Introduction The integumentary system is made of the skin and its appendages. The appendages or modifications of the skin are hair, nails, sebaceous, ceruminous and sweat glands. The word “integument” means a covering, and the skin of an average adult covers well over 3,000 square inches of surface area of the body. The skin weighs about 6 pounds (this is nearly twice the weight of the brain or the liver). It receives approximately one- third of all the blood circulating through the body. It is flexible yet rugged and under normal conditions can repair and generate itself. Our skin is entirely waterproof. It protects us from the ultraviolet rays of the sun through special pigment- producing cells. It is an effective barrier to most harmful chemicals, keeping them from entering our internal environment. It participates in dissipation of water through sweating and helps regulate our body temperature. Ø The Layers of the Skin Our skin consists of two main layers. The epidermis is a layer of epithelial tissue that can further be divided into sublayers. It is found on top of the second layer of the skin called the dermis. This is a layer of dense connective tissue that connects the skin to tissues below it, like fat and muscle. Beneath the dermis is the subcutaneous layer. · The Epidermis The outermost or epidermal layer of the skin is composed of stratified, squamous, keratinized (no nucleus) epithelial cells. The cells are held together by highly convoluted, interlocking cellular links called desmosomes. These desmosomes are responsible for the unique flexibility, entirety and whole continuous structure of the skin. The epidermis is thickest where it receives the most abrasion and weight- on the palms of the hands and the soles of the feet. It is much thinner over the ventral surface of the trunk. The epidermis, which is not vascularized, rests on a basement membrane. The lowermost cells on this membrane divide by mitosis, so new cells push older cells up toward the surface. As they move up, they change shape and chemical composition because they lose most of their water and eventually die. This process is called keratinization because the cells become filled with keratin, a protein material. These dead, outermost cells are constantly being shed. This outermost layers form an effective barrier to substances that would penetrate the skin, and this layer is very resistant to abrasion. The process of keratinization produces distinctive layers of the epidermis called strata (plural) or stratum (singular). There are five layers from the outermost to deep. They are the stratum corneum, commonly called the horny or leathery layer; the stratum lucidum, commonly called the clear layer; stratum granulosum, commonly called the granular layer; the stratum spinosum, commonly called the spiny or prickly layer. The innermost layer and the most important is the stratum germinativum or the regenerative layer. v The Stratum Corneum The Stratum Corneum forms the outermost layer of the epidermis. It consist of dead cells converted to protein. They are called keratinized cells because they have lost most of their fluid. The organelles of the cells are now just masses of the hard protein keratin that gives this layer its structural strength. These cells are also covered and surrounded with lipids to prevent any passage of fluids through this layer. These cells have only about 20% water as compared to cells in the lowermost layer that have 70%. The cells resemble scales in shape and can consists of up to 25 layers. By the time cells reach this layer, the desmosomes have broken apart and, therefore, these cells are constantly being sloughed off. The shedding of these cells from the scalp produces what we call dandruff. This layer also functions as a physical barrier to light and heat waves, microorganisms and most chemicals. The thickness of this layer is determined by the amount of stimulation on the surface by abrasion or weight bearing, hence the thickened palms of hands and soles of the feet. When skin is subjected to an excessive amount of abrasion or friction, a thickened area called a callus will develop. Learners who do a lot of writing will develop small calluses on their fingers that hold their pens. Abrasion on the bony prominences on the foot can produce structures we call corns. v The Stratum Lucidum The Stratum Lucidum lies directly beneath the stratum corneum but is difficult to see in thinner skin. It is only one or two cell layers thick. Its cells are transparent and flat. v The Stratum Granulosum The Stratum Granulosum consists of two or three layers of flattened cells. Because granules tend to accumulate in these cells, it was named the granular layer. These granules have nothing to do with the skin color. This layer is very active in keratinization. In this layer