The structure of the skin

The structure of the skin
The structure of the skin – click to enlarge

The skin is the largest human organ. As a “barrier organ” to the environment, it has numerous important functions. Adult skin has a surface area of approximately 1.5 – 2 m2 and weighs roughly 3 kg (up to 20 kg if the fatty tissue is included). It is between 1.5 and 5 mm thick (excluding the fatty tissue), depending on where it is on the body.

The skin is made up of three layers: the epidermis (the surface skin, as the outer layer), the dermis (corium), and the subcutis (hypodermis, the inner layer). Other important components of the skin are its adnexa: the hair, the nails, the sebaceous glands and the sweat glands, which are deeply embedded in the dermis. 

Epidermis (surface skin) and stratum corneum (horny layer)

The epidermis (surface skin) is a squamous epithelium that forms and carries the outermost horny layer (stratum corneum, see the link below). Keratin-producing cells (keratinocytes) make up the majority of the cells in the epidermis (90 %). It also contains pigment-forming cells (melanocytes) and other important cells of the immune system associated with the skin. The epidermis is approximately 0.1 mm thick and comprises four horizontal layers that sit on a basal membrane as the border to the dermis. The names of the layers are (innermost to outermost): basal cell layer (stratum basale), prickle-cell layer (stratum spinosum), granule cell layer (stratum granulosum) and horny layer (stratum corneum). The epidermis is renewed continuously, the process taking roughly one month.

The stratum corneum (horny layer) forms the outermost boundary of the epidermis and therefore a barrier against the environment. It consists of an impermeable horny layer made up of 10 – 20 layers of flat, bonded corneous cells (corneocytes). These cells are formed from the keratinocytes (keratinizing cells) of the epidermis. The horny layer undergoes continuous external desquamation and internal regeneration (renewal period of the horny layer: approximately two weeks).

Dermis (corium)

The dermis (corium) is the supportive connective tissue layer of the skin and carries the nerves and vessels important for the skin’s nourishment. The dermis is responsible for the high tensile strength and elasticity of the skin and consists of a superficial region (stratum papillare or papillary layer) and a deeper region (stratum reticulare or reticular layer) with collagen fiber bundles (tensile strength) and elastic fibers (elasticity) of differing densities. Embedded in the fiber network are the fiber-forming cells (fibroblasts), immune cells (e.g. mast cells) and other tissue cells. The fibers and cells lie in a ground substance of proteins and sugars (proteoglycans).

 

 

Subcutis (hypodermis)

The subcutis (hypodermis), often referred to as subcutaneous fatty tissue, is a layer of adipose tissue whose principal functions are to provide insulation against the cold, store energy and offer mechanical protection. It is located right underneath the reticular layer of the dermis and consists of lobules of fatty tissue, separated by connective tissue septa. These form the supportive structure of the adipose tissue and have vessels and nerves running through them.

 

 

 

 

Hair

As far as evolution is concerned, hair developed from the scales of fish and reptiles and is specific to mammals. Hair is made up of columns of keratin, complex in structure and differing in pigmentation. Its primary function on the scalp and in the eyebrows and eyelashes of human beings is protection (mechanical, thermal, against UV, etc.) It also has sensory and social functions. Hair is subject to a predetermined cycle of growing and resting phases.

A differentiation is made between long (terminal) hair (scalp), vellus (lanugo) hair (body), curly hair (armpits, pubic hair) and bristle hair (nose, ears, eyes). A human scalp has roughly 100,000 hair follicles. The hair (hair shaft) develops in a tubular invagination of the epidermis (hair follicle). The upper portion of the hair follicle is funnel-shaped (the infundibulum) and the sebaceous glands and sweat glands open out into this.

Sweat glands

Human beings have two types of sweat glands: apocrine and eccrine. Apocrine sweat glands are mainly found in the armpits and genital area and open out into the hair follicle. They do not develop until puberty, as a result of the hormonal changes that take place. Not very much is known about their function and effect. Eccrine sweat glands are distributed all over the body, independent of the hair follicles. They keep the skin’s horny layer moist and supple, serve as an excretory organ, and play an important role in thermoregulation (as the most important mechanism for cooling the skin). The eccrine sweat glands are at their most dense on the soles of the feet (approximately 600/cm²), and at their least dense on the thighs (about 100/cm²). Humans have around 2 – 4 million sweat glands in all, their total weight being roughly that of a kidney (100 g). They can produce a maximum of 10 liters of sweat per day.

Sebaceous glands

The sebaceous glands produce sebum and open out into the upper portion of the hair follicle (the infundibulum), except for the free sebaceous glands in the genitals and mucus membranes of the mouth. As a characteristic blend of lipids, sebum is the main component in the surface oil of the skin. It keeps the hair supple and waterproof and creates an environment that is hostile to germs. Depending on the regional density of the hair follicles, the number of sebaceous glands varies between 100 – 1000/cm². Their size differs considerably from location to location. There are large sebaceous glands in the oily areas of the body, known as the seborrheic areas (e.g. the scalp, face and chest), and small ones in the extremities, for example.

Vellus hair follicles with enormous, multi-lobed sebaceous glands (e.g. in the face) are called sebaceous gland follicles. The composition of sebum is relatively constant, but is subject to age-specific change. The quantity of sebum produced is also subject to extremely individual, regional and age variations. The greatest influencing factor are the male hormones (androgens), which also occur in women. These lead to a growth in the sebaceous glands and to an acceleration of sebum production. The lipids of the sebum are formed in the sebaceous glands. A precursor of cholesterol, known as squalene, is an important sebum lipid in terms of quality. However, the main lipid components are various triglycerides and wax esters.