The Skin’s Three Layers
The epidermis is a thin, protective layer of cells that contain no blood vessels and consists of five layers of cells with no sharp divisions between the layers. Designed to provide protection, it is responsible for the appearance, colour, suppleness and texture of the skin.
The cells shed themselves on a regular basis. The outer layer of dead flat scaly cells made of a tough protein called keratin is constantly being rubbed away by friction and replaced by new cells produced by cell division in the lower living layer. However, the speed of cell regeneration and migration slows down as we age, which in turn slows down the shedding process.
The thickness of the epidermis varies in different parts of the body, being the thickest on the soles of the feet and palms of the hands, and thin in the face, particularly the neck and around the eyes. Thickness also varies in different races, usually being much thicker in dark-skinned people.
The epidermis can be divided into five layers:
- Stratum corneum or the horny cell layer (the skin we can see)
- Stratum lucidum or the clear layer (only found on soles of hands and feet)
- Stratum granulosum or the granular layer
- Stratum spinosum or the prickle cell layer
- Stratum germinativum or the basal cell layer
THE DERMIS LAYER
The dermis is a layer of tissue situated immediately below the epidermis and is the nutrition path to the basal or germinative layer. The dermis supports the epidermis and consists of a complex network of loose connective tissue, blood vessels, lymph vessels and nerve endings are contained within the dermis. These blood vessels are responsible for providing the epidermis with nutrients and oxygen. The strength and support of our skin occurs through elastin, which gives our skin its elasticity and the plumpness in our skin is due to the collagen fibres found in the dermis.
Collagen Fibres – Collagen is the most abundant protein in the body and constitutes about 75% of the dry weight of the dermis. There are 29 identified types of collagen, but in skincare, we are mainly concerned with Collagen I, III, IV, VII, XII and XIII. Collagen serves as the major structural component of the skin, and also plays an essential role in healing wounds and scars. Collagen lies in layers parallel to the skin surface with alternate layers at right angles to each other. Collagen is composed of protein molecules (amino acids) consisting of non-elastic fibres arranged in a folded or wave-like structure. The amount of collagen in dark skin is much greater than in light skin. When collagen fibres deteriorate it usually presents itself on the surface as signs of fine lines and wrinkles and dehydration.
Elastin Fibres – A protein called elastin lies between and runs parallel to collagen fibres. These fibres enable the skin to stretch and to spring back when released giving skin its flexibility. The stretching of the elastic fibres causes the unfolding of the collagen fibres but they normally return to their folded form as the elastic fibres relax. Elastin also contains two unique amino acids, desmosine and isodesmonsine. Collagen and elastin together, much like an elastic textile mix, permit the skin to stretch, and then regain its original shape. With age, the skin’s elastin breaks down and presents itself on the surface as sagging with deep lines and wrinkles.
The Ground Substance – Collagen and elastin are the skin proteins responsible for elasticity, tone and texture, and are embedded in the ground substance (extracellular matrix), which consists of a jelly-like substance called proteoglycan (a compound of protein and sugars) capable of absorbing considerable amounts of water. Glycosaminoglycans (GAG’s or mucopolysaccharides) and proteoglycans hold water in the skin (these are very similar to mucus proteins) and are the true skin moisturisers. GAGs contain special sugars that have high water-holding properties. These are built into larger water-holding chains of sugars such as hyaluronic acid. Proteoglycans are larger molecules with many attached GAG’s which are intensely hydrophilic (water-loving) molecules able to form porous, hydrated gels. Hydrated GAG’s cushion and provide mechanical support to tissues. Ground substance can be fluid, semi fluid, gelatinous, or calcified. As such, ground substance plays an active role in how tissues develop, migrate, proliferate, change shape, and carry out their metabolic functions.
Fibroblasts – Fibroblasts are concerned with the secretion of proteoglycans and glycoproteins for the ground substance of the dermis, and also play a part in the production of collagen, glycosaminoglycans, reticular and elastic fibres, and glycoproteins found in the extracellular matrix. A fibroblast is a type of cell that synthesises and maintains the extracellular matrix of many tissues. Fibroblasts provide a structural framework (stroma) for many tissues, and play a critical role in wound healing. In growing individuals fibroblasts are dividing and synthesising ground substance. We are born with a certain number of fibroblasts which renew themselves no more than 50 to 100 times during a lifetime. During the aging process the fibroblasts become lazier and need stimulation to stay prolific. Tissue damage stimulates fibrocytes and induces the mitosis of fibroblasts.
THE HYPODERMIS LAYER
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 in this layer 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 skin its resilience. Among mammals, only humans and marine mammals such as whales and dolphins have this subcutaneous layer of fat. Sweat glands originate in this layer and excrete waste matter through perspiration. This sweat controls the body’s temperature by evaporating and cooling the skin surface. “Goose-bumps” occur when the fine layer of muscles found in this layer contract.
SKIN IN CROSS SECTION
The diagram below shows the skin in cross section:
Stratum Corneum (outermost layer) is comprised of 25-30 rows of dead cells, approximately 57-150 microns thick. These cells are densely compacted, form a natural barrier and are sloughed off continuously.
Stratum Lucidum has cells that are highly keratinised and contain no nuclei. This layer varies from one cell thick to a discernable thickness on the soles and palms. They are colourless, flat cells, which cushion and protect underlying tissue.
Stratum Granulosum (transitional layer) is the outermost living layer, consisting of two or three rows. At this layer, cells begin to die and lose most of their moisture and lipids (keratinise). These cells progressively begin to flatten as they migrate upward.
Stratum Spinosum (prickle cell or spiny layer) is 8-10 cells thick. This layer of cells is connected by fibrils and migrates upward to the stratum granulosum.
Stratum Germinativum (basal layer) is one cell thick and lies directly on the dermis. This layer is the living portion of epidermis, producing the skin cells that migrate to the surface to become keratinised and eventually sloughed off.
Papillary Dermis is a thin layer of cells, forming the hills and valleys at the bottom of the stratum germinativum. This layer contains lymphatic and capillary vessels, which provide nourishment to the living layer of the epidermis.
Reticular Dermis is the main area of collagen and elastic tissue. This layer contains fibroblasts that are the cells from which collagen and elastin fibres are formed. Vacuum and the exfoliation stimulate fibroblasts into greater production.