INTEGUMENTARY

• Skin


  • Largest single organ of the body
  • 15%-20% of total body weight
• Functions of the Skin

  • Protection
  • Sensory
  • Thermoregulatory
  • Metabolic
  • Sexual Signalling
• Types of Skin


  • Thick Skin - Covers the palms and soles (400 to 1400um (1.4 mm))
  • Thin Skin - Covers the whole body except palms and the soles (varies from 75 to 150 um)

  
• General Layers of the Skin

  • Epidermis
  • Dermis
  • Hypodermis

  
• Epidermis


  • Primary Tissue Composition: Stratified Squamous Keratinized Epithelium
  • Lacks vasculature
  • Receives nutrients only from the dermis
• Cells of the Epidermis

  • Keratinocytes - Primary Cells
  • Melanocytes - Pigment producing Cells
  • Langerhans Cells - Ag-Presenting Cells
  • Merkel Cells - Sensory (Tactile Epithelial Cells)
• Keratinocytes


  • Primary Cells located at Epidermis
  • Originate in the deepest layer of the epidermis, the stratum basale and move up to the final barrier layer of the skin, the stratum corneum

  
• Melanocytes


  • Located at basal epidermis
  • Neural Crest Derived
  • Pale-staining, rounded cell bodies
  • Synthesize dark melanin pigment in melanosomes
  • Protect nuclear DNA from UV damage
  • Eumelanin - brown or black pigment; found in hair follicles
  • Pheomelanin - red hair
• Diseases related to Melanocytes

  • Albinism - A condition wherein there are less melanin than usual in the body; condition that leads to having very light skin, hair, and eyes; Due to defect in the enzyme tyrosinase
  • Vitiligo - a disorder that causes patches of skin to lose pigment or color; happens when melanocytes are attacked and destroyed
• Langerhans Cells


  • APC (Antigen Presenting Cells)
  • 2-8% of cells in epidermis
  • Located at spinous layer [stratum spinosum] of epidermis
  • Bind, process and present antigens to T-lymphocytes
• Merkel Cells


  • Aka Epithelial Tactile Cells
  • Mechanoreceptors for light touch
  • Abundant in fingertips and bases of some hair follicles
  • Characterized by small, Golgi-derived dense-core neurosecretory granules containing peptides
• Layers of the Epidermis

  • Stratum Corneum - Thick and Thin Skin
  • Stratum Lucidum - Thick Skin
  • Stratum Granulosum - Thick and Thin Skin
  • Stratum Spinosum/Stratum Germinativum - Thick and Thin Skin
  • Stratum Basale - Thick and Thin Skin

  
• Stratum Corneum


  • 15-20 layers of dead, flattened, anucleate, keratin-filled keratinocytes called squames
  • Bound by hydrophobic lipid-rich segment that provides protection against friction and water loss
  • Protects against friction and water loss
  • Superficial
• Stratum Lucidum


  • 2-3 layers of anucleate, dead cells
  • Seen only in thick skin

  
• Stratum Granulosum


  • 3-5 layers of keratinocytes
  • Contains kerato-hyaline Granules - intensely basophilic [have intense affinity in basic dyes]
  • Contains lamellar granules - (Golgi-derived), Produces a lipid-rich, impermeable layer around the cells forming a major part of the skin's barrier against water loss
• Stratum Spinosum or Stratum Germinativum


  • THICKEST LAYER
  • Consists of generally polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins
  • Several layers of keratinocytes all joined by desmosomes
  • Contains tonofibrils (keratin filament bundles) - Bind to our desmosomes [thus, forming spines between keratinocytes in the stratum spinosum]
  • Prickle Cells - Keratinocytes in our stratum spinosum
  • Cells may still divide
  • Langerhans cells present
  • Stratum Germinativum - combination of spinosum and basale; layer wherein stratum spinosum and stratum basal meet - Provides germinal cells necessary for the regeneration of the layer of the epidermis
• Stratum Basale


  • Single layer of cuboidal to low columnar cells in contact with basement membrane
  • Mitosis occurs here
  • Melanocytes and Merkel cells also present
• Dermis


  • Layer of connective tissue [Fibro-adipose tissue]
  • Supports epidermis and binds it to hypodermis [physically and metabolically]
  • Contains projections called dermal papillae; connects with epidermal ridges
  • Filled with blood vessels, nerves and sensory receptors
  • Basement Membrane - Connects dermis with epidermis

