Chapter 1 Your Skin And The Sunless Process
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The Skin
Although the skin is less complicated than most other organs, it is still one of the most architecturally advanced of all. It covers the entire body and accounts for about 7% of our total body weight, making it the largest organ. It has been estimated that every square centimeter (cm) of skin contains 70 cm of blood vessels, 55 cm of nerves, 100 sweat glands, 15 oil glands, 230 sensory receptors, and about 500,000 cells that are constantly dying and being replaced.
The skin, which varies in thickness from 1.5 to 4 millimeters (mm) or more in different regions of the body, has two distinct layers. The outer layer is the epidermis, a thick membranous tissue and the layer of skin where the complete sunless process takes place. Located below the epidermis is the dermis, a fibrous connective tissue. And, just below the dermis lies a fatty layer called the hypodermis. Although the hypodermis is usually not thought of as part of the skin, it shares some of the skins functions and will be discussed in this section.
The skin performs many functions, most but not all of are protective. It cushions and insulates the deeper body organs and protects the entire body from physical damage like bumps and cuts. The skin also offers helpful protection from harmful chemicals, thermal damage (heat and cold), and invading bacteria. The epidermis is waterproof, preventing unnecessary loss of water across the body surface. The skins rich abundance of blood flow and sweat glands regulate the loss of heat from the body, helping to control body temperature. The skin also acts as a mini-excretory system: Urea, salts, and water are lost as sweat. Skin also reduces ultraviolet (UV) rays from the sun, and its epidermal cells use these UV rays to synthesize vitamin D. Finally, the skin contains sensory organs called sensory receptors that are associated with nerve endings. By sensing touch, pressure, temperature, and pain, these receptors keep us aware of what is happening at the body surface. As this chapter describes the anatomy of the skin, we will explore its function in greater detail.
HYPODERMIS
Just below the skin is the fatty layer of the hypodermis ("below the skin" in Greek). This layer is also called the subcutaneous layer ("below the skin" in Latin). It consists of both areolar and adipose connective tissue, although the adipose tissue normally dominates. Besides storing fat, the hypodermis anchors the skin to the underlying structures (mostly to muscles) and allows the skin to slide relatively free over those structures. Sliding skin protects us by ensuring most blows just glance off our bodies. The hypodermis is also an insulator: Since fat is a poor conductor of heat, it helps prevent heat loss from the body. The hypodermis thickens distinctly when one gains weight, but this thickening occurs in different body areas in the two sexes. In females, subcutaneous fat accumulates first in the thighs and breasts, whereas in males it first accumulates in the front abdominal area.
DERMIS
The dermis, the second major layer of the skin, is a strong, flexible connective tissue. The cells in the dermis are identical to those of any connective tissue in the body. The dermis binds the body together like a body stocking.
The dermis is richly supplied with nerve fibers and blood vessels. The blood vessels of the dermis are so extensive that it can hold 5% of all blood in the body. When organs, such as exercising muscles, need more blood, the nervous system constricts the blood vessel located in the dermis. This shunts more blood into the general circulation, making it available to the muscles and other organs. On the other hand, the dermal blood vessels swell with warm blood on hot days, allowing heat to radiate from the body creating a cooling effect.
The collagen fibers of the dermis give skin its strength and resilience. Thus, many jabs and scrapes usually do not penetrate the tough dermis. Furthermore, elastic fibers in the dermis provides the skin with stretch and recoil properties.
The deeper part of the dermis is responsible for markings on our skin surface called flexure lines. These lines are easily observed as the deep skin creases on the palms. Flexure lines result from a continual folding of the skin, often over joints, where the dermis attaches tightly to underlying structures. Flexure lines are also visible on the wrists, soles of the feet, fingers, and toes.
EPIDERMIS
It is at this layer the sunless process takes place. The epidermis contains four distinct types of cells: keratinocytes, melanocytes, Merkel cells, and Langerhans cells. Keratinocytes are by far the most abundant cells of these, so we will discuss them first. We will discuss the other types of cells later as we examine the various layers of the epidermis.
Keratinocytes
The chief role of the keratinocytes is to produce keratin, a tough fibrous protein that gives the epidermis its protective properties. Tightly connected to one another by a large number of desmosomes, the keratinocytes arise in the deepest part of the epidermis from cells that undergo almost continuous mitosis, or cell division. As these cells are pushed toward the skin surface by the production of new cells beneath them, they manufacture the keratin that eventually fills their cytoplasm. The cytoplasm makes up the bulk of the cell and is located between the outer layer of the cell and the nucleus.
By the time the keratinocyte reaches the skin surface, they are dead, flat sacs completely filled with keratin. Millions of these dead cells rub off every day, giving us an entirely new epidermis every 30 to 45 days-the average time from "birth" of a keratinocyte to its final wearing away. In the normal healthy epidermis, the production of new cells balances the loss at the surface of the skin. Where the skin experiences friction, both cell production and keratin formation are accelerated.
