Physical resilience of skin
The skin has several mechanisms to protect us. Firstly, the outer epidermis is made up of a constant stream of hardwearing replenishing cells that are filled with the tough protein keratin. On most of our body the epidermis is made up of four layers of squamous cells, a resilient cell type and arrangement that is constantly replenishing as cells are lost from the outermost layer. The outer layers of the epidermis are fully keratinised to provide a tough waterproof layer that is resistant to injury. Maintaining a supple and moisturised epidermis will help prevent skin injury, in part by filling in the junctions between the hard skin cells to create a smooth surface and prevent raised edges where skin cells can be sheared off.
On areas with thicker skin, such as the palms or soles, the epidermis has an additional fifth layer to provide further protection and provide the added resilience to external force. The basal layer of the epidermis is where the melanocytes live, the cells that produce the pigment that protects us against ultraviolet radiation from the sun – melanin. Generally, the thickness of the epidermis ranges from between two to six millimeters so the depth of skin injury sustained may vary by body part for the same force.
Hydration in the deeper epidermis affects its flexibility and ability to resist distortion and injury as well as adequate nutrition and vitamins to allow the escalator of keratinised squamous skin cells to continue to provide a tough resistant outer layer. Aside from keratin, the principal protein that contributes to skin being able to tolerate distortion without injury is collagen, which of course reduces in quality and quantity with age and excessive ultraviolet light exposure, this contributing to a reduction is skin resilience.
Beneath the epidermis lies the non-keratinised and more flexible dermis which houses nerves and blood vessels as well as hair follicles. The junction between epidermis and dermis has a ridged or saw-toothed pattern giving extra structural adherence between the two layers due to a larger surface area of contact and bonds.
Deep to the dermis is the hypodermis, a fatty layer containing fat cells and connective tissue. The relative “sponginess” of this layer provides a deeper layer of resilience to the epidermis by allowing some movement and absorption of force at a deeper level, thus protecting the superficial surface. Thus, the hypodermis is very important in skin injury.
Not surprisingly, parts of the body that lack the fifth layer of epidermis and so are thinner, but also have a thin hypodermis are those areas that are at greatest risk of friction injury. E.g., on forearms and thighs away from the joints.