Layers of the Skin

The epidermis is the outermost layer of the skin.

The epidermis is composed of five layers called the strata.

The uppermost surface layer is the stratum corneum — “the horny cells”, followed by the stratum lucidum — “the clear cells”, stratum granulosum — “the grainy cells”, and stratum spinosum — “the spiny cells”; the bottom layer is the stratum germinativum (the basal layer) — “the germination or growth layer”.

Estheticians are licensed to work only on the epidermis, not the dermis, unless they are working with a physician or another licensed medical practitioner.

Keratinocytes, composed of keratin, comprise 95% of the epidermis. These cells contain both proteins and lipids.

Keratin is a fiberous protein that provides resiliency and protection to the skin.

The stratum corneum, also known as the horny layer, is the top, outermost layer of the epidermis.

The esthetician is primarily concerned with this layer.

Keratinocytes on the surface have hardened into corneocytes, the waterproof, protective cells.

Keratinocytes are continually shed from the skin in a process called desquamation. These cells are replaced by new cells coming to the surface from the lower stratums. This process of desquamation and replacement is known as cell turnover.

The average adult cell turnover rate is ever 28 days depending on a person’s age, lifestyle, and health. The cell turnover rate slows down with age.

Cells and oil combine to form a protective barrier layer on the stratum corneum. This is the acid mantle.

The stratum lucidum is a thin, clear layer of dead skin cells under the stratum corneum. It is a translucent layer made of small cells that let light pass through. This layer is thickest on the palms of the hands and soles of the feet.

The stratum granulosum, also known as granular layer, is composed of cells that resemble granules and are filled with keratin. The production of keratin and intercellular lipids also takes place here.

Natural moisturizing substances such as triglycerides, ceremides, waxes, fatty acids, and other intercellular lipids are made here and are excreted from cells. These water-soluble compounds are referred to as natural moisturizing factors (NMFs) and hydrate the lipid layer surrounding celll, absorb water, and prevent water loss.

The stratum spinosum, also known as spiny layer, is above the stratum germinativum. Cells continue to divide and change shape here, and enzymes are creating lipids and proteins. Cell appendages, which resemble prickly spines, become desmosomes, the intercellular structures that assist in strengthening and holding cells together.

Desmosomes are keratin filaments — the protein bonds that create the junctions between the cells. These strengthen the epidermis and assist in intercellular communication.

Also found here are the Langerhans immune cells, which protect the body from infections by identifying foreign material (antigens). The immune cells help destroy these foreign invaders.

The spinosum is the largest layer of the epidermis.

The stratum germinativum, known as basal cell layer, is located above the dermis, composed of a single layer of basal cells laying on a “basement membrane.” In this active layer, stem cells undergo continuous cel division (mitosis) to replenish the regular loss of skin cells shed from the surface. Stem cells are basically mother cells that divide to produce daughter cells.

Mother cells divide to form two daughter cells. Daughter cells that are not able to divide anymore are now programmed to end up as one specific type of cell. This is known as terminal differentiation.

Merkel cells (sensory cells) are touch receptors also located in the basal layer.

The stratum germinativum also contains melanocytes, which are cells that produce pigment granules in the basal layer. The pigment carrying granules, called melanosomes, then produce a complex protein, melanin, which determines skin, eye, and hair color.