The skin has two layers, the epidermis on the outside and the dermis below, sitting on a layer of fat. Each layer has different cells, blood supply and nerves.
Epidermis
The outer layer is stratified squamous epithelium with no blood vessels. New cells form at the base, move up, fill with keratin and are shed from the surface in about 4 weeks.
Cells of the epidermis
Keratinocytes make the tough surface. Melanocytes make melanin, the pigment that protects against ultraviolet light, and other cells sense touch or take part in immune defense.
Dermis
The dermis is connective tissue rich in collagen and elastic fibers. It holds blood vessels, nerve endings, hair follicles and sweat and oil glands.
Hypodermis
Under the dermis lies the subcutaneous layer of fat and loose connective tissue. It insulates, stores energy and anchors the skin to muscle.
| Layer | Contains | Injection or test here |
|---|---|---|
| Epidermis | Keratinocytes, melanocytes, no vessels | Not injected |
| Dermis | Collagen, vessels, nerves, glands | Intradermal (TB skin test) |
| Hypodermis | Fat and loose tissue | Subcutaneous (insulin, heparin) |
| Muscle (below skin) | Muscle fibers, large vessels | Intramuscular (vaccines) |
At the bedside
Needle angle and length decide which layer a drug reaches: about 10 to 15 degrees for intradermal, 45 to 90 degrees for subcutaneous and 90 degrees for intramuscular. Thin patients have less fat, so a long needle may pass too deep.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).