Anatomy of the skin:

Skin is the largest organ in human organism, which undergoes regular stress related to changes in its external environment. It plays a vital role in protecting the organism from microbes, pollutants and maintains optimal body temperature.

Skin has three layers (see illustration):
  • Epidermis (the outermost layer)
  • Dermis (contains connective tissue, hair follicles and sweat glands)
  • Hypodermis
Three layers of skin

Epidermis:

The epidermis consists of five main layers: the Stratum basale, Stratum spinosum, Stratum granulosum, stratum lucidum and stratum corneum keratinocytes are the major constituent of this layer in addition to melanocytes, Langerhans cells, Merkel cells and inflammatory cells.

The stratum basale contain epidermal stem cells that are able to divide and replenish the upper layers with new cells. Each epidermal stem cell divide to give two daughter cells, one of these daughter cells will remain stem cell and proliferate later to give a second generation of keratinocytes, while the other will undergo a process called differentiation.

Keratinocytes differentiation is a process upon which epidermal stem cells start moving upward to the surface of the skin forming keratinocytes and undergo changes in their structure and content (involucrin and keratin). As they move upward keratinocytes become part of the layers stratum spinosum and granulosum till they complete their differentiation by losing their nucleus and organelles to form corneocytes. Corneocytes are the main constituents of the stratum corneum and will be shed off later on.

At each stage of differentiation, keratinocytes express specific keratins, such as keratin 1, keratin 5, keratin 10 and keratin 14, beside other markers such as involucrin, loricrin, transglutaminase, flaggirin and caspase 14.

Keratinocytes differentiation is mediated by calcium gradient. The highest concentration of Ca+2 is distributed between stratum basale and stratum corneum and plays an important role in the formation of the layers. The lowest concentration of Ca+2 is found in the stratum corneum because corneocytes are dead cells which don’t dissolve Ca+2, the low concentration of calcium allow the cells of stratum corneum to be shed off a process called desquamation.

The stratum corneum is made up of corneocytes, which are surrounded by cell envelope. This cell envelope is made up of insoluble protein structure of loricirn and involucrin. Corneocytes release from their lamellar bodies fatty acids and ceramides that fill the area between them.

Dermis:

The dermis consists of cells called Fibroblasts surrounded by extracellular products. The extracellular environment consists of collagen and elastin fibers floating in a glycoprotein gel. The dermis contains big amount of water and hyaluronic acid one of the constituents of the extracellular matrix plays a crucial role in holding water in the dermis and stabilizing collagen and elastin fibers.

The dermis acts like a cushion that protects the body against stress, gives it volume and protects from stress, strain and plays an important role in healing process.

Hypodermis:

It is the innermost layer of the skin and consists of fibroblasts, adipose tissue and macrophages. It plays an important role as thermo regulator and energy reservoir.