The epidermis is the thin outer layer of the skin which consists of the following three parts:
● Stratum Corneum (horny layer)
This layer consists of fully mature keratinocytes which contain fibrous proteins (keratins). The outermost layer is continuously shed. The stratum corneum prevents the entry of most foreign substances as well as the loss of fluid from the body.
● Keratinocytes (squamous cells)
This layer, just beneath the stratum corneum, contains living keratinocytes (squamous cells), which mature and form the stratum corneum. Keratinocytes release antimicrobial peptides which protect the skin against viral, bacterial and fungal infections.
● Basal Layer
The basal layer is the deepest layer of the epidermis, containing basal cells. Basal cells continually divide, forming new keratinocytes, replacing the old ones that are shed from the skin’s surface.
The epidermis also contains melanocytes, which are cells that produce melanin (skin pigment).
Skin Needs To Hold Internal Moisture
The dermis is the connective tissue matrix of the skin, providing structural strength, storing water, and interacting with the epidermis. In humans, dermal thickness varies from 1 mm to 3 mm. Its principal components include elastin (0.2 to 0.6 percent by volume ; 4 percent by dry weight), collagen (27 to 39 percent by volume; 75 to 80 percent by dry weight), glycosaminoglycans (0.03 to 0 .35 percent by volume), water (60 to 72 percent by volume), and cells.
Skin Needs Nutrition
The vasculature of human skin is composed of two distinct parts, the nutritional capillaries and the thermoregulatory blood vessels. The nutritional capillaries are organized into vertical capillary loops in the papillary dermis, providing nutrients to the upper dermis and basement membrane.
Structure of the Skin
The skin is divided into three main areas or layers the hypodermis, the dermis, and the epidermis. The hypodermis is loose connective tissue where the formation and storage of fat occurs. Deep rooted hair follicles and sweat glands also originate here. The fatty layer acts as a thermal regulator, food control, and cushioning. The hypodermis plays an integral role in supporting blood vessels and nerves that travel to and through the dermis.
The dermis is a layer of connective tissue rich in blood and nerve supply. The dermis has internal layers, the papillary (connects the dermis to epidermis) and reticular sublayers (connects the dermis to hypodermis). Thickness of the dermis varies from area to area, being the thickest on the back and thinnest on the eyelids. Its main function is to offer support to the skin and its annexes, such as hair and nails.
The epidermis is the outermost layer and is highly significant to skin integrity. The layers of the epidermis include the stratum germinativum (basale), stratum spinosum (prickle), stratum granulosum (granular), stratum lucidum (lucid, only present in thick-skinned areas such as palms and soles), and stratum corneum. Each layer has proliferated keratinocytes that travel upwards while cornification occurs. Cornification is the process of forming an epidermal barrier in stratified squamous epithelial tissue. As they move to the top and reach the final layer, the keratinocytes flatten, lose their nuclei, and fill with keratin fibers. The dead keratinocytes are now corneocytes. Eventually, corneocytes are shed. In healthy skin, this process of epidermal differentiation takes approximately 14 days.
Intracellular lipids stabilize corneocytes in a wall-like structure. Lipids also move upwards from lower epidermis levels and have a vital role in retaining water in the body. They are converted from phospholipids to nonpolar lipids (ceramides, free sterols, free fatty acids, and cholesterol). These intercorneocyte lipids are arranged in layers and are important for cellular cohesion. They play an important role in the keratinization process and the moisturization of the skin. Any disruption in structure can cause moderate to severe dry skin.
Major functions of the skin are to protect the body from physical and chemical harm and to prevent loss of body water and other substances. The protein enriched corneocytes and lipid-enriched intracellular domains establish the function of the stratum corneum, as they literally form a barrier between the interior body and external world. The function of the stratum corneum is maintained by water, which ensures that the stratum corneum is soft and flexible. If water is lost and skin is dry, the stratum corneum becomes hard and brittle, losing its ability to function and clinically presenting with irritating symptoms.
Low molecular humectants called natural moisturizing factor are contained within the corneocytes. They are water soluble and account for 15% to 20% of the stratum corneum. Natural moisturizing factor play a crucial role in the water holding capacity of the skin and increase its elasticity as well. Without natural moisturizing factor, the skin’s water binding capacity is lost or diminished. Water is also necessary to maintain metabolic processes of the skin.