The skin or integument is the largest organ in both surface area and in weight. It comprises 16% of the total body weight. The skin is highly visible. It is seen every day and others see it every day. Consequently a lot of time is spent devoted to its appearance-shaving, applying deodorant, cologne, washing, etc. These activities modify the skins appearance and properties. When there are problems of the skin, they are immediately apparent. Physicians look for obvious signs as well as the general skin appearance such as color, flexibility, elasticity, sensitivity. Changes may indicate a dysfunction in another system.
The skin is made of 2 major components-cutaneous membrane-skin & accessory structures-hair, nails, exocrine glands. The cutaneous membrane has 2 parts, an outer epidermisand a dermis. The accessory structures begin in the dermis and protrude through the epidermis to the skin surface. Deep to the dermis is loose connective tissue in a subcutaneous layer, the hypodermis or subcutaneous layer. This layerseparates the integument from the deep fascia around other organs. It is not considered part of the integument, but it will be discussed in this unit, since the connective tissue fibers are interwoven with those of the dermis.
The skin has several very important functions. The first of those is protection. The skin is continually abused, abraided, attached by microorganisms, irradiated & exposed to chemicals. It is the first line of protection against those irritants. Another function is excretion. The skin exudessalts, water & organic wastes. The skin also is important in maintaining body temperature. It can both insulate & cool. Vitamin D3 synthesis is another function of the skin. This vitamin is essential for Ca++ metabolism. The skin has a storage function. It storesnutrients-lipids in adipose tissue found in the dermis & subcutaneous layers. Finally the skin is essential for detection. It contains sensory receptors for touch, pain, pressure & temperature which relay information to the nervous system.
The epidermis is composed of keratinized stratified squamous epithelium.It is avascular and serves as a protective barrier against ultra-violet light, bacteria, chemicals & abrasion. The epidermis has 4celltypes located in 4-5 layers. There are 4 layers in thin skin and five layers in thick skin. Thickness varies: from 0.07-0.12 mm over most of the body to 0.8 mm on palms and 1.4 mm-soles.
Layers of Epidermis
The stratum germinativum or stratum basale forms the inner most layer. Hemi-desmosomes attach this layer to the basement membrane. It separates the epidermis from the dermis. The layer forms epidermal ridges which extend into the dermis. Dermal projections called dermal papillae extend between adjacent epidermal ridges. This layer consists of one row of cuboidal to columnar shaped, large, basal or germinative cellsàdivideàreplace superficial keratinocytes which push up toward the surface to become part of the more superficial layers. 10-25% cells are melanocytes. These have long branching processes that extend, reaching into the stratum spinosum. Melanocytes synthesize melanin, a brown pigment that absorbs UV light. In skin surfaces without hair Merkel cellscan be found. These aresensitive to touch.
Stratum spinosum is the next layer. It is composed of 8-10 layers and is called spinosum or spiny layer because in histological preparations the cells shrinkand look prickly like little spines hence spinosum. The main cell type is the keratinocyte. Cells pushed into this layer from the s. germinativium, may continue to divide Langerhans'cells or epidermal dendritic cells are found in this layer. They participate in the immune response and provide defense against microorganisms. The S. germinativum & s. spinosum are located adjacent to the dermis and therefore receive adequate nourishment by diffusion from the dermis.
The next layer is the stratum granulosum. This is agrainy layer composed of 3-4 layers pushed up from the s. spinosum. Cells do not divide. They are undergoing apoptosis or programmed cell death. They begin to secrete keratohyalin and keratin, afibrous protein and a basic component of hair and nails. As the cells are pushed upward, away from the source of nutrition, they gradually die or become keratinized. As keratin fibers develops cells become thinner & flatter. As the cell membrane thickens, it becomes less permeable. Keratohyalin forms dense granulesàpromote dehydration, aggregation &cross linkage of keratin fibers. Nuclei & other organelles disintegrateàcell dies. Further dehydration produces a tightly interlocked layer of cells made of keratin fibers surrounded by keratohyalin.
The stratum lucidum is a clear section composed of 4-6 layers of clear, dead cells. It is only found in thick skin located in the soles, palms and fingertips. Cells are flattened, densely packed & filled with keratin.
The stratumcorneum is the outermost layer. It is composed of 25-30 layers of flattened, dead, keratinized cells. The cells are keratinized or cornified. The cells and the keratin form a protective, superficial layer that keeps the skin dry and more or less waterproof. It resists surface evaporation and therefore prevents excessive water loss. It helps waterproof and protect the skin and underlying tissues from heat, microbes, abrasion & chemicals. This layer is found on all exposed surfaces except at the anterior surface of the eyes. Dead cells are connected by desmosomes so tight, they are shed in groups. It takes 15-30 days for cells to move from s. germinativium to s. corneum. Dead cells remain in the s. corneum about 2 weeks before being shed. The surface is maintained by coating it with lipid secretions from sebaceous glands.
The dermis is found between the epidermis and the subcutaneous layer. It consists of dense, irregular connective tissue. It can be stretched and recoils easily. It is thicker than the epidermis and had fibroblasts as its primary cell type. It is divided into two layers, a think papillary layer and a thicker, deeper reticular layer. The papillary layer is comprised of areolar tissue having both collagen and fine elastic fibers present. Dermal papilla are found here which project into the epidermis. Some of these contain touch receptors called Meissner’s Corpuscles. Some have free nerve endings which detect pain, coolness, itching and tickling. The reticular layer comprises4/5th of the dermis and is attached to the subcutaneous layer. It is made of dense irregular connective tissue containing thick bundles of interlacing collagen and elastic fibers. Collagen fibers are easily bent or twisted. They give strength and extensibility-ability to stretch. Elastic fibers stretch and recoil to original length. They provide elasticity-gives the skin the ability to return to its original shape after stretching. Collagen fibers extend into the subcutaneous layer.
