The Basics of Skin

Skin is tough but flexible armor that keeps microbes, chemicals and strong rays of light away from more delicate inner tissues. It also provides sensory perception and other functions such as temperature regulation and immune surveillance.

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Use gentle cleansers and moisturizers, avoiding those that dry or strip the skin of its natural oils. Pat skin dry rather than rub to avoid tugging and losing elasticity.

The Epidermis

The epidermis is the outermost layer of skin, the part that provides protection from physical damage, pathogens and UV light. It is composed of stratified squamous epithelium. This consists of layers of flattened cells with an impermeable surface made of a tough protein called keratin.

The cell that forms the top of this layer is a specialised epithelial cell called a keratinocyte. Its flattened, cube-shaped cells are tightly packed together and have a clear cytoplasm that looks like water on H&E staining. The keratinocytes produce, or synthesise, a thick fibrous protein that is deposited in the form of hardened clumps (known as corneocytes). The corneocytes have a tight bond between them and an impermeable barrier of keratin.

Above the keratinocytes is a layer known as the stratum lucidum, found only in thickened skin on the palms of the hands and feet, and on some parts of the face and soles of the feet. This layer is a thin, translucent layer of dead keratinocytes which are densely filled with a clear fluid called lipids. The lipids are derived from the membrane of a secretory product (the stratum granulosum) produced by keratinocytes in the stratum basal.

Other cells in the epidermis include mast cells, which contain granules that are packed with histamine and other chemicals. When an allergen is encountered, these cells break it into smaller particles and migrate through the epidermis into the dermis where they are presented to immune cells that recognise and destroy the allergen. Finally, there are special epidermis cells, called Merkel cells, whose exact role is not fully understood. These have segmented nuclei and appear pink on H&E staining.

The Dermis

The dermis (also called corium) is a dense layer of connective tissue that cushions subcutaneous tissues and protects the body from stress and strain. The dermis also provides elasticity to the skin and support for hair follicles, glands, blood vessels and nerves. It is primarily composed of fibrous, tough collagen fibres and elastin fibres that bind to a gel-like ground substance.

The top area of the dermis adjacent to the epidermis is known as the papillary layer and the bottom thicker area is called the reticular layer. The papillary layer is well supplied with blood vessels. These blood vessels expand when you are hot to bring your body heat up toward the surface of the skin and contract when you are cold to reduce blood flow and thereby maintain your skin’s temperature. This process is regulated by the endocrine system through release of the hormone cortisol.

The reticular layer is richly supplied with blood vessels and contains most of the other structures in your dermis, including glands, hair follicles, lymphatics and nerves. The reticular layer is thicker than the papillary layer and appears reticulated because of the close meshwork of elastin and collagen fibres that gives your skin strength and elasticity. Nerve endings in your dermis allow you to feel different sensations, such as pressure, pain, heat and cold. Your dermis also contains sweat glands, which produce sweat to help control your body temperature. And, your sebaceous glands secrete a oily lubricant called sebum to keep your skin supple and shiny.

The Hypodermis

The adipose tissue in the hypodermis acts as an initial barrier against pathogens, UV light and chemicals and protects from mechanical injury. It is also a source of energy. It consists of fat cells that form lobules with fibrous septa in between. It produces a hormone called leptin that regulates your body’s energy balance and prevents you from overeating.

The epidermis consists of four to five layers of keratinized stratified squamous epithelium, depending on its location in the body. It has no blood vessels in it (it is avascular) and has the following layering:

In the uppermost, most superficial layer of the epidermis, the cellular material becomes hardened and dry, a process called keratinization. This leads to the formation of a protective and waterproof barrier known as the stratum corneum. The cells in the stratum corneum do not have nuclei, giving them a dead appearance, and are densely packed with eleiden, a clear protein rich in lipids which provides a lipid barrier to water.

The dermis consists of loose connective tissue, which differs from site to site and forms gliding layers or large pockets of adipose tissue that insulate and protect it. The underlying reticular dermis appears reticulated due to a tight meshwork of collagen fibers that provide structure, tensile strength and some elasticity. It also binds water to the skin’s surface, allowing it to retain moisture and flexibility 79.

The Cicatrix

A cicatrix is a scar that remains after the development of new tissue over a recovering wound or sore. It is distinguished from other scars by a change in texture, a lack of flexibility, and the presence of nerve pressure or circulation disturbance.

The best way to think of a cicatrix is the raised mark you get on your skin where you burned yourself on the pan a few years ago. Cicatrix can also refer to a large number of medical procedures, including cicatrix removal, contracture release, division and elevation of platysma flaps, and skin grafts.

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