3 INTRODUCTION TO THE SKIN The integumentary system consists of the skin and the derivatives of the epidermis such as hair, glands and nails. The skin is also classified as a cutaneous epithelial membrane.A. Stratum corneum is a dead upper portion of the epidermisB. Living epidermis consists of several layers of cells which manufacture keratin.C. Dermis is the layer of skin containing blood vessels, glands and nerve endingsABC
4 EPIDERMIS - THIN SKINThe epidermis of skin is a stratified squamous epithelium consisting of several layers of cells. These cells are called keratinocytes because of their ability to produce the tough, fibrous protein keratin. The surface of the epidermis is covered with a layer of dead tissue called the stratum corneum. On some of the surfaces of the body, the epidermis is thin and delicate, e.g., the under surfaces of the upper arms.
5 * KERATIN FORMATION * sc Keratin fiber is formed from keratohyaline. Keratohyaline is formed in the stratum granulosum (black asterisk). It is in this region that the keratinocytes die. Immediately above this layer is a clear translucent layer, the stratum lucidum (red asterisk). The stratum corneum (sc) is the uppermost layersc**
6 Keratinocytes – Most abundant cell type Keratinocytes – Most abundant cell type. Cells are held together by desmosomes and are organized into layers.1. Stratum germinativum – Cells found just above the basement membrane which undergo rapid cell divisions Stratum spinosum is a middle portion of the epidermis several cells thick.. The cells have a "spiny appearance".3. Stratum granulosum is a layer just above the spinosum in which large amounts of keratohyalin is being synthesized Stratum lucidum - The keratinocytes in this layer are filled with keratohyalin and a fibrous protein, keratin. Keratin is tough, durable and water resistant. Keratin is also the chief component in hair and nails.5. Stratum corneum is the outermost layer of the epidermis composed of flattened, dead, keratin-filled cells. This dead layer provides protection from abrasion, strong chemicals, microbial invasion and dehydration.
7 Other cell types in the epidermis include: 1. Langerhans cells are T lymphocytes (cells that mount an immune response) located within the stratum spinosum. These cells will initiate an immune response against microbes and epidermal skin cancers.2. Merkel cells are located among the cells of the germinativium. They are sensitive to touch.3. Melanocytes are also located in the germinativium often under the basal cells just above the basement membrane. They manufacture a brown pigment called melanin. Melanin is packaged within vesicles called melanosomes in the cytoplasm of the melanocytes. These vesicles are transferred into the overlying keratinocytes, darkening the skin. Melanin is a helpful material in the skin. It tends to surround the nucleus of the keratinocyte protecting the DNA of that cell from damage due to ultraviolet light from the sun. It also seems to neutralize damaging chemicals called free radicals that accumulate in traumatized tissues. Cancer of the melanocytes, melanoma, is an extremely dangerous and malignant neoplasm.
8 Changes in Skin Color Tanning Effect –Due to the activity of melanocytes Reddening of skin occurs due to an increased blood flow in the underlying dermis. For example, an increase in body temperature which leads to a vasodilation of blood vessels in the dermis.3. Strong emotional states will lead to an increase in blood flow in the dermis.4. Blanching of Skin ("turning pale") occurs due to decreased blood flow to the skin. This decreased blood flow may be caused by a sudden drop in blood pressure, hypothermia and emotional states.5. Cyanosis – bluish color in skin due to sustained reduction in blood supply to the skin 6. Jaundice – When liver function is interrupted due to cirrhosis, liver cancer or blockage of bile flow, yellow bile pigments and bilirubin accumulate in the skin and whites of the eyes.
9 The Dermis1. Located below the epidermis it contains all of the accessory organs of epidermal origin, such as, hair follicles and glands It also has extensive networks of blood vessels, lymphatic vessels nerve endings and nerve fibers The dermis consists of two major layers:a. The Papillary layer contains loose (areolar) connective tissue with a rich supply of blood capillaries. It also contains the nerve endings for touch and pain.b. The Reticular layer contains dense irregular bundles of collagen, elastic and reticular fibers. These fibrous bundles blend into the papillary layer above and into the underlying subcutaneous layer.
10 Dermal Blood Supply The Cutaneous Plexus When arteries supplying the skin reach the subcutaneous layer, they form a network of branches called the cutaneous plexus. Branches from this plexus supply the subcutaneous fat and various structures in the dermis such as hair follicles and sweat glands.2. The Papillary plexus – capillary beds that follow the contours of the epidermal-dermal boundary. Interruptions in this circulatory flow can result in epidermal and dermal deterioration and necrosis, e.g., decubitis ulcers and diabetic foot.
