The integumentary system is the body's outer covering, comprising skin and its associated hair, nails and glands. Skin consists of an outer epidermis and a deeper dermis, with the hypodermis beneath them. Together, these structures provide a barrier, help regulate temperature, receive sensory information and participate in vitamin D production.
What are the main layers of skin?
The epidermis is the outer layer of skin. It is an epithelium, a closely packed sheet of cells covering a surface. Its cells are arranged in layers, with new keratinocytes forming deeply and moving towards the surface as they mature.
The dermis lies beneath the epidermis. It is connective tissue, a supporting tissue in which cells lie within a framework of fibres and surrounding material. It contains blood vessels, nerves, glands and the deeper portions of hair follicles, which are tubular structures that produce hair.
The hypodermis, also called subcutaneous tissue, lies beneath the dermis and is not part of skin itself. It contains loose connective tissue and variable amounts of adipose tissue, meaning tissue that stores fat. It connects skin to underlying structures while allowing movement between them.
These layers differ in both structure and blood supply. The epidermis contains no blood vessels; nutrients reach it by diffusion, the movement of substances through surrounding material, from vessels in the dermis.
What are the layers of the epidermis?
The epidermis is a keratinised stratified squamous epithelium: it has multiple cell layers, flattened surface cells and a protective accumulation of keratin. Keratin is a tough structural protein. Most epidermal cells are keratinocytes, the cells that produce it.
From deepest to most superficial, the named layers are:
- Stratum basale: the basal layer, next to the dermis, where dividing cells replace cells lost at the surface.
- Stratum spinosum: the layer above the basal layer, where strong connections between keratinocytes give them a spiny appearance in tissue preparations.
- Stratum granulosum: the granular layer, where cells contain granules associated with keratin formation and contribute to the skin's water barrier.
- Stratum lucidum: a clear layer found in thick skin, between the granular layer and the outermost layer.
- Stratum corneum: the outer layer of flattened, dead, keratin-filled cells that are gradually shed.
The stratum lucidum is not a distinct layer in thin skin. Although the surface consists of dead cells, the deeper epidermis is living tissue that continually renews this protective covering.
The epidermis also contains melanocytes, cells that produce the pigment melanin. Melanin contributes to skin colour and helps protect underlying cells from ultraviolet radiation, the short-wavelength radiation present in sunlight.
How do the dermis and hypodermis support skin?
The dermis has a superficial papillary layer and a deeper reticular layer. The papillary layer forms small projections towards the epidermis and contains fine connective tissue, small blood vessels and sensory nerve endings. These projections help connect the two layers and support the overlying epidermis.
The reticular layer contains a denser network of collagen and elastic fibres. Collagen provides tensile strength, meaning resistance to being pulled apart, while elastic fibres allow tissue to stretch and recoil. Most of the dermis's thickness lies in this deeper layer.
Blood vessels in the dermis supply its tissues and support the epidermis above. Nerves carry sensory information, while other nerve fibres influence glands and small muscles associated with hair follicles.
The hypodermis provides cushioning, insulation and an energy store through its adipose tissue. Larger vessels and nerves pass through it towards the dermis. Its loose tissue also allows skin to slide over deeper structures rather than being rigidly fixed to them.
How are hair and nails related to skin?
A skin appendage is a specialised structure associated with skin and derived from the epidermis. Hair, nails, sweat glands and sebaceous glands belong to this group, although much of an appendage may extend into the dermis or hypodermis.
A hair has a shaft above the skin surface and a root within a follicle. At the follicle's expanded base, dividing cells form the hair, which becomes keratinised as it grows. A connective-tissue projection into this base carries vessels that support the growing cells.
Hair follicles are distributed unevenly, and hair varies in length, thickness and pigmentation between regions. Fine vellus hairs cover much of the body, whereas coarser terminal hairs occur on the scalp and, after puberty, in regions such as the axillae and pubic region. The hair of trunk varies in prominence across the chest, abdomen and back. Palms and soles lack hair follicles.
Small smooth muscles called arrector pili muscles attach to many follicles. Smooth muscle is involuntary muscle without the striped appearance of skeletal muscle; contraction of these small muscles raises the hairs.
Nails are hard plates of keratin on the ends of fingers and toes. The visible nail plate rests on the nail bed, while the nail matrix, the growth region beneath the proximal nail fold, produces the plate. Here, proximal means nearer the base of the digit.
What do sweat and sebaceous glands do?
Sweat glands are coiled glands whose ducts, the channels carrying their secretions, lead towards the skin surface. Eccrine sweat glands open directly onto the surface and are widely distributed, including on the palms and soles. Their watery secretion supports heat loss through evaporation.
Apocrine sweat glands occur in particular regions, including the armpits and around the external genitalia. Their ducts usually open into hair follicles. Unlike eccrine glands, they are not the principal glands responsible for cooling the body.
Sebaceous glands produce sebum, an oily secretion that lubricates skin and hair. Most open into hair follicles, although some open directly onto the surface. They are absent from the palms and soles.
What are the main functions of skin?
Skin's functions arise from the combined organisation of its surface cells, connective tissue, vessels, nerves and appendages:
- Barrier: the keratinised surface and lipids, or fatty substances, between its cells limit water loss and resist the entry of external substances. The dermis adds mechanical strength.
- Temperature regulation: evaporation of sweat removes heat, while changes in blood flow through skin vessels alter heat exchange at the surface. Subcutaneous fat helps insulate deeper tissues.
- Sensation: free nerve endings and specialised sensory receptors detect stimuli such as touch, pressure and temperature. A receptor is a structure that responds to a particular stimulus.
- Vitamin D production: ultraviolet radiation initiates vitamin D formation in the epidermis. Further processing occurs in other organs.
These roles explain why skin is more than a passive covering. Its outer barrier depends on continued cell renewal, while its deeper tissues connect that barrier to the body's circulation and nervous system.
What is the difference between thick and thin skin?
In anatomy, thick skin and thin skin refer chiefly to differences in the epidermis, not to the total thickness of all tissues beneath the surface. Thick skin covers the palms and soles; thin skin covers the remaining body surface.
| Feature | Thick skin | Thin skin |
|---|---|---|
| Location | Palms and soles | Remaining body surface |
| Epidermis | Thick, with a prominent stratum corneum | Thinner, with a less prominent stratum corneum |
| Stratum lucidum | Present | Not distinct |
| Hair follicles and sebaceous glands | Absent | Generally present, with regional exceptions |
| Eccrine sweat glands | Numerous | Present, with distribution varying by region |
The thick outer layer on palms and soles suits surfaces exposed to repeated contact and friction. Thin skin is not necessarily hair-bearing at every location, so the absence of visible hair alone does not identify thick skin.
How can skin be explored in the 3D model?
In the 3D model next to this article, select skin and fade it to make it translucent, then rotate or zoom to explore its relationship to deeper structures. You can also hide it or switch layers on and off to compare the outer covering with the structures beneath it.
Use rotation, zooming and layer switching to compare the positions and spatial relationships of selected structures. The epidermal layers described above are microscopic tissue layers; studying their organisation is distinct from using these controls to explore spatial relationships.