Skin

Skin

The skin is also known as the integument. This is where the name integumentary system is derived. The skin varies in thickness between 1.5 and 4 mm, depending on which part of the body it covers. The general functions of the skin are:

■■ Protection of the underlying organs and tissues against abrasion, chemicals, fluid loss, and impact

■■ Detection of stimuli (pain, pressure, temperature, and touch) and relay of this information to the nervous system

■■ Maintenance of normal body temperature, as needed, via insulation or evaporative cooling

■■ Melanin production, to protect underlying tissue from ultraviolet (UV) radiation.

■■ Keratin production, to serve as a water repellant and to protect against abrasion

■■ Vitamin D₃ synthesis; this steroid is converted to calcitriol, a hormone vital for normal calcium metabolism

■■ Lipid storage, in the adipocytes of the dermis and in the adipose tissue of the subcutaneous layer

■■ Excretion of organic wastes, salts, and water via the integumentary glands

The two main layers of skin are the epidermis and dermis. The epidermis, the outer layer, is made up of keratinized stratified squamous epithelium (FIGURE 6-1). It is also called the superficial epithe-lium. It has four primary cell types and four or five layers, depending on body location (four layers in most body areas and five layers on the palms, fin-gertips, and soles of the feet). Unlike the dermis, the epidermis is not vascularized. Nutrients must dif-fuse through dermal blood vessels and tissue fluid to reach the epidermis. The dermis, the inner layer, is much thicker than the epidermis and consists of papillary and reticular regions. The papillary region contains fine elastic fibers and dermal papillae. The reticular region is composed of connective tissue containing collagen, elastic fibers, fat tissue, hair fol-licles, nerves, sebaceous (oil) glands, and the ducts of sweat glands. The epidermis is connected to the dermis by a basement membrane.

Loose connective tissue below the dermis binds the skin to the organs underneath. This tissue, which is pre-dominantly adipose (fatty), forms the subcutaneous layer, also known as the hypodermis or superficial fascia. It is deep below the dermis and not actually part of the skin. This adipose tissue insulates the body, conserving inner heat and helping to keep excessive heat from out-side the body from entering. The major blood vessels that supply the skin and ­adipose tissue are ­containe within the subcutaneous layer. The hypodermis is loose enough that the skin slides with ease over its underlying structures. It also acts as a shock absorber and becomes much thicker when weight is gained. This tissue first accumulates in the anterior abdomen in males and in the thighs and breasts in females. The hypodermis lends its name to the term “hypodermic,” which is where sub-cutaneous injections are made via hypodermic needles.

Epidermis

The epidermis is the outermost layer of the skin and is composed of stratified squamous epithelium. The epidermis does not contain blood vessels, although its deepest layer, the stratum basale, receives blood via the dermal blood vessels. Epidermal cells require diffusion of oxygen and nutrients from the capillaries within the dermis. Cells that have a higher metabolic demand are located closer to the basement mem-brane. Cells in this layer of the epidermis divide and grow, moving toward the skin surface and away from the dermis below. As they move upward, they receive fewer nutrients and eventually die. Older cells are called keratinocytes, which harden with age in the process known as keratinization. Keratin protein fills the cytoplasm of these skin cells, which collectively form a layer called the stratum corneum. Dead skin cells in this layer are eventually shed from the body.

Epidermis Layers

There are basically five layers of the epidermis: ­stratum germinativum (stratum basale), stratum spi-nosum, stratum granulosum, stratum lucidum, and stratum corneum. Most of the body surface is covered by thin skin, consisting of four layers of keratinocytes that total only 0.08 mm in thickness. Areas of thick skin (the palms and soles) contain a fifth layer, the stratum lucidum, and the stratum corneum in these areas is much thicker. Therefore, on the palms and soles, the epidermal layers total about 0.5 mm in thickness. The five individual layers are explained as:

■■ The stratum germinativum (“germinative” or “basal layer”) is the innermost epidermal layer and is also known as the stratum basale (basal layer). It is interlocked with the underlying ­dermis via hemidesmosomes, which are tiny pin-like structures. This layer forms the epidermal ridges, extending into the dermis, which are adjacent to dermal projections (dermal papillae). The attach-ment of the stratum basale to the dermis is along a wavy borderline. Epidermal ridges are import-ant because the strength of the attachment of the layer is proportional to the surface area of the basal lamina. Ridge shapes are genetically deter-mined, and the pattern of epidermal ridges does not change during the life span of an individual (FIGURE 6- 2).