  
• Layers of the Dermis

  • Papillary Layer
  • Reticular Layer
• Papillary Layer


  • Connects to epidermis
  • More superficial layer of dermis
  • Contains subpapillary vascular plexus
  • Contains: Loose Connective Tissue, Type I and III Collagen, Mast cells, Dendritic cells, Fibrils of Type VII Collagen [inserted into the basal lamina; help in binding the dermis into the epidermis]

  
• Reticular Layer


  • Thicker than Papillary Layer
  • Deeper layer of dermis
  • Dense irregular connective tissue surrounding hair follicles, skin glands [sebaceous glands and sweat glands], nerves, and deep plexus of blood vessels extending into subcutaneous layer
  • Contains: Dense connective tissue-mostly type I collagen, Fewer cells than papillary layer, Elastic fibers

  
• Both dermal regions contain a rich network of blood and lymphatic vessels
• Nutritive vessels form two major plexuses

  • Subpapillary Vascular Plexus [Superficial Vascular Plexus] - Between the papillary and reticular dermal layers, Form a rich, nutritive capillary network just below the epidermis
  • Deep Plexus - With larger blood and lymphatic vessels lie near the interface of the dermis and the subcutaneous layer
• Thermoregulatory Function of the dermis


  Done by arteriovenous anastomoses located between both plexuses
• Subcutaneous Layer or Hypodermis

  • Layer beneath the dermis and usually consist of adipose tissue and loose connective tissue
  • Contains the larger vessels which supply and drain the dermal blood vasculature
• Sensory Receptors


  • Simple nerve endings with no Schwann cell or collagenous coverings (Unencapsulated)
  • More complex structures with sensory fibers enclosed by glia and delicate connective tissue capsules (Capsulated)
• Unencapsulated Receptors


  • Merkel Cells - tonic receptors for sustained light touch and for sensing an object's texture
  • Free Nerve Endings - in the papillary dermis; respond primarily to high and low temperatures, pain, and itching
  • Root Hair Plexus - surrounding the bases of hair follicles in the reticular dermis that detects movements of the hairs

  
• Capsulated Sensory Receptors


  • Meissner Corpuscles - initiate impulses when there is light-touch or low-frequency stimuli against skin
  • Lamellated (Pacinian) Corpuscles - for detection of pressure or firm touch
  • Krause End Bulbs - simpler encapsulated, ovoid structures, with extremely thin, collagenous capsules penetrated by a sensory fiber
  • Ruffini Corpuscles - collagenous, fusiform capsules anchored firmly to the surrounding connective tissue
• Epidermal Appendages


  • Hair
  • Nails
  • Glands - Sweat Glands, Sebaceous Glands
• Hair


  • Keratinized Structures forming within epidermal evaginations called hair follicles
  • All skin has at least minimal hair except the glabrous skin of the palms, soles, lips, glans penis, clitoris, and labia minora
  • Rapidly undergoing keratinization to form the medulla, cortex, and cuticle of a hair root

  
• Parts of Hair


  • Hair Bulb
  • Dermal Papilla
  • Hair Root
  • Arrector Pili Muscle
• Hair Bulb


  • Terminal dilation; connects to the dermal papilla
  • Internal Root Sheath - surrounds initial part of bulb
  • External Root Sheath - covers IRS; extends all the way to epidermis
• Dermal Papilla


  • Contains capillary network; sustains follicle
• Hair Root


  • Forms base of hair follicle
  • Medulla - large, vacuolated, and moderately keratinized cells; center of root
  • Cortex - densely packed, heavily keratinized; surrounds medulla
  • Cuticle - squamous, thin layer; heavily keratinized; outermost layer of hair root
• Arrector Pili Muscle


  • Consists of smooth muscle; pulls the shaft in erect position
• Nails


  • Nail Root
  • Nail Plate
  • Nail Bed
  • Nail Matrix
• Nail Root


  • Proximal part of the nail
• Nail Plate


  • Bound to a bed of epidermis, covered by a fold of skin, from which the epidermal stratum corneum extends as the cuticle, or eponychium
• Nail Bed


  • Contains only the basal and spinous epidermal layers
• Nail Matrix


  • Cells divide, move distally, and become keratinized in a process somewhat similar to hair formation but without keratohyaline granules