NOTE: It is the most outer layer of the epidermis that most directly reacts with sunless solution to create a sunless tan. Specifically, DHA reacts quickly in the stratum corneum.
Layers of the Epidermis
In thick skin, which covers the palms of the hand and soles of the feet, the thickened epidermis consists of five layers, or strata. In thin skin, which covers the rest of the body, only four strata are present.
1. Stratum Basale (Basal Layer)
The stratum basale, the deepest layer of the epidermis, is firmly attached to the dermis along a wavy borderline. Also called the germinating layer, this stratum consists of a single row of cells representing the youngest keratinocytes. These cells divide rapidly. Occasional Merkel cells are seen among the keratinocytes. Each semi-circular Merkel cell is closely associated with a disc-like sensory nerve ending and may serve as a receptor for touch.
Between 5 percent and 10 percent of the cells in the stratum basale are melanocytes ("melanin cells"). These make the dark skin pigment melanin. The spider-shaped melanocytes have many branching processes that reach and touch all of the keratinocytes in the basal layer. Melanin is made in membrane-lined granules and then transferred through the cell processes to nearby keratinocytes. Consequently, the basale keratinocytes contain more melanin than do the melanocytes themselves. The melanin granules accumulate on the surface of each keratinocyte, forming a shield of pigment over the nucleus. In Caucasians, the melanin disappears a short distance above the basal layer, where it is digested by lysosomes in the keratinocytes. In black skinned individuals, no such digestion occurs, so melanin occupies keratinocytes throughout the epidermis. Although black skinned individuals have darker melanin and more pigment in each melanocyte, they do not have more melanocytes in their skin. In all but the darkest people, melanin builds up in response to ultraviolet rays, the response that we know as suntanning.
2. Stratum Spinosum (Spiny Layer)
The stratum spinosum is several cell layers thick. Mitosis, or cell production through division, occurs here, but less frequently than in the basal layer. Under microscopic imaging, the keratinocytes in this layer have many spine-like extensions.
Scattered among the keratinocytes of the stratum spinosum are Langerhans cells. These star-shaped cells are particulate ingesting microphages that help activate the immune system.
3. Stratum Granulosum (Granular Layer)
The thin stratum granulosum consists of three to five layers of flattened kearatinocytes. These keratinocytes contribute to the formation of keratin in the upper layers of the epidermis. This keratin contains a waterproofing material that is secreted into the areas between the cells and is the major factor for slowing water loss from the epidermis. Further more, the external wall of the cells thicken, so that they become more resistant to destruction. You might say that the keratinocytes are 'toughening up' to make the outer layers of the epidermis the strongest.
4. Stratum Lucida (Clear Layer)
The stratum lucida only occurs in thick skin, not thin skin. This layer consists of a few rows of flat dead keratinocytes. Electron microscopes show that its cells are identical to cells at the stratum corneum, the next layer.
5. Stratum Corneum (Horny Layer)
*DHA reacts specifically in the stratum corneum.
The most external part of the epidermis, the stratum corneum, is many cells thick. This layer is far thicker in thick skin than in thin skin. Its dead cells are flat sacs completely filled with keratin, because their nuclei and organelles were digested away by the lysosome enzymes upon cell death. Both the keratin and the thickened plasma membranes of the cells in the stratum corneum protect the skin against abrasion and penetration. It is amazing that a dead layer of cells can still perform such important functions.
The cells of the stratum corneum are referred to as horny cells. These cells are the dandruff shed from the scalp and the flakes that come off dry skin. The average person sheds about 40 pounds of these flakes in a lifetime. The common saying 'Beauty is only skin deep' is especially interesting in the light of the fact that nearly everything we see when we look at someone is dead!
Skin Color
Not including the sunless process, three pigments contribute to the skins natural tanned and untanned color: melanin, carotene, and hemoglobin. Carotene is a yellow to orange pigment derived from certain plant products, such as carrots. It tends to accumulate in the stratum corneum of the epidermis and in fat tissue of the hypodermis. Color derived from carotene is most obvious in the palms and soles, where the stratum corneum is thickest.
The pink tone of Caucasian skin reflects the red color of oxygenated hemoglobin in the capillaries of the dermis. Since Caucasian skin contains little melanin, the epidermis is nearly transparent in untanned individuals and allows blood's color to show through more predominantly.
Melanin, the most prominent pigment and is made from an amino acid called tyrosine. Melanin ranges in color from yellow to reddish to brown to black. Its production depends on an enzyme in melanocytes called tyrosinase. Freckles and pigmented moles are localized accumulations of melanin.
