THE SKIN AND THE STRUCTURES that develop from it-hair, glands, and nails-form the integumentary
system. The skin, or integument, is also known as the cutaneous membrane, one of the three types of epithelial membranes. The skin is a pliable, tough, waterproof, self-repairing barrier that separates deeper tissues and organs from the external environment. Although it often gets little respect, the skin is vital for maintaining homeostasis.
Functions of The Skin
- Resistance to trauma and infection. The skin suffers the most physical injuries to the body, but it resists and recovers from trauma better than other organs do. The epidermal cells are packed with the tough protein keratin and linked by strong desmosomes that give this epithelium its durability. Few infectious organisms can penetrate the intact skin. Bacteria and fungi colonize the surface, but their numbers are kept in check by its relative dryness and slight acidity (pH 4-6). This protective, acidic film is called the acid mantle.
- Other barrier functions. The skin is important as a barrier to water. It prevents the body from absorbing excess water when you are swimming or bathing, but even more importantly, it prevents the body from losing excess water. The epidermis is also a barrier to ultraviolet (UV) radiation, blocking much of this cancer-causing radiation from reaching deeper tissue layers; and it is a barrier to many potentially harmful chemicals. It is, however, permeable to several drugs and poisons.
- Vitamin D synthesis. The skin carries out the first step in the synthesis of vitamin D, which is needed for bone development and maintenance. The liver and kidneys complete the process.
- Sensation. The skin is our most extensive sense organ. It is equipped with a variety of nerve endings that react to heat, cold, touch, texture, pressure, vibration, and tissue injury. These sensory receptors are especially abundant on the face, palms, fingers, soles, nipples, and genitals. There are relatively few on the back and in skin overlying joints such as the knees and elbows.
- Thermoregulation. In response to chilling, the skin helps to retain heat. The dermis has nerve endings called thermoreceptors that transmit signals to the brain, and the brain sends signals back to the dermal blood vessels. Vasoconstriction, or narrowing of these blood vessels, reduces the flow of blood close to the skin surface and thus reduces heat loss. When one is overheated, vasodilation or widening of the dermal blood vessels increases cutaneous blood flow and increases heat loss. If this is not enough to restore normal temperature, the brain also triggers sweating.
- Nonverbal communication. The skin is an important means of nonverbal communication. Humans, like most other primates, have much more expressive faces than other mammals. Complex skeletal muscles insert on dermal collagen fibers and pull on the skin to create subtle and varied facial expressions. The general appearance of the skin, hair, and nails is also important to social acceptance and to a person’s self-image and emotional state—whether the ravages of adolescent acne, the presence of a birthmark or scar or just a “bad hair day.”
Structure of The Skin And Subcutaneous Tissue
The skin is thickest in areas subjected to wear and tear (abrasion), such as the soles of the feet, where it may be 6 mm in thickness. It is thinnest on the eyelids, eardrums, and external genitalia, where it averages about 0.5 mm in thickness.
The skin consists of two major layers: the epidermis and the dermis. The epidermis, the thinner superficial layer, is composed of an epithelium. The dermis, the thicker deep layer, is composed of connective tissue. The subcutaneous tissue, located deep in the dermis, is not part of the skin but is considered here because of its close association with the skin. Figure 5.1 shows the arrangement of the epidermis, dermis, and subcutaneous tissue as well as accessory organs of the skin. Table 5.1 summarizes these three tissue layers.
The epidermis is the thin, stratified outer skin layer extending downward to the subepidermal basement membrane. Keratinocytes are the principal cells of the epidermis and produce keratin. The cells replicate in the basal cell layer and migrate upward toward the skin surface. On the surface, they are sloughed off or lost by abrasion. Thus, the epidermis constantly regenerates itself, providing a tough keratinized barrier.
See Epidermis for more.
The dermis is a connective tissue layer that gives the skin most of its substance and structure. The dermis, the deep layer of the skin, can be divided into two regions: the superficial papillary layer and the deeper reticular layer.
The papillary layer of the dermis is adjacent to the epidermis and is composed of areolar connective tissue. The most notable features of this region are dermal papillae, nipple-like projections of the dermis that extend superficially into the epidermis. The dermal papillae contain numerous blood vessels that are used to supply nutrients to and remove wastes from the adjacent epidermal cells through diffusion. They contain touch receptors called the tactile (Meissner)
corpuscles. The epidermal ridges and grooves that produce the fingerprints and toe prints unique to each person are formed by the dermal papillae. Epidermal ridges provide a textured surface that increases traction on these gripping surfaces, in addition to the man-made application of personal identification. The dermal papillae and epidermal ridges also help to interlock the epidermis and dermis, so that they move as a unit.
The reticular layer of the dermis is deeper and thicker than the papillary layer, making up 70-80% of the total thickness of the dermis. The dense irregular connective tissue within this region possesses an abundance of collagen and elastic fibers. The collagen provides the dermis with strength and toughness, while the elastic fibers provide extensibility (ability to stretch) and elasticity (ability to return to its original shape). Numerous pressure, pain, and temperature receptors are located here. For example, the lamellated (Pacinian) corpsucles that are used to detect pressure are found within the deeper areas of the reticular layer. Free nerve endings responsible for touch, pain, and temperature are located throughout both the dermis and the epidermis. The blood vessels found within this region play an important role in temperature regulation.
Subcutaneous Tissue Or Hypodermis
The subcutaneous tissue also called the hypodermis, attaches the skin to deeper tissues and organs. The subcutaneous hypodermis layer is a specialized layer of connective tissue containing adipocytes. It consists primarily of areolar connective tissue and adipose tissue. It is the site used for subcutaneous injections and where white blood cells attack pathogens that have penetrated the skin. Subcutaneous adipose tissue absorbs the forces created by impact to the skin, which protects deeper structures, and serves as a storage site for fat. It insulates the body by conserving body heat and limits the penetration of external heat into the body. Blood vessels and nerves within the subcutaneous tissue give off branches that supply the dermis.