The Different Types of Skin

Your skin provides a barrier against heat, light, chemicals and physical injury. It also helps maintain a normal internal body temperature and gathers sensory information.

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Your outer layer of skin, the epidermis, is constantly replacing itself with new cells. This process is called keratinization. Skin cells called keratinocytes produce the protein known as keratin and also create melanin, which helps regulate temperature.

The Epidermis

The epidermis, which consists of keratinized stratified squamous epithelium, is the outer most layer of skin and is responsible for providing protection from outside substances. It is tough and relatively impermeable and it expresses various proteins that serve important protective functions. It also contains a strong fibrous protein called keratin that provides a range of structural properties, including impact resistance (as in hooves and nails), the ability to resist aerodynamic forces (horns and claws), and the capacity for the growth of new cells and regeneration after injury or disease.

The principal cells in the epidermis are keratinocytes. These proliferate during mitosis and migrate up through a series of layers, becoming more specialized as they go. When they reach the epidermis’ bottom layer, the stratum basale, they become keratinized by secreting the protein keratin into the intercellular space. As they move up through the stratum basale, the keratinized cells become flattened and stick together. When they reach the top of the epidermis, they become a hardened layer called the stratum corneum. The dead cells are eventually shed from the surface of the skin.

The epidermis also contains melanocytes, which produce the pigment that gives skin its color. Other types of cells in the epidermis include dendritic cells that act as part of the body’s immune system, and Merkel cells that provide a sense of gentle touch. This layer is continuous with — and a component of — the dermis, which contains blood vessels, sensory nerves, epithelial invaginations for hair follicles, sebaceous glands, and sweat glands.

The Dermis

The dermis is the middle layer of your skin. It provides padding, support and elasticity. It also houses blood vessels, glands, hair follicles and nerves. In addition to this, the dermis contains fat cells. It is also known as the subcutis. Your body’s fat is stored in the subcutis and functions as an insulator, conserving heat and cushioning the inner organs against shock and injury.

The dermis has two layers of connective tissue, the papillary and reticular layers. These layers merge into the hypodermis below without a clear demarcation. The papillary layer is thinner and contains loosely arranged collagen fibers. The reticular layer is thicker and denser, and it is made up of dense bundles of collagen fibers. It is this layer that houses sweat glands, hair follicles, nerve endings and adipose lobules.

The adipose lobules, which contain fat for insulation and storage, are located deep in the dermis beneath the reticular layer. They are surrounded by a network of blood vessels and lymph vessels that transport nutrients and eliminate wastes from the cells in the epidermis and dermis. The reticular layer also has a bed of capillaries that supplies nourishment to hair follicles and nerve cells. There are also sebaceous glands in the reticular layer that secrete the oil that coats and protects the hair follicles.

The Hypodermis

A layer of fat and connective tissue, it carries out the cushioning and insulation functions of the skin, as well as anchoring the skin to underlying muscles and tissues. It also stores energy and has a role in regulating body temperature. Fibroblasts and adipose cells are the primary cells of the hypodermis.

Your skin is the largest organ in your body, and it is a complex and dynamic structure. Its form and shape are determined by the fractal geometric patterns of the cellular structures in it, which deform with movement to translate forces through the dermal fibrillar network into a pattern of topographical lines that appear on your skin.

The hypodermis is also the site of hair follicles, sweat glands and some glands that produce prolactin and other pituitary hormones. It also contains sensory nerves and blood vessels.

The lower, reticular layer of the hypodermis is denser than the papillary layer and appears reticulated because of its tightly knit meshwork of collagen fibers. The reticular layer supports the epidermis, and its fibers provide some of the skin’s elasticity. It also binds water, a function that helps keep the skin hydrated. It is also the primary site for fat storage in your body, and it contains the nerves, blood vessels and lymph vessels that pass through the skin to reach underlying muscle.

The Endothelium

The endothelium is the inner lining of blood and lymph vessels. It is an active organ that responds to and secretes chemical signals. It controls the movement of substances into and out of the blood vessels, maintains the balance between fluid and cell trafficking and plays an important role in controlling vascular tone (the balance between constriction and dilation).

The lining is composed of a single layer of squamous cells that are stacked on top of one another. It has the appearance of scales and was named squamous because of its resemblance to fish or snake skin. Your body has more than 1 trillion of these specialized cells. They line all blood and lymph vessels except for those in direct contact with the outside environment (e.g. the cornea and lungs). The cells that come into direct contact with blood are called vascular endothelial cells; those that line lymphatic vessels are called lymphatic endothelial cells.

The endothelium forms a selectively permeable layer that allows passive diffusion of molecules smaller than 3 nm. It also regulates cellular and nutrient trafficking, contributes to blood fluidity, mediates the balance between pro- and anti-inflammatory factors and thrombosis, orchestrates tissue development, participates in innate and acquired immunity and undergoes programmed cell death. It does this by responding to a wide range of vasoactive chemicals including nitric oxide, acetylcholine and norepinephrine. It also produces the chemical prostacyclin that inhibits platelet aggregation and the protein von Willebrand factor which binds to clotting factors to prevent blood clots from forming in a vessel.