VI. The Integumentary System
The integumentary system consists of the skin and accessory structures such as hair, nails, and glands. In terms of area, the skin is the largest organ in the body. It is continuous with the mucous membranes which line the bodies openings, but is different in structure from these membranes.
l. Protection - The skin serves as a mechanical barrier between the body and the environment. It protects against water loss due to evaporation and invasion of microorganisms.
2. Temperature regulation - The sweat glands of the skin produce perspiration and the evaporation of this perspiration from the surface of the skin is a major means of temperature regulation.
3. Excretion - Small quantities of waste products are excreted with the perspiration.
4. Sensation - The skin is very rich in nerves and sense organs which provide information about the external environment.
5. Vitamin D production - Ultraviolet light converts a cholesterol derivative into vitamin D.
6. Defense against infection - The skin play a significant role in the bodies's defensive response to microorganisms. This will be discussed later in the course.
B. Anatomy - The skin consists of two layers, and outer epidermis and inner dermis. The skin is connected to the underlying musculature by a layer of loose connective tissue known as the hypodermis (superficial fascia). The hypodermis is rich in adipose cells and represents an area in which obesity occurs. In women this is usually most obvious in the hip and breasts while in men is takes on the appearance of a "spare tire."
l. Epidermis - This layer consists of five different sublayers. Cells originate by mitosis in the inner most sublayer and gradually move through the remaining four upper layers. Each sublayer represents a part of the life history of an epidermal cell. From the outside to the inside, these epidermal layers are as follows.
a. Stratum corneum - This is composed of 25 to 30 flat, densely packed, rows of dead cells. The cytoplasm of each cell has been replaced by a fibrous protein, keratin. It is keratin that gives skin its waterproofing and makes it a barrier to microorganisms. These cells are constantly being shed and replaced. These cells are a major component of the ring around your bathtub.
b. Stratum lucidum - A clear layer in which the cells have lost all of their cytoplasmic inclusions except for keratin fibrils and eleidin. Eleidin is gradually converted into keratin and is in turn derived from a precursor molecule known as keratohyalin.
c. Stratum granulosum - This layer is made up of two or three rows of flattened cells which contain keratohyalin, the starting point for keratin synthesis. These cells are in the process of dying.
d. Stratum spinosum - There are 8 to l0 rows of cells that present a prickly appearance in microscopic preparations of skin. The prickly appearance is responsible for the layer's name.
e. Stratum basale - This innermost layer is but a single layer of columnar cells which gives rise to all of the others by mitosis. The stratum spinosum and basale are collectively referred to as the stratum germinativum.
2. Dermis - This layer contains elastic, collagenous, and reticular fibers. It also is rich in nerves and blood vessels. It is the dermis that nourishes the epidermis which is avascular. The dermis consists of two irregular layers.
a. Papillary layer - This fits against the basalar layer of the epidermis. It derives its name from projections (papilla) into the epidermis. These projections from ridges on the palms and soles of the feet and are therefore responsible for finger prints.
b. Reticular layer - This is a deep layer and consists largely of bundles of collagenous fibers.
3. Skin thickness - Most skin is thin (1.5 mm) but on the palms soles the skin may be thick (4 mm). This is due to increased thickness of the epidermis. All five layers of the epidermis can usually only be found on thick skin.
4. Epidermal derivatives - These are skin structures that develop from the same embryonic tissue as the epidermis. They include the following structures.
a. Skin glands - There are two major types.
(l) Sweat glands (sudoriferous) - These glands function in the production of perspiration. There are two types.
(a) Eccrine - These are merocrine type glands and are distributed over most of the body.
(2) Sebaceous (oil) glands - These glands develop from the epidermis, and empty into, hair follicles. They produce a lipid rich substance called sebum which prevents the skin and hair from drying out. Sebum also contains antimicrobial substances.
b. Hair - This is distributed over most parts of the body except for the palms of the hands, soles of the feet, nipples, and certain regions of the external genitalia.
(l) Structure - Each hair develops in a follicle by means of mitosis of cells located at the bottom of the follicle.
(a) The follicle extends from the surface of the epidermis down into the dermis.
(b) The follicle is bounded by an external root sheath. This develops from a down growth of the epidermis.
(c) An internal root sheath lines the follicle from its bottom to the level of the sebaceous glands.
(d) Papilla containing blood vessels that nourish the follicle cells protrude into the bottom of the follicle from the dermis.
(e) Each hair consists of a root and a shaft. The root is the mitotically active region immediately over the papilla. The shaft consists of dead, keratinized cells. A cross section of a shaft shows the following regions.
/1/ Medulla - This is the central region and is composed of loosely packed cells.
/2/ Cortex - This region surrounds the medulla and is composed of more densely packed cells.
/3/ Cuticle - This is the bounding region and is composed of extremely hard keratinized cells.
(f) Arrector pili - This is smooth muscle which is attached to the follicle. When they contract, the follicle, which is oriented obliquely, is pulled upward and the hair stands up. It is contraction of these muscles that is responsible for "goose bumps."
(g) Sebaceous glands - Each follicle has a gland which secretes sebum into the follicle.
(2) Hair color - This is due to the presence of the pigment melanin in the cortex of the shaft. Different hair color is due to differences in the types and quantities of melanin. White hair is due to a loss of pigment and its replacement by air.
c. Nails - These are formed by heavy cornification of the stratum corneum and lucidum at the ends of the digits. The major components of the nails are as follows.
(l) Nail bed - This consists of the stratum germinativum upon which the nail lies.
(2) Lunula - This is the whitish area at the proximal end of the nail. It is formed by a thickening of the germinativum and is termed the nail matrix. This is where nail growth occurs.
(3) Eponychium (cuticle) - A fold of the epidermis that extends over the proximal end of the nail.
(4) Hyponychium - Thickened stratum corneum that lies beneath the free edge of the nail.
(5) Nail color - Nails tend to be pinkish which is due to the flow of blood showing through the nail. Pressing the nail blocks the flow and causes the nail to turn white. This fact can be used to detect poor circulation. If the color does not return immediately after the removal of the pressure it is indicative of reduced blood flow.
C. Skin color - This is due to pigment in the epidermis and the circulation in the dermis.
l. Melanin - This is a dark pigment produced by cells termed melanocytes. After it is produced, melanin moves into the other epidermal cells. Dark skin has a lot of melanin spread through most of the epidermal layers.
3. Circulation - The blood is what gives the pinkish color to light colored skin. Low levels of melanin permit the color of the blood to show through.
4. Exposure of the skin to UV light promotes an increase in the amount of melanin and a darker skin, a condition known as tanning. It must be remembered that increasing the amount of melanin is dependent upon have the appropriate genes for melanin production. Albinos (genetic lack of melanin) cannot tan!