The ridge patterns on the tip of each finger are instrumental in the forming of finger-prints. Each person’s fingerprints are unique, including those of identical twins. As a result, fingerprints are commonly used in criminal cases to identify individuals. Large basal (germinative) cells dominate the stratum germinativum. Stem cells are usually in single rows, with divisions that replace superficial keratinocytes that are lost or shed on the epithelial surface. When a stem cell divides into daughter cells, they are pushed from the stratum germinativum upward into the next layer, the stratum spinosum. There are many mitotic cell nuclei in this layer, reflecting rapid cell division. Approximately, 10% to 25% of these cells are melanocytes that reach into the stratum spinosum. The ridges on the palms and soles also increase the surface area of the skin and help us to grip objects due to the increased friction. In areas where the skin surface does not have hair, there are specialized epithelial cells known as Merkel or tactile cells. They are sensitive to touch, releasing chemicals that stimulate sensory nerve endings when they are compressed. The brownish color of the skin comes from melanocytes, distributed in the stratum germinativum. They have cell processes that extend into the more superficial layers.

■■ The stratum spinosum (“spinous” or “prickle cell layer”) is made up of 8–10 layers of keratinocytes that are bound together by desmosomes. It contains cells that look like tiny pin cushions, because of exposure to chemicals that caused the keratinocyte cytoplasm to shrink slightly. However, the desmosomes and elements of the cytoskeleton remained intact. Some entering cells from the stratum basale continue dividing, which increases the thickness of the epithelium. Additionally, this layer contains Langerhans cells, also known as dendritic cells, which stimulate immune defenses against microorganisms and superficial skin cancers.

■■ The stratum granulosum (“granular layer”) is the third layer and consists of only three to six layers of keratinocytes. Cells in this layer have mostly stopped dividing and begin to make the proteins keratin and keratohyalin. Keratin is tough and fibrous, making up hairs and nails. Developing keratin fibers become flatter and thinner, as their membranes thicken and lose permeability. Keratohyalin forms cytoplasmic granules that dehydrate cells and aggregate and cross-link keratin fibers. The cells die as the nuclei and other organelles disintegrate. Continued dehydration causes this layer to become extremely interlocked. Nutrients are brought via capillaries in the dermis. However, above this layer, the cells are too distant from the dermal capillaries and glycol lipids coating them keep nutrients from being supplied, hence their normal death.The stratum lucidum (“clear layer”) is the fourth region, which is only found on the palms of the hands and soles of the feet, with a glassy or clear appearance. Therefore, the overall skin of the palms and soles is thicker than on other parts of the body. In this layer, the cells are flattened and densely packed, have few organelles, and are filled with keratin. The stratum lucidum is microscopically viewed only in thick skin, appearing as a thin translucent band above the stratum granulosum. It consists only of two or three rows of flat and dead keratinocytes that have indistinct boundaries.

■■ The stratum corneum (“horny layer”) makes up the surface of the skin and contains 15–30 layers of keratinized cells that are protective and filled with keratin, they are the most numerous epidermal cells. The process of keratinization is also known as cornification. It occurs on all exposed body surfaces, except the anterior eye surfaces.The dead cells of the stratum corneum are tightly interconnected by desmosomes. Because of this interconnection, keratinized cells of this layer are shed in large sheets rather than individually. Cells move from the stratum germinativum to the stratum corneum in 7–30 days, remaining in the stratum corneum for about two weeks before being washed away or naturally shed. The dryness­ of the stratum corneum reduces the amount of potential microbial growth, and this layer is coated with lipid secretions from the sebaceous glands.This layer is water resistant but not waterproof. Water from the interstitial fluids eventually penetrates to the surface. About 500 mL of water is lost from this layer via evaporation every day in a process known as ­insensible perspiration. This differs from sensible perspiration, which is produced by merocrine sweat glands. The rate of insensible perspiration may be increased when the epidermis is damaged and sometimes can be dangerous, such as when severe burns excessively damage the epidermis. Oppositely, being immersed in fresh (hypotonic) water for a long time causes the epidermal cells to swell up to four times their normal volume. This is most noticeable on the palms and soles. Immersion in ocean (hypertonic) watercauses water to leave the epidermal cells, eventually resulting in dehydration. FIGURE 6-3 shows how healthy skin balances the production of ­epidermal cells with the loss of dead cells. The stratum corneum makes up nearly three-fourths of the total epidermal thickness. Keratin ­protects it against abrasion and ­glycolipids cause its water resistance. ­Approximately, 50,000 dead cells are shed from this layer every minute. In an average lifetime, you will lose 40 pounds of these dead skin cells.

