidermal junction.
Hypodermis. Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue under the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, activity level, body region, and gender. It is also called subcutaneous fascia and, where it is thick enough, the panniculus adiposus.
Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epithelial-mesenchymal interactions between the epidermis and dermis for their formation and maintenance.
Hypodermis. Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue under the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, activity level, body region, and gender. It is also called subcutaneous fascia and, where it is thick enough, the panniculus adiposus.