the cells lose their nuclei and become compact and brittle. v The Stratum Spinosum The Stratum Spinosum consists of several layers of the prickly or spiny shaped cells that are polyhedron in structure. In this layer, desmosomes are still quite prevalent. The outline caused by the polyhedral shapes causes the cell’s outlines to look spiny, hence the name. In some classification schemes, this layer is included with the germinativum. v The Stratum Germinativum The Stratum Germinativum is the deepest and most important layer of the skin because it contains the only cells of the epidermis that are capable of dividing by mitosis. When new cells are formed they undergo morphologic and nuclear changes as they get pushed upward by the dividing cells beneath them. Therefore, these cells give rise to ll the other upper layers of the epidermis. The epidermis will regenerate itself only as long as the stratum germinativum remains intact. Its basal layer, called the stratum basale, rest on the basement membrane. The stratum germinativum also contains cells called melanocytes, which are responsible for producing skin color. Melanocytes are irregularly shaped with long processes that extend between the other epithelial cells of this layer. They produce a pigment called melanin; which is responsible for variations in skin pigmentation. All races have the same number of melanocytes, but the different races have specific genes that determine the amount of melanin produced by the melanocytes. Even through many genes are responsible for skin color, one mutation can cause the absence of the skin color by preventing the production of melanin. This condition is called albinism and results from a recessive gene that cause the absence of melanin. The Dermis The dermis is also known as the corium. It lies directly beneath the epidermis and is often referred to as the true skin. It is composed of dense connective tissue with tough white collagenous fibers and yellow elastin fibers. Blood vessels, nerves, lymph vessels, smooth muscles, sweat glands, hair follicles and sebaceous glands are all embedded in the dermis. The dermis can be divided into two portions. The papillary portion is the area adjacent to the epidermis, and the reticular portion is found between the papillary portion and the fatty subcutaneous tissue beneath. A sheet of areolar tissue, usually containing fat (adipose tissue), is known as the subcutaneous tissue or superficial fascia and attaches the dermis to underlying structures like muscle or bone. This subcutaneous tissue is sometimes referred to as the hypodermis. It is into this area that hypodermic injections are given. The pink tint of light-skinned individuals is embarrassed, blood vessels in the dermis dilate. This causes “blushing” or reddish tint seen in the facial area. When a light-skinned individual suffocates or drowns, carbon dioxide in the blood to take on a bluish tinge. This results in the bluish discoloration of skin or cyanosis caused by lack of oxygen in the blood. When a dark-skinned individual suffocates or drowns, the same condition occurs but results in a grayish or ashy tinge to the skin rather than a bluish tinge. Ø THE ACCESSORY STRUCTURES OF THE SKIN The structures associated with the skin include hair, nails, sebaceous glands, ceruminous glands or wax glands in the ear canal and sweat glands. · Hair Hair, in the addition to mammary glands, is a main characteristic of all mammals. When the hair is very thick and covers most of the surface of the body, as on a dog or cat, it is called fur. Even on humans, hair covers the entire body except the palms of the hands, the soles of the feet and certain portions of the external genitalia (e.g., the head of the penis). In some parts of the body, the hair is so small that it appears invisible, yet in other places it is very obvious as on the head, in the armpits and around the genitalia. The amount of hair a person develops is related to complex genetic factors. Each individual hair is composed of three parts : the cuticle, the cortex and the medulla. The outermost portion is the the cuticle, which consists of several layers of overlapping scalelike cells. The cortex is the principal portion of the hair. Its cells are elongated and united to form flattened fibers. In people with dark hair, these fibers contain pigment granules. The middle or central part of the hair is called the medulla. It is composed of the cells with many sides. These cells frequently contain air spaces. There are other parts to the anatomy of a hair. The shaft is the visible portion of the hair. The root is found in an epidermal tube called the hair follicle. The follicle is made of an outer connective tissue sheath and an inner epithelial membrane continuous with the stratum germinativum. Attached to the hair follicle is