Integumentary System


I. Anatomy & Physiology

The integumentary system (skin) has been called a membrane and an organ but, it is generally considered a system because it has organs that work together as a system. It is sometimes considered an organ because it contains several types of tissues and a membrane and it covers the body. 

The skin is the largest organ of the body and includes associated organs and derivatives of the skin such as hair, nails, glands, and specialized nerve endings. One square centimeter of skin contains approximately 70 cm of blood vessels, over 100 glands, and well over 200 sensory receptors!  For this reason, it is virtually impossible to find an area of the skin that is insensitive to sensations of touch, pressure, heat, cold, pain, or vibration.

Skin serves the primary function of protection. It also cushions internal organs and serves as the first line of defense from infection and injury. It is waterproof, stretchable, and capable of repairing itself. 

Three main layers make up the skin:

1. Epidermis: the outer layer of skin. It is made up of 5-6 layers with no blood vessels. The layers (outer to innermost) are the stratum corneumstratum granulosumstratum spinosum, and the stratum germinativum which replaces the outer layers with new cells. The stratum lucidum is found primarily in the palms of the hands and the soles of the feet. It adds additional thickness to the skin where greater friction typically occurs.

2. Dermis: also called corium or "true skin". The dermis contains elastic connective tissue, blood vessels, involuntary muscle, sweat and oil glands, and hair follicles. The dermis consists of two principal portions. The more superficial portion consists of areolar connective tissue with many fine elastic fibers.  Papillae cover the top of the dermis which fit into ridges on the stratum germinativum. These ridges form lines (fingerprints or footprints) on the skin which is unique to each individual. This region is called the papillary region. The deeper portion of the dermis is the reticular region which consists of dense, irregular connective tissue intertwined with course elastic fibers. This is the region from which the hair and glands of the skin arise.

3. Hypodermis: also called subcutaneous fascia. It is the innermost layer and is made up of loose, connective tissues like areolar and adipose (fatty) tissue. About half the body's supply of adipose tissue is found in the subcutaneous fat. It is an excellent insulator and shock absorber, and anchors the skin to the organs below.

The integumentary system has two main types of glands: sudoriferous (sweat) glands and sebaceous (oil) glands. 

The principal types of sudoriferous glands are eccrine, apocrine, ceruminous, and mammary. Eccrine are the most common and function primarily in thermoregulation by the production of sweat. The sweat (perspiration) eliminated by these glands contain water, salts, and some body wastes. Apocrine are confined to the anogenital and axillary regions of the body and produce sweat that contains fatty substances and proteins and may be analogous to sexual scent glands of animals. Ceruminous are modified apocrine glands found in the lining of the external acoustic meatus that produce cerumen (ear wax). Mammary are specialized sweat glands found in females that secrete milk.  

The sebaceous glands produce a lipid based secretion called sebum, an oil that keeps the skin and hair from becoming dry and brittle ans serves as a bactericide. When an oil gland becomes plugged, the accumulation of dirt and oil results in a blackhead or pimple.

Hair consists of a root, which grows in a hollow tube called a follicle, and a shaft. Hair helps protect the body. 

Nails are made of tightly packed, keratinized cells. The nail body is the visible portion with a free edge which may extend past the distal end of the digit. The nail root extends into a fold of skin and is continuous with the stratum basale of the epidermis. A thickened area of the nail bed, called the nail matrix is responsible for growth. The region of the nail body that overlies the nail matrix appears as a white crescent called the lanula. Nails protect the end of the digits and serve as "tools" for the manipulation of small objects. Skin color is produced by three pigments: melanin, carotene, and hemoglobin. Melanin is produced in the melanocytes and dispersed to the keratinocytes. The difference in skin color due to melanin is typically not due to a difference in the number of melanocytes, but in the amount of pigment that they produce. Increased exposure to UV light tends to increase their activity and darken the skin. Carotene is a precursor of vitamin A and tends to accumulate in the cells of the stratum corneum. The amount of dilation of the capillary nets in the dermal papillae adds to the hue of the skin by exposing greater or lesser blood flow in the immediate area of the skin. An albino is a person unable to synthesize melanin.

The skin produces thermal regulation in two principal ways. The perspiration secreted by sweat glands evaporates to lower the epidermal temperature. Dilation or constriction of the blood flow to the capillary nets of the papillary layer will alter the amount of heat loss from the blood to the external environment.

The protective functions of the skin include the chemical, physical, and biological barriers. The chemical secretion of sebum serves as a bactericide and the production of melanin serves to absorb UV radiation. The physical overlapping, layering structure of stratified squamous epithelium, like the shingles of a roof, reduce the likelihood of infection while effectively blocking the diffusion of water and water-soluble substances. In addition, these keratinized cells are resistant to the wear and tear from the environment. The Langerhans cells and macrophages found in the epidermis and dermis are active elements in the biological immune system. 

Deep wound healing begins with the inflammatory phase in which a blood clot begins to form. Vasodilation and increased permeability deliver white blood cells such as macrophages and neutrophils that serve to phagocytize invading microbes. In addition, mesenchymal cells are delivered which develop into fibroblasts. In the migratory phase, the clot becomes a scab as epithelial cells migrate beneath the scab to bridge the wound. Fibroblasts begin to synthesize the scar tissue (collagen fibers), and the wound fills during the granulation phase. The proliferation phase is characterized by growth of epithelium and growth of blood vessels. The healing process ends with the maturation phase in which the scab is sloughed off as the epithelium is restored to near normal organization.

Review Questions:

1. Explain differences in skin color.

2.  What is the function of dermal papillae?

3. What are the three barriers produced by the skin that provide protection and describe them.

4. What is the excretory function of the integument?

5. Describe the stages involved in the healing of a deep skin wound.