Technically the skin is composed of three layers: the epidermis (the top layer), the dermis (the middle layer) and the subcutaneous (bottom layer).
The epidermis is the thinnest skin layer with a maximum thickness of 1 mm. It is composed of 4 distinct layers of about 0,25 mm each. The top outer layer called "corneal layer" is made of dead, hard, tough cells that form the hard skin surface. The 3 lower layers produce millions of new cells every day, at the bottom of the epidermis. These new cells start as soft cells and then undergo a differentiation process and are transformed into the hard, flat cells of the outermost layers as they are pushed upwards to the surface while the dead cells are continuously sloughed off and replaced by new ones from below. In this way the epidermis is replaced about once a month.
The dermis is a thick, supple and sturdy layer of connective tissue and makes up about 90 percent of the skin's thickness. The dermis is a dense meshwork of collagen and elastin fibres, two connecting proteins. This meshwork supports tiny lymph- and blood vessels that allow the skin to breathe and be nourished as well as the nerves, muscle cells, sweat and sebaceous glands and hair follicles. This layer contains special cells that repair the skin, such as fibroblasts.
The subcutis and subcutaneous adipose tissues
The subcutis is the deepest layer of the skin, composed primarily of fat. The subcutaneous layer manages the skin's functions of feeding, excreting and heat exchange. The key cells are fat cells or adipocytes that provide energy, serve as a heat insulator for the body and act as a shock absorber to protect underlying tissue against mechanical trauma and help give the skin its resilience.
Major skin molecules: collagen, elastin, GAG's and proteoglycans
Collagen forms the structural network of the skin and is the most abundant protein in the human body. It is one of the strongest proteins in nature and gives the skin its strength and durability.
Elastin is similar to collagen but it is a more stretchable protein that maintains the skin's elasticity. It provides the matrix that holds indvidual skin cells in place.
GAG's contain special sugars that have high water holding properties. These sugars are built into larger water holding chains of sugar such as hyaluronic acid.
Proteoglycans are larger molecules with many attached GAG's.