The Epidermis

As the outermost layer of the skin, the epidermis forms the actual protective covering against environmental influences such as toxins, bacteria and loss of fluids. Over most parts of the body, it is only about 0.1mm thick. On the surface, it is even thinner, at 0.02mm. On the feet, it is considerably thicker at 1 to 5 mm.

The epidermis is largely made up of special cells called keratinocytes, which received their name because they product a fibrous protein called keratin found in the outer layer of the skin, nails and hair. Keratinocytes are produced in the innermost layer of the epidermis and migrate upward to the surface of the skin. As they do this, the cells undergo a series of changes and make up five distinct layers:

• Horny layer (stratum corneum)
• Clear layer (stratum lucidum)
• Granular layer (stratum granulosum)
• Prickle layer (stratum spinosum)
• Basal layer (stratum basale)

(6 shows the Basal membrane)

Diagram of the epidermis: the basal cells change, through differentiation, into flat horny skin cells that are without nuclei.

1. Horny layer (stratum corneum)

2. Clear layer (stratum lucidum)

3. Granular layer (stratum granuolsum)

4. Prickle layer (stratum spinosum)

5. Basal layer (stratum basale)

6. Basal Membrane

Basal layer (stratum basale)
The innermost layer of the epidermis, the basal cells lie directly on a membrane that separates the epidermis from the dermis. Basal cells act as ‘mother cells’ by continuously reproducing, through cell division, additional basal cells. This constant reproduction causes the upward push of basal cells. As they are pushed upward, basal cells undergo a series of changes, forming the distinct types of cells found in the four additional layers in the epidermis. Melanocytes, melanin or skin pigment producing cells, live in the basal layer.

Prickle layer (stratum spinosum)
As the basal cells move upward, they change and become spindle-shaped. Desmosomes (dense plaques that play a role in holding the cells together) develop at this point and form ‘spines’ between the cells (hence the name spinous layer). The cells in this layer also produce keratin or protein fibers.

Granular layer (stratum granulosum)
As the cells move further upward, the process of keratinisation begins. The cytoplasm of the cells produce hard, granule-like proteins made up of keratohyalin, hen the name stratum granulosum. As the cells push upward, keratohyalin changes into keratin.

Clear layer (stratum lucidium)
Just above the granular layer, the cells become so tightly compressed that they stick together and lose their nuclei. This makes cells indistinguishable from one another. In addition, these keratinizing cells contain a substance called eleidin, which has a high light-refracting capacity. This gives the layer a ‘translucent’ appearance (hence, the name clear layer and stratum lucidium)

Horny layer (stratum corneum)
The outermost layer of the epidermis, the horny layer consist of about 20 layers of flattened, dead cells with no nuclei. By the time the cells reach this layer, they are fully keratinized. Epidermal cells lie in between the cells. Skin cells from the layers closest to the surface are regularly sloughed off in a process called desquamation. The tightly packed inner layers of cells form a permeability barrier that protects the skin against environmental factors.

Electron microscope image of scaling horny skin cells