11 B. Innervation of the skin – The skin is richly innervated B. Innervation of the skin – The skin is richly innervated. The functions of these nerves include: 1. Control of blood flow through the skin.2. Adjusting the rate of glandular secretions.3. Sensory reception – The sensory receptors of the skin respond to two basic types of stimulation, mechanical change and pain.Pacinian CorpuscleMeissner’s Corpuscle
12 Mechanoreceptors of the skin 1. Meissner’s corpuscles – located within the dermal papillae, they respond to light touch. Meissner’s corpuscles are located immediately under the epidermis within the dermal papillae. These nerve endings respond to light touch.2. Pacinian corpuscles – located deep within the reticular layer, Pacinian corpuscles are sensitive to deep pressure and vibrations. They are located in the deep dermis or subcutaneous layer. They are also common in the glans penis of the male and in the pancreas. The role they may play in the latter organ has never been determined.3. Merkel’s disks – are specialized nerve endings found at the ends of nerve fibers. These disks are closely associated with the epidermal Merkel’s cells and respond to fine touch.
13 Nociceptors – Pain receptors are especially abundant in the upper skin, joint capsules, the periosteum of bone and the walls of blood vessels. Very few pain receptors are located in visceral organs or deep tissues. There are three types of pain receptors: 1. Those sensitive to temperature extremes. 2. Those sensitive to mechanical damage. 3. Those sensitive to chemicals, e.g., metabolites from traumatized tissues such as aracidonic acid and prostaglandins. Very strong pain stimuli will excite all three types of receptors. For example, severe trauma of any kind (burns, cuts, corrosive chemicals) will often be described as "burning" or acute pain.
14 Hair Follicle –Hair follicles project deep into the dermis and subcutaneous fat from the surface of the skin. Each hair follicle consists of the following:1. Hair Papilla – a small clump of connective tissue, capillaries and nerves which supports the growth of the hair.2. Hair Bulb – consists of the epithelium that surrounds the papilla. This structure is an invagination of the epidermis. Cells from the basal layer of the bulb divide and are pushed up into the root of the hair.3. Hair shaft – begins at a point about midway between the papilla and the skin surface.4. The arrector pili are small bands of smooth muscle which extend from the connective tissue sheath of the follicle and anchor in the papillary layer of the dermis. This muscle is stimulated to contract by strong emotional states or cold temperatures. These stimuli operate through the nervous system to cause the hairs to become erect.
15 SUBCUTANEOUS LAYERThe subcutaneous layer of skin contains abundant adipose tissue. In addition, many blood vessels, nerve endings and hair follicles can usually be seen.Adipose tissueHair Follicle
16 HISTOLOGY OF THE HAIR FOLLICLE The hair follicle is an epidermal sheath that surrounds the hair. Sebaceous glands are usually attached to the side of a follicle. Their oily secretion enters the follicle and follows it to the surface. The hair grows from the bulb, the swollen lower end of the follicle. The bulb is invested with blood vessels and nerves which are essential for the continued growth of the hair.
17 Glands in the Skin – The skin contains a number of exocrine glands Sebaceous glands – Holocrine glands which discharge an oily secretion called sebum.The gland cells originate in the periphery of the gland. As they mature, the cells manufacture sebum, a mixture of triglycerides, cholesterol, proteins and electrolytes.As the cells reach the opening or lumen of the gland, they rupture releasing their product (holocrine secretion).There are two types of sebaceous glands: 1. Simple branched alveolar glands – empty their secretion into the follicle of a hair. 2. Sebaceous follicles – large sebaceous glands that are connected directly to the epidermis and are not associated with a hair. They are found in the skin of the face, back, chest and nipples.Sebum functions by lubricating the skin and retarding the growth of bacteria.
18 ACNEAcne is a condition of the skin which is shared by individuals of all ages. Each pimple is in fact an inflammed sebaceous gland. The causes of the inflammation include inadequate cleansing of the skin to remove superficial oils, endocrine changes during puberty and pregnancy, dietary effects and emotional stress. It is easy to understand why teenagers so often exhibit this problem.
