Chapter 14
Skin Deep: Skin, Glands,
rid the body of waste products. It also guards deeper tissues of the body against excessive loss of water, salts, and heat. Secretions from the skin are somewhat acidic in nature and contribute to its ability to fight off bacterial invasion.
The skin has the large responsibility of keeping you cool. The many different tissues in the skin help maintain the body temperature. Nerve fibers coordi- nate this thermoregulation by carrying messages to the skin from heat centers in the brain that are sensitive to changes in the body temperature. Nerve impulses cause blood vessels to dilate to bring blood to the surface where the heat can dissipate and cause sweat glands to produce the watery secre- tion that evaporates, thereby acting as your cooling system.
The skin is the most important player in the integumentary system, and is made up of three layers. The outer layer is the epidermis, a thin, cellular membrane layer. The second layer is the dermis, dense fibrous connective tissue. The third layer is the subcutaneous tissue, fat-containing tissue that joins the skin to underlying muscle.
The structure of skin varies throughout the body. It is stretchable and tough and has different thicknesses. It is thick on the palms of the hands and soles of the feet, but thin on the eyelids. The skin is initially firm and elastic, but with age, becomes wrinkled, drier, and saggy, especially around the eyes, mouth, and neck. The skin covering palms of the hand and soles of the feet is different from that covering the rest of the body. The skin on fingers and toes has patterns of ridges that never change and are unique for each individual, and, as you know from crime dramas, provide a basis for the use of finger- prints as a means of positive identification.
Epidermis
The epidermis is the outer, totally cellular layer of skin. It is composed of epi- thelium. Epithelium covers both internal and external surfaces of the body.
The epidermis has no blood vessels, lymphatic vessels, connective tissue, cartilage, or fat. It depends on the deeper dermis, or corium, layer and its network of capillaries for nourishment. Oxygen and nutrients from the capil- laries in the dermis pass through tissue fluid, supplying nourishment to the deeper levels of the epidermis.
The deepest layer of the epidermis is called the basal layer. Cells in the basal layer are always growing and multiplying. As the basal layer cells divide, they are pushed upwards and away from the blood supply of the dermis layer by a steady stream of younger cells. These cells shrink, lose their nuclei, die, and
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Chapter 14: Skin Deep: Skin, Glands, Nails, and Hair
become filled with a hard protein called keratin. They are then called horny cells, reflecting their composition of keratin. Within a 3–4 week period after living as a basal cell in the deepest part of the epidermis, the horny kerati- nized cell is sloughed off from the surface of the skin. The epidermis, then, is constantly renewing itself.
Cells die at the same rate at which they are born. The basal layer of the epidermis contains cells called melanocytes. Melanocytes contain a black pigment called melanin. The amount of melanin accounts for the color differ- ences in skin. Darker skin possesses more active melanocytes, not a greater number of melanocytes. Melanin in the epidermis is vital for protection against harmful ultraviolet radiation, which can manifest as skin cancers.
Individuals, who through a flaw in their chemical make-up are incapable of forming melanin, are called albino, meaning white. Their skin and hair are white and their eyes are red (because of the absence of pigment, the tiny blood vessels are visible in the iris).
Dermis
The dermis, the second layer, below the epidermis, is also called the corium.
The dermis differs from the epidermis in that it is living tissue composed of blood, lymph vessels, and nerve fibers, as well as the accessory organs of the skin. These accessories include the hair follicles, sweat glands, and oil glands.
To support this system of nerves, vessels, and glands, the dermis contains connective tissue cells and fibers. The dermis is composed of different types of connective tissue cells: fibroblasts, histiocytes, and mast cells. Fibroblast cells act to repair an injury to the skin. Histiocytes protect the body by sur- rounding foreign materials. Mast cells contain histamine, a substance released in allergies that causes itching.
Fibers in the dermis are composed of collagen. Collagen, meaning “glue,” is a fibrous protein material found in bone, cartilage, tendons, and ligaments as well as the skin. It is tough, but also flexible. In an infant, collagen is loose and delicate but it becomes harder as the body ages. Collagen fibers support and protect the blood and nerve networks that pass through the dermis.
Hair shafts in the dermis have bundles of involuntary muscle called arrector pili attached to hair follicles. When you are frightened or cold, these muscles contract, the hair stands up, and “goosebumps” appear on the skin.
Subcutaneous layer
The subcutaneous layer of the skin is made up of connective tissue that spe- cializes in the formation of fat. Lipocytes, plentiful in the subcutaneous layer, manufacture and store large amounts of fat. Areas of the body vary as far as fat deposition is concerned. This layer of skin is important in protecting deeper tissues of the body and also acts as a heat insulator. The subcutane- ous layer connects the dermis to the muscles and organs below it. Its fat tissue insulates inner structures from temperature extremes.
Glands, Both Sebaceous and Sudoriferous
The skin has two types of glands that, as accessory organs, produce impor- tant secretions. These glands under the skin’s surface are called the seba- ceous (oil) glands and the sudoriferous (sweat) glands.
The sebaceous glands produce an oily secretion called sebum, whereas the sudoriferous glands produce a watery secretion called sweat. Sebum and sweat are carried to the outer edges of the skin by ducts and excreted through openings in the skin called pores. Sebum helps lubricate the skin.
Sebaceous glands are closely associated with hair follicles, and their ducts open into the hair follicle through which the sebum is released.
Sebaceous glands are influenced by sex hormones. This causes them to be overactive at puberty and underactive in old age. This explains the excess oil production of the skin at puberty and gradual drying of the skin as we age.