Skin is very thin. On most parts of the body, it’s about 0.04 to 0.08 inches thick (1.0 to 2.0 millimeters). The skin on the palms of the hands and the soles of the feet is a little thicker- nearly 0.25 inches (6.0 mm). Skin is made up of three even thinner layers known as the epidermis, the dermis, and the hypodermis. The hypodermis is also called the subcutis or the subcutaneous tissue.
EPIDERMIS
The epidermis is the outer layer of the skin, the part that we see and touch. Unlike most parts of the body, the epidermis has no blood vessels. Even though the epidermis is only thirty to fifty cell s thick, it is formed of five layers. From the innermost layer (stratum) to the outermost, they are: the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum.
The stratum basale is a very thin layer of cells that lies over the basement membrane. This membrane separates the lowest level of the epidermis from the upper level of the dermis. The stratum basale contains the cells- know n as basal cells- that make new skin cells. The basal cells and the new skin cells they make are collectively called epithelial cells. These cells cover our bodies on t he outside and line many of our organs on the inside.
The millions of new epithelial cells formed in the stratum basale are continually moving upward to replace older cells. This explains why our skin easily heals from minor scrapes and scratches without scarring. The new epithelial cells migrate upward through the spinosum, granulosum, lucidum, and the corneum layers. As the epithelial cells move into the spinosum and granulosum layers, they mature into the oblong-shaped cells called squamous cells. Squamous cells are the most common cells in the epidermis. The majority of skin cancers start in the basal and squamous cells.
It takes about seven weeks for living epithelial cells produced in the stratum basale to move upward through the epidermis to the stratum corneum, the outermost layer of skin. By the time the epithelial cells reach the stratum corneum, they are basically dead. Cells in the stratum corneum no longer have a nucleus (the control center of a cell that holds the cell’s genetic material).
The dead cells are flat and held together by lipids. Lipids are fatty, waxy substances that help to waterproof the skin. Every day we shed many thousands of dead, dried-up skin cells from the stratum corneum. These flecks of skin constantly fall from our bodies like microscopic snowflakes, landing on the floor, in our clothing, and at the bottom of the shower.
Other cells and structures reside within the epidermis. These are called non-epithelial cells. They are very different from the cells that make up most of the epidermis.
- Melanocytes are located in the bas al layer of the epidermis. They make up 5 to 10 percent of cells in that layer. Melanocytes produce the pigment melanin, which gives skin its color. People with little melanin are light-skinned. Those with more melanin have darker skin. All people have about the same number of melanocytes. The difference in the amount of melanin produced determines skin color. The reddish hemoglobin found in red blood cells and the yellowish pigment carotene also influences skin color. Melanin helps to protect skin against the ultraviolet radiation in sun light. Excess melanin product ion makes people tan. It’s a signal that the skin is working hard to protect itself from the sun’s ultraviolet radiation. The dangerous skin cancer known as melanoma starts in melanocytes.
- Langerhan s cells are par t of the body’s immune system. These cells work to fight skin infections. When Langerhans cells come across invading pathogens (organisms that cause disease) such as bacteria, the cells capture the invaders and shunt them off to the nearest lymph node (small organs the size and shape of beans that are part of the immune system). Once in the lymph node, other parts of the immune system do their part 10 finish off the bacteria.
- Merkel cells are sensory cells found in the basal layer of the epidermis. They sense touch and hair movement. For example, the Merkel cells can feel a nearly weightless mosquito landing on your arm and moving a hair or two as it crawls over your skin. A very rare and dangerous skin cancer called Merkel cell carcinoma may form in these cells.
DERMIS
The dermis lies below the epidermis. The dermis is the thickest skin layer and help s to cushion and protect the body. Proteins called collagen and elastin in the dermis give skin its strength and flexibility. Blood vessels, nerves, glands, and other types of tissues make up the dermis. The following is a list of these and their functions:
- Sebaceous glands produce and secrete, or give off, sebum, the skin’s natural oil.
- Apocrine glands produce and secrete sweat.
- Pacinian corpuscles sense deep pressure.
- Meissner’s corpuscles sense light pressure.
- Krause end bulbs detect cold temperatures, as well as touch and pressure.
- Ruffini’s corpuscles detect warm temperatures and skin stretching, as well as touch and pressure.
- Nerve endings detect pain.
- Hair roots and hair follicles originate in the dermis. The hair shaft reaches from the dermis through the epidermis and extends outside the skin.
- Arrector pili muscles control the movement of each hair.
- When people feel their “hair standing on end,” these muscles are working.
HYPODERMIS
The hypoderm is, or subcutaneous, tissue lies beneath the dermis. The hypodermis is composed of adjpose tissue (fat), fatty lipids, connective tissue (tough, elastic like fibers that help to hold cells and organs together), and blood vessels. The hypodermis holds the dermis to underlying muscle. It also cushions the body and regulates temperature. Women store more fat in the hypodermis than men. This extra subcutaneous fat gives women a soft body shape.