Epidermal Cells

The epidermis protects the underlying tissues against the effects of harmful chemicals, excess water loss, mechanical injury, and pathogenic microorganisms. Layers of pigment in the epidermis help protect­ both epidermal and dermal tissues. Melanin is a brown, yellow-brown, or black pigment produced by spider-shaped melanocytes located in the stratum germinativum, either between or deeply rooted in the epithelial cells (FIGURE 6-4). It is made of tyrosine amino acids and has two forms that range in color (from red-yellow to brown-black). Synthesis of mela-nin is based on an enzyme called tyrosinase. Melanin accumulates in granules that are bound to membranes called melanosomes. Lysosomes eventually break down melanosomes, meaning that melanin pigment is found only in the deeper layers of the epidermis, regardless of race, all human beings have about the same number of melanocytes. Motor proteins move down actin filaments to reach the ends of each mela-nocyte’s processes. Then, they move to nearby kerat-inocytes, accumulating on the superficial side of the keratinocyte nucleus. Normal exposure of the skin to sunlight causes the keratinocytes to secrete chemi-cals that stimulate the melanocytes. Melanin absorbs UV radiation from sunlight, protecting the epidermis and dermis from its harmful effects. It builds up from sun exposure, absorbing rays, dissipating this energy as heat, and protecting DNA of viable skin cells from UV radiation. However, sunlight contains extremely significant amounts of UV radiation. Although small amounts of UV radiation are beneficial because they stimulate the epidermal production of a compound required for calcium ion homeostasis (the produc-tion of vitamin D), larger amounts damage DNA. This causes mutations, promoting the development of cancer. UV radiation can also produce burns. When severe, they can damage the epidermis and the dermis.

Keratinocytes produce keratin, which is the fibrous protein that aids the epidermis in protecting the body. Most epidermal cells are keratinocytes, which arise in the stratum basale. Upon reaching the skin surface, they are already dead. At this time, they have a scale-like appearance and are basically plasma membranes filled with keratin. Every day, millions of dead kera-tinocytes rub off. Therefore, the epidermis is totally replaced every 25–45 days.

Star-shaped dendritic cells or Langerhans cells from the bone marrow eventually move to the epider-mis. They consume foreign substances and play a key role in activating the immune system. Tactile cells or Merkel cells are located at the epidermal–dermal junction. They have spiked shapes and combine with disc-like sensory nerve endings to form tactile discs, which are receptors for the sense of touch.

Differences in skin color are based on the amount of melanin produced and how it is distributed through-out the skin. Skin color is based on a person’s genetics, which regulates the amount of melanin produced by the melanocytes. Other factors that affect skin color include sunlight, UV light, and X-rays. Dermal blood vessel also affects the color of the skin. Well-oxygenated blood makes light-skinned people appear pinker, whereas poorly oxygenated blood makes them appear bluer, as in the condition known as cyanosis. Diet also affects skin color, as do biochemical imbalances. For example, the buildup of the substance known as bili-rubin makes the skin appear yellowish, as in the con-dition called jaundice.

Also contributing to skin color are the pigments carotene and hemoglobin. Carotene is a yellow to orange pigment that primarily accumulates in the stratum corneum and the hypodermic fatty tissue. It is also found in plant products such as carrots and other orange-colored vegetables. The color of carotene is most easily seen in the palms and soles, especially in lighter skinned individuals, where the stratum cor-neum is present in thicker cellular levels. It intensi-fies in the body when large amounts of foods rich in carotene are consumed. Carotene can be converted to vitamin A (essential for normal vision), which aids in the health of the epidermis. Carotene, along with variations in melanin, contributes to the skin color of people from certain Asian countries. Hemoglobin is the red pigment inside red blood cells. As it circulates throughout the dermal capillaries, it gives off a pink color that is easily seen in people with fairer skin.

1. List and discuss the five layers of the epidermis.

2. Explain melanin, melanocytes, and melanosomes.

3.Which layer of the epidermis is vascular?

4. What are the functions of the subcutaneous layer