Skin (Integumentary System) Wheater, Chap. 9
Skin (Integument) Consists of skin and associated derivatives Largest organ of body (21 ft 2 ; 9 lbs.; has 11 miles of blood vessels) Functions: Protection protects body from external environment and prevents water loss from body Sensory provides information concerning external environment Thermoregulation helps to regulate internal body temperature Production of Vitamin D
Skin (integument) consists of: Epidermis Dermis Hypodermis Epidermal derivatives: Hair and hair follicles Sweat glands Sebaceous glands Mammary glands Nails
Skin Layers of the skin Epidermis consists of keratinized stratified squamous epithelium Dermis consists of loose irregular and dense irregular connective tissues Hypodermis (subcutaneous tissue; superficial fascia) consists of loose irregular connective tissue and varying amounts of adipose tissue Types of skin Thin skin covers most of body Thick skin palm of hand; sole of foot Difference determined by thickness of epidermis only, not by thickness of dermis or hypodermis
Skin, thin epidermis
Epidermis Keratinized stratified squamous epithelium Has 5 layers (in thick skin); 4 layers in thin skin Each layer represents a stage of differentiation that epithelial cells pass through during their life cycle (normally 50-60 days total) Epidermal Layers: Stratum basale (stratum germinativum) Stratum spinosum Stratum granulosum Stratum lucidum seen only in thick skin Stratum corneum
Epidermis Stratum basale (stratum germinativum) Deepest layer Consists of single layer of undifferentiated cells, all of which sit on the basement membrane Cuboidal cells that stain basophilic Undergo constant cell division One cell stays as part of stratum basale Other cell pushed toward surface becomes keratinocyte
Stratum spinosum Several cell layers in thickness Keratinocytes have numerous spiny cell processes Cell processes of adjacent cells interdigitate Processes joined by numerous desmosomes anchor cells together to resist friction Area with cell processes seen as pale region surrounding each cell Keratinocytes become flatter as they move upward
Thick Skin stratum corneum stratum lucidum stratum granulosum stratum spinosum
Stratum granulosum Thick skin 1-2 cells in thickness Thin skin forms variable or discontinuous layer Cells accumulate numerous cytoplasmic keratohyaline granules Granules cause cells to stain basophilic Cells also secrete lipid material into spaces between cells makes skin waterproof This is most superficial of non-keratinized (living) cell layers (keratinocytes still have nuclei) Stratum lucidum Thin, uniform staining layer seen in thick skin only Cells rapidly pass through this stage of differentiation, so lucidum layer of thin skin is very thin and thus indistinct Keratohyaline granules released into cytoplasm forms keratin Cytoplasm fills with keratin Keratinocytes lose cell organelles and nucleus cell dies Cells are now said to be keratinized or cornified
Stratum corneum Consists of multiple layers of flattened (squamousshaped), dead, keratinized cells Dead cells constantly lost from skin surface (body looses 50,000 cells per minute) Keratinized cell layers provide mechanical and waterproof barriers for the body Total thickness of stratum corneum is variable Thin in thin skin and very thick in thick skin Thickness of stratum corneum is defining feature that determines thick versus thin skin
Skin, thick L Low magnification view of thick skin. B = stratum basale; S = stratum spinosum; G = stratum granulosum; L = stratum lucidum; C = stratum corneum High magnification view of thin skin.
