The Integumentary System
Skin Structure The skin of cutaneous membranes covers the external surface of the body. It is the largest organ of the body It covers 2 square meters and weighs 4.5 5.5 pounds Consist of two main parts: Epidermis: supperficial, inner portion composed of epithelial tissue. Dermis: deeper and thicker connective tissue. Its fibers extend and anchor the skin to the subcutaneous layer.
Subcutaneous Layer Hypodermis Deep in the dermis but not part of the skin is the subcutaneous layer of hypodermis that consist of areolar and adipose tissue. The subcutaneous layer serves as a storage of fat and contains large blood vessels that supply the skin. This region contains nerve endings called Lamellated (Pacinian) corpuscles that are sensitive to pressure.
Epidermis The epidermis is composed of keratinized stratified squamous epithelium. Contains four types of cells: Keratinocytes: 90% of epidermal cells. They are arranged in 4 to 5 layers and produce keratin. Melanocytes: 8 % of epidermal cells, produce the pigment melanin. Their long, slender projections extend between the keratinocytes and transfer melanin to them. Melanin is a brown black pigment that contributes to skin color and absorbs UV light. Langerhans cells Important in Immune system, responds to microbes that invade skin. Can be damaged by UV light. Merkell cells contact the flattened process of nerve cells, a structure called tactile Merkel discs, touch sensations.
Strata of the Epidermis The epidermis is divided in four strata or layers: Stratum Basale: The deepest of the layers. Composed of a single layer of cuboidal or columnar keratinocytes. Some of these cells are stem cells that undergo division constantly. Stratum Spinosum: Somehow flatenned cells. It is formed by 8 to 10 layers many sided keratinocytes that fit very close together.
Strata of the Epidermis Stratum Granulosum: Consist of three or five layers of flattened keratinocytes that undergo apoptosis, genetically programmed death. It has keratin in it. Also present in the keratinocytes are membrane enclosed lamellar granules, which release a lipid rich secretion that acts as water repellent. Stratum Lucidum: Only present in the thick skin of the fingertips, palms and soles. It consists of three to five layers of flattened clear, dead keratinocytes that contain large amount of keratin.
Strata and Epidermis Stratum Corneum_ Consist of 25 to 30 layers of flattened cells keratinocytes. These cells are continually shed and replace by cells of different strata. The interior of the cell contains mostly keratin. Constant exposure to friction creates a callus. Continually reproducing through a process of keratinization, keratinized cells are continually shed. Every 4 weeks a person develops a new skin.
Skin Dermis The second deepest part of the skin It is composed mainly of connective tissue containing collagen and elastin fibers. It makes about 1/5 of thickness of the whole skin. It consist of areolar connective tissue containing fine elastic fibers. Its surface area is greatly increased by small fingerlike projections called dermal papillae. These projections are indented in the epidermis and contain blood capillaries. Other dermal papillae are contain tactile receptors called corpuscles of touch or Meissner corpuscles, that are sensitive to touch.
Skin Dermis Present in the dermal papillae are free nerve endings that are associated with sensations of warmth, pain, coolness, tickling The deeper part of the dermis, which is attached to the subcutaneous layers, consist of dense irregular connective tissue containing collagen and elastic tissue. This two types of tissue gives the elasticity and extensibility needed for pregnancy or gaining weight. Extreme stretching is going to produce striae.
Skin Color Three pigments in the skin Melanin Carotene Hemoglobin The amount of melanin causes the skin s color from pale yellow to tan or black. Melanin accumulation produce freckles Age spots are accumulation of melanin too. UV stimulates the production of melanin. Albinism is the inherited inability of an individual to produce melanin.
Skin Color Carotene Is a yellow orange pigment, it is the precursor of vitamin A, needed for vision. When little melanin or carotene is present, the skin seems translucent. The skin of white people appears from pink to red according on the amount of oxygen content of the blood moving through the capillaries of the dermis. The red color is due to the hemoglobin.
Accessories structures of the skin The accessory structures of the skin are: hair, glands and nails. Hair: Protects body, it is present on most skin surfaces except in palms, palmar of fingers or toes. Genes and hormones determine the amount of hair of a person. The hair in the head protects against UV light, and the hair in the eyebrows and eyelids protect against bacteria penetrating eyes.
Hair Each hair is formed by fused, dead, keratinized cells that consist in a shaft and a root. The shaft is the superficial portion, projects over the surface of skin The root is the portion below the surface. The hair follicle surrounds the root. The hair follicle has two layers of epidermal cells: external and internal root sheaths surrounded by connective tissue. Surrounding each hair follicle are nerve endings, called hair root plexuses, that are sensitive to touch.
Hair The base of each follicle is enlarged into an onion shaped structure, the bulb. The bulb has an indentation, the papilla of the hair that contains blood vessels. The bulb contains a region called matrix, which produces hair division. Smooth muscle cells are associated with hairs, the smooth muscle is called arrector pili. It extends from the upper dermis to the side of the hair follicle. The color of the hair depends in the amount of melanin. Red hair contains more sulfur, white hair contains bubbles in the hair shaft, and gray hair has a decline of melanocytes. Alopecia Baldness, is due to genetically predisposition of androgenes
Glands Three types: sebaceous, sudoriferous and Ceruminous glands Sebaceous or oil glands: They are connected to hair follicles. The secreting portions of the gland lie in the dermis and open into the hair follicles onto skin surface. They secrete sebum, that keeps hair from drying and evaporation of water. Blackheads are accumulation of sebum, produced by oxidezed oil. Pimples or boils appear because they have nutrients for certain bacteria
Sudoriferous glands Two types: Eccrine cells: numerous, all over body except in nail beds and around lips. Sweat is made of water, uric acid, ammonia, glucose, and lactic acid. Apocrine Sweat glands: Found mainly in axilla, groin, areolae of breast and bearded regions of male. The secretory product is viscous and contain same components than sweat. They do not begin to work until puberty. They are stimulated during stress periods.
Ceraminous glands Produce wax They are located in the auditory canal Secretion is cerumen: ear wax Protects against bacteria.
Nails They are tightly packed, hard, keratinized cells of the epidermis. Each nail consist of : Nail body: visible portion Free edge: Part that extends to the end of the finger or toe. Nail root: Portion that it is not visible, lots of blood capillaries Lunula: The moon, it appears white because the vascular tissue underneath does not show because stratum basale. Nail matrix is where the nail grow Cuticle consist of stratum corneum
Functions of the Skin Major functions of the skin: Body Temperature regulation: Sweat helps to cool down body. Protection: Keratin protects skin from bacteria, heat Sensations: pain, touch and pressure Excretion and Absorption: Elimination of substances throughout sweat, the passage of materials from external environment into body cells. Synthesis of vitamin D. Vitamin D is converted into calcitriol that aids in the absorption of phosphorus and calcium
Aging and Skin Most children do not have problems with their skin Adolescents have acne Most of age related issues develop in the dermis. The collagen fibers are lost with age, making the skin flaccid and produce wrinkles. Macrophages become less efficient and more skin infections are produced. Decrease in melanocyte, gray hair. Aged skin is thinner. Growth of hair and nails is slower and brittle.