The skin is the largest organ composed of three layers, each with its own important function: the epidermis, the dermis, and the hypodermis – the deeper subcutaneous tissue made of fat and connective tissue are found under the dermis.
- The EPIDERMIS is the outer layer which is accounts for 10% of the total thickness of the skin. The epidermis is a thin, flat layer of skin. It is the only layer that we have direct contact with. It protects the body from toxins, bacteria, and fluid loss. It does not contain any blood vessels, is dependent on the dermis, the layer of skin beneath it, which provides the epidermis with nutrients. The epidermis is always compromised of a number of layers of epithelial cells, densely packed one on top of the other, creating this compact layer. The number of layers differs depending on the different areas of the body, for example, in humans, the number of layers is greater on the palms of the hands and soles of the feet (on the elbows and back it is 0.3mm thick, and on the palms and soles it’s up to 4mm thick). Every day, thousands of dead cells are shed, but the skin does not degenerate as these cells are being constantly regenerated. The epidermis regeneration period lasts 28 days. Throughout a lifetime, a person sheds approximately 18kg of “dead” skin. In addition to keratinocytes, which make up 90% of the cells of the epidermis, this layer of the skin is also comprised of melanocytes, Langerhans cells, and Merkel cells. The epidermis consists of four layers according to cell maturation:
1- Basal layer (stratum basale)
2- Spinous layer (stratum spinosum)
3- Granular layer (stratum spinosum)
4- Horny layer (stratum corneum)
The innermost layer – the basal layer is comprised of one row of cells which are constantly working. All other cells are produced from these cells – keratinocytes, which migrate from the innermost layer towards the surface, mature, and go through a series of changes. This process of keratinocyte maturation differentiates the sublayers of the epidermis and is called keratinization (cornification).
- The DERMIS is located beneath the epidermis and contains the blood vessels, nerve endings, sweat and sebaceous glands, follicles (openings through which hairs, collagen, and elastin grow). The dermis is the second and thickest layer of the three major layers of skin, located between the epidermis (top payer) and the hypodermis (lower layer). Both the epidermis and dermis are made up of layers of cells and tissue, but the dermis is a lot thicker than the epidermis, even though the dermis only has 2 layers and the epidermis has five layers. That’s because the cells in the epidermis are smaller and flatter, while those in the dermis are larger. The dermis is made up of fibroblast cells that produce two proteins, collagen, and elastin, that give your skin both strength and flexibility. In addition, collagen binds water to keep the skin hydrated. Next to fibroblast cells and the proteins collagen and elastin, the dermis contains nerve endings, sweat glands, oil (sebaceous) glands, hair follicles, and blood vessels. The dermis is rich with blood vessels, however, although none penetrate the living layer of the epidermis. The site where the dermis and the epidermis meet is called the dermal-epidermal junction, where the network of blood vessels obtains its nutrients and oxygen from, and which expands from the dermis towards the epidermis. Little blood vessels within the dermis act as a transport system that allows nutrients to feed the skin. The epidermis gets its nutrients from the dermis and oxygen by means of diffusion which is the transport of substances from the environments with higher to environments with lower concentration.
- The hypodermis, subcutaneous tissue, serves as an insulator and a place where energy is stored. The hypodermis is the innermost and thickest layer of the skin, located directly beneath the dermis. It is also known as the subcutaneous layer of subcutaneous tissue. This layer is composed of that tissue which consists of fat cells (adipocytes) and connective tissue barriers, as well as blood and lymph vessels and nerves. The main function of the hypodermis energy storage, necessary for the normal functioning of the organism. This layer is active in regulating body temperature by trapping in heat or cold due to the blood vessels and nerve endings that are woven throughout it. In addition, it connects the skin with the motion structures, muscles, and bones that are situated below the skin surface. Another function of this layer of the skin is to amortize the mechanical traumas in order to reasonably reduce the possibility of body injury. Where the fat is deposited and accumulates within the hypodermis depends on hormones, as well as genetic factors. Fat distribution changes as our bodies mature and age. Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women ( breasts, hips, thighs, and buttocks).