a bundle of smooth muscle fibers that make up the arrector pili muscle. This muscle causes the goose flesh appearance on our skin when we get scared or when we get a chill. The muscle is involuntary and when it contracts it pulls on the hair follicle, causing the hair to “stand on its end.” We see the goose flesh appearance where is scarce. When dogs or cats get angry, their hairs stand up on the nape of their necks. This is all the result of contraction of the arrector pill muscles. v Hair Growth Hair growth is similar to the growth of epidermis. Note that the hair follicle is an involution of the epidermis. The deeper cell layers at the base of the hair follicle are responsible for the production of new cells by mitosis. The epithelial cells of the hair follicle divide by mitosis and get pushed upward because of the basement membrane. As the cells move upward, they keratinize and form the layers of the hair shaft.Hair growth begins in the hair bulb. Blood vessels in the hair bulb provide the nourishment to produce the hair. v Hair Texture We classify hair texture as straight, curly or tightly curly (kinky). This is due to the genetic factors controlling the nature of the keratin of the hair. The keratin of the cortex of the hair is polymerized and cross- linked chemically in characteristic folded configuration called alpha keratin, making the fibers elastic. The alpha keratin chain is some individuals produces straight hair, in others curly and in still others tightly curled. v Hair Color Hair color is also determined by the complex genetic factors. For example some people turn gray in their youth, yet others turn gray in their 40s, 50s, or even as late as their 60s. · Nails At the end of the fingers and toes, we have nails. Other animals have claws or hooves. The nail is a modification of horny (leathery) epidermal cells composed of very hard keratin. Air mixed in the keratin matrix forms the white crescent at the proximal end of each nail called lunula. Again the size of the lunula will vary from person to person and sometimes from nail to nail due to the genetic factors. The nail body is the visible part of the nail. The nail root is the part of the nail body attached to the nail bed from which the nail grows approximately 1 mm per week unless inhibited by disease. · Sebaceous Glands Sebaceous glands develop along the walls of hair follicles and produce sebum. Sebaceous secretions consists or entire cells containing the sebum. As the cells disintegrate, the sebum moves along the hair shaft to the surface of the skin where it produces a cosmetic gloss. Sebaceous secretion is under the control of the endocrine system. It increases in puberty, resulting in acne problems in the adolescents, and it decreases in later life resulting in dry skin problems. It also increases in late pregnancy. · Sweat Glands Sweat glands are simple tubular glands found in most parts of the body. They are not found on the margins of the lips or the head of the penis. They are most numerous in the palms of our hands and in the soles of our feet. It has been estimated that there are 3000 sweat glands per square inch on the palms of our hands. Ø THE FUNCTIONS OF THE INTEGUMENTARY SYSTEM The skin functions in sensation, protection, thermoregulation and secretion. · Sensation Receptor sites in the skin detect changes in the external environment for the temperature and pressure. Receptor sites are in contact with sensory neurons that transmit the impulse to the brain and spinal cord. Temperature receptors produce the sensations of hot and cold. The pressure receptor sites allow us to interpret excessive pressure. · Protection The skin is an elastic, resistant covering. It prevents passage of harmful physical and chemical agents. The melanin produced by the melanocytes in the stratum germinativum darkens our skin and protects us from the damaging ultraviolet rays of sunlight. · Thermoregulation Temperature regulation is critical to our survival because changes in temperature affect the functioning enzymes. The presence of enzymes is critical for normal chemical reactions to occur in our cells. When people get high fevers, they can die because the heat of a fever destroys the enzymes by breaking up their chemical structure. Without enzymes, chemical reactions cannot occur and our cellular machinery breaks down and death results. · Secretion The skin produces two secretions: sebum and sweat. Sebum imparts a cosmetic gloss to our skin and and moisturizing our skin, it has both antifungal and antibacterial properties. It helps prevent infections and maintains the texture and integrity of the skin. Sweat is essential for the cooling process of the body. It also contains waste products such ammonia and uric acid. References: "Anatomy and Physiology" by Dr. Donald P. Rizzo www.google.com |