The hypodermis or subcutaneous layer is not part of the skin. It stabilizes the skin’s position in relation to underlying tissues while allowing for independent movement. The reticular layer of the dermis and the connective tissue fiber of the hypodermis are interwoven .The boundary is indistinct. It consists of adipose tissue or subcutaneous fat and areolar tissue. The hypodermis has protectivefunctions. It stores fat, helps prevent heat loss and acts as a shock absorber; it anchors the skin to underlying structures allowing the skin to slide almost freely over them. This layer contains sensory endings for touch and Pacinian or lamellated corpuscles for deep pressure. Hair follicles and oil glands begin in this layer.
Skin color is due to an interaction of epidermal pigmentation and dermal circulation. Epidermis contains 2 main pigments-carotene & melanin. Carotene is an orange-yellow pigment which accumulates in epidermal cells. It is most apparent in the s. corneum. Melanin is abrown, yellow-brown or black pigment-made by melanocytes in the s. germinativium. It is packed in melanosomes Differences in skin color are due to the amount of pigment that the melanocytes produce. Melanin protects the epidermis and dermis from UV radiation. Some UV is neededàvitamin D. However UV light can damage DNAàmutation & cancer. Melanosomes concentrate around the nucleus to protect DNA. Melanocytes respond to UV exposure by increasing their activity. Response is slow; it cannot prevent sunburn. After UV exposure, melanin synthesis accelerates slowly peaking in about 10 days after initial exposure.
Dermal circulation is another factor that effects skincolor.Blood contains RBCs which contain hemoglobin-a red pigment. It binds and transports O2. When bound to O2àbright red-blood vessels in the dermis take on a reddish tint-when dilated. Circulatory system decreasesàskinàpale-may turn white. Sustained reduction in circulatory system-tissue O2 decreasesàhemoglobin releases O2àturns darker red-seen from surfaceàbluish-cyanosis. This is most apparent in thin skin such as the lips and beneath the nails.
Accessory structures include hair, sweat glands, sebaceous glands, nailsandteeth enamel. These are located in the dermis and project through the epidermis to the surface. Hairprojects above the surface of the skin almost everywhere except the sides and soles of the feet & palms, the sides of fingers, toes and the lips. 98% of the general body surface has hair. Hair first appears by the fifth month, prenatally. This hair is termed larugo; it is fine, unpigmented and shed before birth. It is replaced by vellus or terminal hair. Vellus hair is fine body hair; sometimes called peach fuzz. Terminal hair is coarser, heavy and deeply pigmented. It is found on the scalp, eyebrows, etc.
The primary function of hair isprotection. It protects the scalp from injury, sun rays and decreases heat loss. Eyebrows & eyelashes protect the eye from foreign particles. Nose hair filters inhaled air and ear hair prevents foreign particles from penetrating into the ear canal. Hair is produced in a hair follicle. Follicle means bag which extends into the dermis, where it expands forming a hair bulb. The hair root anchors the hair into the skin, beginning at the base of the hair at the bulb. The hair shaft projects from the surface. The shape of the shaft determines the curliness of hair-round shaftàstraight; ovalàwavy; flatàkinky. The cuticleconsists of alayer of dead keratinized cells which overlap like shingles on a roof. It covers the intermediate layer or cortex which consists ofseveral layers of cells and hard keratin. It contains pigmentindark hair; different forms of melanin produce different hair colors. The medulla is thecore. It consists of 2-3 rows of cells containing pigment, air spaces and soft keratin. Around each hair follicle there are sensory nerve endings, the root hair plexus which surrounds the bulb and allows us to feel movement of one shaft of hair. Associated with each hair follicle is a tiny band of smooth muscle, the arrector pili muscle. These muscles are attached to the papillary layer of the dermis and to the connective tissue root sheath of the follicle. When the arrector pili m. is stimulated to contract by the nervous system from stress or cold, the hair is pulled perpendicular to the skin’s surface. This traps more airàwarmerà goose bumps. Touch receptors are associated with each hair follicle.
Another accessory structure of the skin are skin glands. Glands are specialized epithelial cells Skin glands are exocrine. They secrete sweat, oil and wax. Gland that secrete oil are called sebaceous glands. These are branched, acinar type gland and are typically found attached to hair follicles. They secrete an oily substance called sebum. Sebum (seb = oil) is a mixture of triglycerides, cholesterol, proteins and electrolytes which is usually secreted into the hair folliclein the lips & mammary papilla and are directly secreted onto the skin on the face, back and chest, areas which have large sebaceous glands not associated with hair follicles. Functionally oil glands are holocrine. They burst when they secrete. The secretions help to inhibit bacterial growth and they lubricate and protect and condition the skin.
Sweat glands are termed sudoriferous glands. There are 2 kinds: apocrine and eccrine. Eccrine are simple coiled tubular glands. The secretory part lies in the deep dermis. These glands make about 600mls of sweat each day. This sweat helps with thermoregulation and is called thermoregulatory sweating. Sweat that evaporates before it is noticed is called insensible perspiration. Sweat that is felt as moisture is termed sensible perspiration. Sweat can be released due to emotional stimulation. This is often called a cold sweat and is termed emotional sweating.
Apocrine sweat glands are found mainly in the armpits and anogenital areas. The secretory portion is located in the dermis or hypodermis. The sweat produced is sticky and cloudy and has the potential to become odorous. The name is a misnomer, it secretes by eccrine type secretion, i.e. by exocytosis.
Wax or cerumen is made by glands. These are found in the ear canal.