to many different environments; the frigid temperatures of Alaska for example, or the oppressive heat of the open deserts of Arizona. In hot environments, excretion of water and salt decreases as the body becomes acclimated to the external environment. The same applies to colder environments. Over time, the body adapts (to a certain degree) to the colder temperatures.
Skin is also involved in vitamin D synthesis. This occurs through exposure to ultraviolet rays or sunlight. Vitamin D is an essential nutrient required for metabolism of other compounds in the body that include but are not limited to phosphorus and calcium.
Skin and skin appendages such as our fingernails and hair not only provide us with a sense of individuality in regard to our appearance, but also serve specific functions. Body hair also protects the skin from a number of environmental dangers.
Our hair often provides a visible indication of health and wellness. For example, hypertrichosis (excessive growth of hair) can be an indication of changing hormone levels in the body. Another type of excessive hair growth in women called hirsutism may be caused by adrenal dysfunction, tumors of the ovaries or exogenous introduction of androgens into the body, such as the case of anabolic-androgenic steroid use. Of course, sometimes it’s also related to genetics.
Alopecia (hair loss) is not that unusual in aging men, but abnormal and unexpected hair loss may be the cause of ill health, stress, substance abuse, thyroid issues, or hormonal imbalances.
Development of skin lesions, the surface appearance of fingernails, and changes in skin color or texture are also indications of possible illness or nutritional deficiencies. Some of the most common skin problems and threats to skin health and wellness will be discussed later in this chapter.
The skin also serves as a source of lubrication and cushioning for the body. Skin moves; it’s pliable and can stretch, such as during a woman’s progression through pregnancy, and then revert to its normal shape. It accommodates growth of muscle and grows with us as we age from infancy to old age.
■Skin Structure
The skin is composed of two layers: the epidermis and the dermis. The epidermis defines the outer layer of skin.
The dermis is considered the second layer, inside which belongs a unique network or framework of connective tissues. These two layers (epidermis and dermis) are generally referred to as “our skin”.
However, a third layer, known as subcutaneous tissue (also called the hypodermis) is not always considered part of the skin per se, depending on source. The third layer of skin contains fat deposits that are modified within the connective tissues. Some texts refer to the hypodermis/subcutaneous layer as superficial facia. This is due to its location being “superficial” to connective tissues that wrap skeletal muscle (facia) but consists of adipose tissue.4 These fat deposits are known as adipose tissue.
Four types of cells of the epidermis
The outermost layer of the skin – the epidermis – is constructed of keratinocytes that are able to regenerate continuously. Keratinocytes are also known as squamous cells. The keratinocytes produce keratin. Keratin is defined as a fiber-like protein that protects the uppermost surface of the skin. Keratin cells shed on a regular basis, typically every month, but are consistently replaced by brand new keratinocytes.
Another structure found in the outer layer of the skin are melanocytes. Melanocytes are responsible for the formation of pigment granules (melanosomes), which give our skin, eyes, and hair color. Melanocytes are also vital in protecting the skin from ultraviolet (UV) light.
Another component of the epidermis is the presence of Langerhans cells, also known as dendritic cells. They’re named after German pathologist and biologist Paul Langerhans (1847–1888).5 Langerhans is also recognized for the discovery of the Islets of Langerhans (pancreas). While more typically associated with bone marrow, Langerhans cells in the skin aid immune system function, most specifically in regard to antigens. These cells assimilate antigens in the peripheral tissues of the body, where they are then transported to the nearest lymph nodes to fight presence of bacteria. They’re also involved in antimicrobial immunity.6
Merkel cells are found in the deepest layer of the epidermis and typically behave as sensory receptors. Merkel cells were first identified by Friedrich Sigmund Merkel in 1875.7 Merkel cells, also known as Merkel-Ranvier cells and tactile cells, are ovoid in shape and located in the basal layer of the epidermis. They are most commonly known as “touch cells” and are found in close proximity to nerve tissues. These cells are thought to be unique and sensitive receptor cells that respond to pressure. The greatest number of Merkel cells are found in the mouth and mucosal areas of the body that are involved in tactile perception. Merkel cells can be found more often in skin that is exposed to sunlight than covered skin.8
Five layers of the epidermis
The epidermis itself is comprised of five layers, each independent on the stage of reproductive activity of the specific keratinocyte. From the outer to the inner:
•Stratum corneum (horny layer)
•Stratum lucidum (clear layer)
•Stratum granulosum (granular layer)
•Stratum spinosum (prickly layer)
•Stratum basale (basal layer)
Figure 3-2 Epidermal layers.
Stratum corneum – the outermost layer. This layer of skin is formed by flat skin cells that contain keratin, a fiber-like protein material as mentioned earlier. This layer of the epidermis encompasses most of its thickness, approximately 20 to 30 cell layers thick.9 This outermost layer of skin is actually constructed of dead skin cells that have made their way to the surface from inner layers of skin. For this reason, the cells, which overlay each other much like roofing shingles, have a horny appearance, hence their name or nickname as horny cells. Dead skin cells are constantly replaced by new cells that push the old, dead skin cells outward or upward from deeper layers found in the epidermis. We mostly refer to these dead skin cells as dry skin, dander, or dandruff.
The skin can shed as many as 50,000 dead cells every minute. The average person will shed approximately 40 pounds of dead skin flakes in their lifetime.10
The stratum corneum also contains granules of pigment known as melanin. Melanin aids in the protection of the body from over-exposure to ultraviolet light. The amount of melanin found in the epidermis differs between races as well as geographical regions.
Stratum lucidum – This layer of skin is known as the clear layer, which under microscopic evaluation appears as a translucent layer or band of skin that serves as a boundary of sorts between the stratum corneum and the next layer of skin, the stratum granulosum. It’s much narrower than that of stratum corneum, maybe three cells thick.
Stratum granulosum – Known as the granular layer, stratum granulosum is also relatively thin and averages one to five layers in cell thickness. Within this skin layer, keratinocytes are constantly morphing due to a process known as keratinization. During this process, the cells change shape; they flatten while their organelles and nuclei disintegrate. During this process, they collect