19 SEBACEOUS AND SWEAT GLANDS Sebaceous glands (Sb) are usually associated with a hair and release their holocrine secretion (sebum) into the hair shaft which carries it to the surface. Sweat glands come in two varieties. The most numerous are the eccrine glands (Sw). The watery merocrine product of these glands travels through a duct to the surface of the skin. Here it evaporates and cools the body. Apocrine sweat glands are found around the areola of the nipple, in the labia majora and axilla. Their secretion is thicker and more odiferous than the eccrine secretion.SbSbSwSw
20 Merocrine (eccrine) sweat glands 1. Very numerous Merocrine (eccrine) sweat glands 1. Very numerous. In the adult, the skin may contain 2 to 5 million merocrine glands per square inch. Palms and the soles of the feet have the highest concentration. 2. Merocrine glands are smaller than the apocrine glands. They produce a watery sweat containing electrolytes, lysozymes, antibodies and other ingredients. 3. The functions of these glands include: a. Removing heat from body’s surface to lower body temperature. b. Excretion of water, electrolytes and nitrogenous wastes. c. Protection from chemical and microbialApocrine glandsLocated in the armpits, groin and around the nipples.They produce a sticky, cloudy, odorous secretion into a hair follicle. These secretions become intensified at the time of puberty under the influence of the nervous and endocrine systems.
21 SWEAT GLANDSSweat glands are located in the deep dermis. They are usually associated with small blood vessels and nerves. In response to hyperthermia, the eccrine sweat glands release a serous liquid to the surface of the skin via a duct. The evaporation of this liquid from the surface of the skin cools the body. In the arm pits and groin, apocrine sweat glands secrete an oily material. Sweat glands (sg) are unbranched, coiled, tubular glands distributed throughout the skin. They are not found in the nail beds, margins of the lips, glans penis or ear drum. They are most numerous in the palms of the hands and soles of the feet.
22 Functions of the Skin A. Regulation of body temperature 1. During hyperthermia blood vessels (arteries) in the skin dilate due to nerve stimulation from the brain. Blood flow to the skin increases.2. Warm water moves from the blood to the merocrine sweat glands by filtration. This warm sweat now moves to the skin’s surface by the duct from the gland.3. The warm sweat evaporates from the skin’s surface taking excess body heat with it.4. Body temperature comes down and sweating is reduced (negative feedback).
23 During hypothermia1. Blood vessels in the skin constrict due to nerve stimulation from the brain. This reduces blood flow to the skin.2. Radiant heat loss is reduced from the skin.Sweat production decreases.3. Body heat is retained in the trunk and head or "core" of the body.4. Shivering, an involuntary contraction of skeletal muscles due to nerve stimulation, helps to generate body heat. This is the result of the breakdown or "burning" of glucose in the skeletal muscles.
24 B. Protection from ultraviolet light 1. Ultraviolet light penetrates the epidermis and stimulates the melanocytes to increase their production of melanin. 2. Melanocytes inject packages of melanin called melanosomes into cells of the lower epidermis. As these darkened cells divide, they are pushed to the top of the epidermis Eventually, the entire epidermis becomes a "sunscreen" blocking out much of the harmful ultraviolet light.The arrow indicates the layer of melanocytes in the lowest level of the epidermis.
25 Cancer of the Melanocytes MELANOMACancer of the MelanocytesMelanoma is a neoplasm of the melanin producing cells. The cancer usually begins when a mole (actually a benign tumor) becomes malignant. Overexposure to sunlight is thought to be a cause of melanoma. However, some forms may have a hereditary basis. Melanoma is one of the most metastatic and invasive cancers. The prognosis for an individual with advanced melanoma is not good. Any change in a mole or nevus should be followed up with a visit to the dermatologist.
26 C. Protection from infection by microbes 1. The tough impervious epidermis forms a barrier to microbes, as long as it is intact.2. The formation of sweat and sebum flushes away microbes on the surface of the skin, as well as those in sweat gland pores and hair follicles. The pH of sweat and sebum tends to retard the growth of bacteria. Sweat also contains lysozymes and antibodies which attack microbes.3. The blood-rich dermis reacts quickly to microbial invasion through the activities of various cells. Tissue basophils release chemicals which increase the blood supply to an infected region of skin. Phagocytic cells such as the macrophages and neutrophils physically attack microbes. The combined reaction of these cells is referred to as inflammation.
27 D. Protection from excessive water loss or dehydration D. Protection from excessive water loss or dehydration. The tough, keratinized epidermis forms a watertight cover on the body keeping body water in.E. Production of Vitamin D - Some of the ultraviolet light striking the skin passes through the melanin sunscreen and causes a chemical change in the blood leading to the formation of vitamin D. This vitamin is required for the body to absorb calcium from the intestine. Calcium is essential for the development of teeth and bones.