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5 The Integumentary System Pages ,
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The Skin and the Hypodermis
Skin—our largest organ Accounts for 7% of body weight Varies in thickness from 1.5–4.4mm Divided into two distinct layers Epidermis Dermis Hypodermis—lies deep to the dermis
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Skin Structure Hair shaft Dermal papillae Epidermis
Subpapillary vascular plexus Papillary layer Pore Appendages of skin Dermis Reticular layer Eccrine sweat gland Arrector pili muscle Sebaceous (oil) gland Hair follicle Hair root Hypodermis (superficial fascia) Nervous structures Sensory nerve fiber Dermal vascular plexus Lamellar (Pacinian) corpuscle Adipose tissue Hair follicle receptor (root hair plexus) Figure 5.1
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Figure 5.2 Gross structure of skin and underlying tissues.
Epidermis Dermis Hypodermis Deep fascia Muscle 4
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The Skin and Hypodermis
Functions Protection—cushions organs and protects from bumps, chemicals, water loss, UV radiation Regulation of body temperature Excretion—urea, salts, and water lost through sweat
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The Skin and Hypodermis
Functions (continued) Production of vitamin D Sensory reception—keeps us aware of conditions at the body’s surface
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Epidermis Contains four main cell types Keratinocytes Melanocytes
Produce keratin a fibrous protein Melanocytes Location—basal layer; manufacture and secrete pigment
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Epidermis Contains four main cell types (continued)
Tactile epithelial cells Location—basal layer; attached to sensory nerve endings Dendritic cells Location—stratum spinosum; part of immune system; macrophage-like
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Epidermis Keratinocytes—most abundant cell type in epidermis
Arise from deepest layer of epidermis Produce keratin, a tough fibrous protein Produce antibodies and enzymes Keratinocytes are dead at skin's surface
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Layers of the Epidermis
Stratum basale (stratum germinativum) Stratum spinosum Stratum granulosum Stratum lucidum (only in thick skin) Stratum corneum
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Figure 5.3a Epidermal cells and layers of the epidermis.
Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum One to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Desmosomes Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and dendritic cells. Dermis 11
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Figure 5.3b Epidermal cells and layers of the epidermis.
Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Keratinocytes Stratum granulosum One to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Desmosomes Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and dendritic cells. Dermis Dendritic cell Sensory nerve ending Melanocyte Melanin granule Tactile epithelial cell 12
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Layers of the Epidermis
Stratum basale Deepest layer of epidermis Attached to underlying dermis Cells actively divide Stratum basale contains Merkel cells—associated with sensory nerve ending Melanocytes—secrete the pigment melanin
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Layers of the Epidermis
Stratum spinosum (spiny layer) “Spiny” appearance caused by: Artifacts of histological preparation Contains thick bundles of intermediate filaments (tonofilaments) Resist tension Contain protein prekeratin Contains star-shaped dendritic cells A type of macrophage Function in immune system
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Figure 5.3b Epidermal cells and layers of the epidermis.
Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Keratinocytes Stratum granulosum One to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Desmosomes Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and dendritic cells. Dermis Dendritic cell Sensory nerve ending Melanocyte Melanin granule Tactile epithelial cell 15
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Layers of the Epidermis
Stratum granulosum Consists of keratinocytes and tonofilaments Tonofilaments contain: Keratohyaline granules—help form keratin Lamellated granules—contain a waterproofing glycolipid
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Figure 5.3b Epidermal cells and layers of the epidermis.
Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Keratinocytes Stratum granulosum One to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Desmosomes Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and dendritic cells. Dermis Dendritic cell Sensory nerve ending Melanocyte Melanin granule Tactile epithelial cell 17
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Layers of the Epidermis
Stratum lucidum (clear layer) Occurs only in thick skin Locations of thick skin—palms and soles Composed of a few rows of flat, dead keratinocytes
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Epidermis Stratum corneum Stratum lucidum Stratum granulosum
Figure 5.4 Thick skin. Epidermis Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Dermis Stratum basale Papillary dermis Dermal papilla Reticular dermis Collagen fibers 19
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Layers of the Epidermis
Stratum corneum (horny layer) Thick layer of dead keratinocytes and thickened plasma membranes Protects skin against abrasion and penetration
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Dermis Second major layer of the skin
Strong, flexible connective tissue Richly supplied with blood vessels and nerves Has two layers Papillary layer—includes dermal papillae Reticular layer Deeper layer—80% of thickness of dermis
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Epidermis Stratum corneum Stratum lucidum Stratum granulosum
Figure 5.4 Thick skin. Epidermis Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Dermis Stratum basale Papillary dermis Dermal papilla Reticular dermis Collagen fibers 22
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The Two Regions of the Dermis
(b) Papillary layer of dermis, SEM (570) (a) Light micrograph of thick skin identifying the extent of the dermis, (100) (c) Reticular layer of dermis, SEM (430)
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Hypodermis Deep to the skin—also called superficial fascia
Contains areolar and adipose CT Anchors skin to underlying structures Helps insulate the body
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Hair shaft Dermal papillae Epidermis Subpapillary vascular plexus Papillary layer Pore Appendages of skin Dermis Reticular layer Eccrine sweat gland Arrector pili muscle Sebaceous (oil) gland Hair follicle Hair root Hypodermis (superficial fascia) Nervous structures Sensory nerve fiber Dermal vascular plexus Lamellar (Pacinian) corpuscle Adipose tissue Hair follicle receptor (root hair plexus) Figure 5.1
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Nails Nails—scalelike modification of epidermis Made of hard keratin
Parts of the nail Free edge Body Root Nail folds Eponychium—cuticle
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Structure of a Nail Lunule Lateral nail fold (a) Free edge of nail
Body of nail Eponychium (cuticle) Root of nail Proximal nail fold Nail matrix (b) Nail bed Phalanx (bone of fingertip) Figure 5.6
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Appendages of the Skin Hair Flexible strand of dead, keratinized cells
Hard keratin—tough and durable Chief parts of a hair Root—imbedded in the skin Shaft—projects above skin's surface
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Appendages of the Skin Hair has three concentric layers of keratinized cells Medulla—central core Cortex—surrounds medulla Cuticle—outermost layer
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Figure 5.7a Structure of a hair and hair follicle.
Hair shaft Arrector pili Sebaceous gland Follicle wall Peripheral connective tissue (fibrous) sheath Hair root Glassy membrane Hair bulb Epithelial root sheath External root sheath Internal root sheath Hair Cuticle Cortex Medulla Diagram of a cross section of a hair within its follicle 30
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Figure 5.7c Structure of a hair and hair follicle.
Hair shaft Follicle wall Peripheral connective tissue (fibrous) sheath Glassy membrane Arrector pili Epithelial root sheath External root sheath Sebaceous gland Internal root sheath Hair root Hair root Cuticle Cortex Hair bulb Medulla Hair matrix Dermal papilla Melanocyte Subcutaneous adipose tissue Diagram of a longitudinal view of the expanded hair bulb of the follicle, which encloses the matrix 31
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Appendages of the Skin Hair follicles Hair bulb Root plexus
Extend from epidermis into dermis Hair bulb Deep, expanded end of the hair follicle Root plexus Knot of sensory nerves around hair bulb
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Skin Structure Hair shaft Dermal papillae Epidermis
Subpapillary vascular plexus Papillary layer Pore Appendages of skin Dermis Reticular layer Eccrine sweat gland Arrector pili muscle Sebaceous (oil) gland Hair follicle Hair root Hypodermis (superficial fascia) Nervous structures Sensory nerve fiber Dermal vascular plexus Lamellar (Pacinian) corpuscle Adipose tissue Hair follicle receptor (root hair plexus) Figure 5.1
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Appendages of the Skin Arrector pili muscle Bundle of smooth muscle
Hair stands erect when arrector pili contracts
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Sebaceous Glands Occur over entire body
Except palms and soles Secrete sebum—an oily substance Most associated with a hair follicle Functions of sebum Collects dirt; softens and lubricates hair and skin
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Dermal connective tissue Hair in hair follicle
Figure 5.8a Skin glands. Sebaceous gland Sweat pore Sebaceous gland duct Dermal connective tissue Hair in hair follicle Eccrine gland Secretory cells Photomicrograph of a sectioned sebaceous gland (90) 36
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Sweat Glands Sweat glands (sudoriferous glands) widely distributed on body Sweat—is a blood filtrate 99% water with some salts Contains traces of metabolic wastes About 2% urea
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Dermal connective tissue
Figure 5.8b Skin glands. Sebaceous gland Sweat pore Eccrine gland Duct Dermal connective tissue Secretory cells Photomicrograph of a sectioned eccrine gland (140) 38
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Sweat Glands Two types of sweat gland Eccrine gland Apocrine gland
Most numerous—these produce true sweat Apocrine gland Confined to axillary, anal, and genital areas Produce a special kind of sweat
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Subpapillary vascular plexus Papillary dermis
Figure 5.1 Skin structure. Hair shaft Dermal papillae Epidermis Subpapillary vascular plexus Papillary dermis Sweat pore Appendages of skin Dermis Reticular dermis Eccrine sweat gland Arrector pili muscle Sebaceous (oil) gland Hair follicle Hair root Hypodermis (superficial fascia) Nervous structures Sensory nerve fiber with free nerve endings Dermal vascular plexus Lamellar corpuscle Adipose tissue Hair follicle receptor (root hair plexus) 40
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