The largest organ of the body by area and weight, roughly two square metres in an adult
Skin — the integument.
Anatomy · Glands & the immune system
Skin
The body's largest organ
Integumentary system · whole body surface
The body's largest organ, covering about 1.5–2 square metres in an adult. It has three layers: a thin, tough, waterproof epidermis of stratified squamous epithelium with no blood vessels of its own; a thicker dermis of connective tissue containing vessels, nerves, glands and hair follicles; and beneath both a hypodermis of loose connective tissue and fat that anchors it to the deeper tissues. Its jobs include protection, waterproofing, temperature control, sensation and making vitamin D.
How to spot them
- ▸Three layers in section: a thin dark-staining top layer, a thick pink middle layer and a pale fatty base
- ▸Epidermis is stratified squamous epithelium whose surface cells are flat, dead and full of keratin
- ▸Epidermis has five layers in the palms and soles: basale, spinosum, granulosum, lucidum and corneum
- ▸Wavy junction where dermal papillae push up into the epidermis, forming ridges such as fingerprints
- ▸Dermis holds hair follicles, sweat glands, sebaceous glands, nerve endings and blood vessels; the fat beneath is the hypodermis
What to know
From deepest to surface: basale, spinosum, granulosum, lucidum, corneum
Skin — the five layers (strata) of thick epidermis, with the lucidum present only on palms and soles.
A layer with no blood vessels of its own, fed by diffusion from the tissue below, and renewed roughly every four weeks
Skin — the epidermis; the basal layer's stem cells divide and the cells rise to be shed.
The middle layer, of tough connective tissue in two zones, papillary and reticular, whose peg-like projections make fingerprints
Skin — the dermis; its dermal papillae push into the epidermis.
The deepest layer of fat and loose tissue, which insulates and cushions and into which injections are given
Skin — the hypodermis (subcutaneous layer).
Cells in its basal layer make melanin and pass it to neighbouring keratinocytes as a natural sunshade
Skin — melanocytes, in the stratum basale.
It makes vitamin D when its cells are exposed to ultraviolet B light
Skin — 7-dehydrocholesterol is converted to previtamin D3.
Key ideas
- Keratinocytes are born in the deepest layer, then move outward as they mature, flatten, fill with keratin and finally die and flake off from the surface layer, the stratum corneum.
- The dermis gives strength and stretch through collagen and elastic fibres, and its papillary layer anchors and nourishes the avascular epidermis.
Quick-fire facts
- •Covers about 1.5–2 square metres and, with its fat layer, forms a considerable fraction of body weight
- •Epidermal layers from surface to depth: corneum, lucidum (thick only), granulosum, spinosum, basale — remembered by 'Come, Let's Get Sun Burned'
- •The thickest epidermis is on the palms and soles; the thinnest is on the eyelids
- •Around 90% of epidermal cells are keratinocytes; the rest include melanocytes, Merkel cells (touch) and Langerhans cells (immune)
- •Replacement of the epidermis takes about 4 weeks (OpenStax) — some sources say 4–6
- •The dermis is packed with collagen, and the network of vessels beneath it is used to lose or conserve heat
- •The hypodermis (subcutis) is not strictly part of the skin, but it binds it to muscle and bone and stores fat
- •Skin appendages include hair, nails, sebaceous glands and sweat glands, all derived from epidermis but lying in the dermis
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