The Epidermis
As the outermost skin layer the epidermis forms the actual protective covering against environment. Its average thickness is 0.1 mm. On the face it is only 0.02 mm, while on the soles of the feet between 1 and 5 mm.
On the skin surface are the sweat gland pores (100-200/cm2) and the openings of the sebaceous glands (i) (50-100/cm2). Their secretions provide the skin with moisture and lipids and thus maintain the hydrolipid film. The epidermis itself has no blood vessels, so nutrients are supplied through the fine blood vessels in the dermal papillae.
The epidermis consists of up to 90 percent keratinocytes, the actual epidermal cells that are held together by what are called desmosomes. The epidermis has five different layers:
- Horny layer (stratum corneum)
- Clear layer (stratum lucidum)
- Granular layer (stratum granulosum)
- Prickle-cell layer (stratum spinosum)
- Basal layer (stratum basale)
Schematic diagram of the epidermis:
During differentiation the basal cells change into flat horny skin cells without nuclei.
Horny layer (stratum corneum)
Clear layer (stratum lucidum)
Granular layer (stratum granulosum)
Prickle-cell layer (stratum spinosum)
Basal layer (stratum basale)
Basal membrane
Compostition of the epidermis
Basal layer (stratum basale)
The stratum basale (basal = base, ground/lat.) is the bottom layer of the epidermis. The basal cells lie directly on the basal membrane, which forms a well-defined border between the dermis and epidermis. The basal cells act as mother cells, ensuring continuous regeneration of the skin by cell division (proliferation). The daughter cells are slowly pushed by the actively dividing cells into the outer lying layers where they go through various stages of development. Also found in the basal layer are the melanocytes, which are the pigment-producing cells.
Prickle-cell layer (stratum spinosum)
The stratum spinosum (spino = thorn, prickle/lat.) or prickle-cell layer lies above the basal layer. In it the membrane-bounded vacuoles (Odland bodies), become visible for the first time. They contain the precursors of the epidermal lipids in the form of lamellar (arranged in thin plates) bilayer lipid membranes.
Granular layer (stratum granulosum)
Above the prickle-cell layer is the stratum granulosum (granula = grain/Lat.), where cornification (keratinization) of the keratinocytes begins. It gets its name from its appearance, which is due to the presence of what are known as keratohyaline (i) granules which are composed primarily of the protein profilaggrin and keratin intermediate filaments.
Clear layer (stratum lucidium)
The stratum lucidium is also called the clear layer as it is highly refractive. The cells have been extremely flattened and are closely packed. The cell boundaries are no longer recognizable.
Horny layer (stratum corneum)
The stratum corneum (cornea = horny skin/Lat.) is the uppermost layer of the epidermis. Between the cornified cells (corneocytes) lie the epidermal lipids. The horny layer - especially the bottom third - forms the permeability barrier (i), which is the skin's true barrier against exogenous factors and endogenous water loss.
- Electron microscope image of shedding corneocytes.