Cells of the Epidermis Keratinocytes Langerhans cells Phagocytic cells Part of Mononuclear Phagocytic System Antigen-presenting cells involved in immune responses of skin (such as contact allergic dermatitis) Merkel cells Modified epithelial cells that serve as mechanoreceptors for touch and pressure Closely associated with afferent nerve ending Found in skin for all areas of body Melanocytes Pigment producing cells
Melanocytes Cells that produce melanin (dark skin pigment) Cell body of melanocyte is located in stratum basale Send long cell processes up into stratum spinosum Cell processes extend between keratinocytes Melanin packaged within melanocytes into granules called melanosomes Melanosomes transported from melanocyte cell body out to tips of cell processes Ends of cell processes containing melanosomes bud off and are phagocytized by surrounding keratinocytes Example of apocrine secretory mechanism by melanocytes Melanin retained in keratinocytes for variable amount of time Eventually degraded by keratinocyte
Melanocytes melanocyte
Melanin protects keratinocytes of stratum spinosum and mitotic cells of stratum basale from damage by UV radiation from sun UV radiation can damage DNA may lead to skin cancer Melanin absorbs UV radiation Melanin production increased following regular exposure to sunlight produces tan Sunburn is inflammatory response of skin following UV damage to unprotected cells
Skin Color Melanin contributes to normal skin color Same number of melanocytes regardless of skin color Lighter skin color keratinocytes rapidly degrade melanosomes Darker skin color melanosomes retained longer, thus keratinocytes accumulate more melanin Albino can t produce melanin due to genetic mutation Red color of blood in underlying dermis also contributes to skin color Melanocytes also found at base of growing hairs Provide melanosomes to hair cells gives hair color Various kinds of hair melanin dark brown/black to red/yellow
Dermis Thick connective tissue layer underlying epidermis Basement membrane mediates attachment of epidermis to dermis Epidermal cells of stratum basale anchored to basement membrane via hemidesmosomes and focal adhesions Reticular lamina layer blends into upper dermis (papillary layer of dermis) dermal papillae epidermis dermis
Uneven boundary between dermis and epidermis Dermal papillae upward projections of dermis into epidermis that serve to increase surface area of contact between dermis and epidermis Increases strength of attachment larger papillae found in areas of higher stress Increases surface area for diffusion of nutrients into avascular epidermis from capillaries located in dermal papillae Dermal ridges long, continuous line of dermal papillae; elevate overlying epidermis into similar ridges (produces fingerprints)
Dermis varies in thickness in different areas of body - Thickest on back and buttocks (more friction) - Thickness of dermis not used to define thick and thin skin Dermis divided into 2 regions: - Papillary layer - Reticular layer
Papillary layer of dermis Thin layer located immediately deep to epithelium Fills area of dermal papillae Consists of loose irregular connective tissue Many cells (per unit area) and thin collagen fibers Merges with reticular lamina of basement membrane Has numerous capillaries and sensory nerve endings Reticular layer of dermis Much thicker layer that extends down to hypodermis Consists of dense irregular connective tissue Fewer cells and thick collagen fibers
Sensory Nerve Endings Free nerve endings Penetrate into epidermis For temperature, pain, itch In tongue, activated by capsaicin (active ingredient in hot peppers) Free nerve endings also associated with hair follicles (respond to hair movement) Meissner s corpuscles Receptors for fine touch Located at top of dermal papillae (in papillary layer of dermis, just below epithelium) Innervated by an afferent nerve ending Most common in fingertips, palm, sole of foot, & lips Pacinian corpuscles Pressure, vibration receptors located in deep dermis and hypodermis Common in palm, sole of foot, and abdomen
Meissner s corpuscle formed by sensory nerve endings located between stacked layers of cells; found under epidermis, at top of dermal papillae (Note: you are not responsible for visual identification of these structures) Pacinian corpuscle consists of multiple cell layers, separated by fluid-filled spaces, with a sensory nerve ending located at the center; found in deep dermis or hypodermis
Skin Appendages Derived from epithelial invaginations into dermis during embryonic development Sweat glands, Hair, Sebaceous glands, Nails, Mammary glands Sweat Glands Produce sweat to cool body via evaporation Coiled tubular glands Secretory portion Secretory cells single layer of epithelial cells Myoepithelial cells epithelial-derived contractile cells; cell processes surround and squeeze secretory part of gland Duct portion Stratified cuboidal epithelium
Hair Hair follicle invagination of epithelial cells Continuous with epidermis layer of skin Extends deeply into dermis or hypodermis Dermal sheath thin layer of dense irregular connective tissue surrounding outer margin of hair follicle Hair shaft composed of dead, keratinized cells Produced by dividing cells located at base of hair follicle Cell growth pushes hair shaft upward Cells become keratinized and die
shaft hair matrix hair hair papilla
Hair bulb expanded, terminal end of hair follicle; has 2 parts: Hair matrix Mass of dividing epithelial cells that produce hair shaft and walls of hair follicle Avascular Also contains melanocytes transfer melanosomes containing melanin to cells of growing hair shaft (give color to the hair shaft) Hair papillae Loose irregular connective tissue that invaginates into center of hair matrix Contains numerous capillaries to supply epithelial cells of hair matrix
Arrector pili muscle Small bundle of smooth muscle Runs diagonally from upper dermis to insert onto side of hair follicle Serves to raise hair
Sebaceous Gland Cluster of large, pale-staining secretory cells Duct empties into lateral side of hair follicle Secretes sebum Oily secretion Dissolved out during slide preparation leaves cells with empty appearance Coats hair and skin surface Makes hair less brittle and waterproofs skin Holocrine secretory mechanism Cells fill with sebum, die, and break open Entire contents of cell is secretory product
Skin Diseases Contact allergic dermatitis allergic response to environmental antigen (poison ivy) Melanoma skin cancer, usually associated with excessive exposure to sunlight or UV light