Histology

• histology:

  • study of microscopic structures of tissues and organs of the body
  • requires use of microscope
• tissue:

  • group of cells organised to perform one or more specific functions
• HE staining:

  • hematoxylin in conjunction with counterstain easin = stain
  • hematoxylin = base that binds acidic components of the cell such as DNA and RNA, staining them blue, referred to as basophilic
  • eosin = acid that binds to cytoplasmic constituents that have basic pH, staining them pink, referred to as acidophilic
• four basic tissues types:

  • epithelial = covers body surfaces and forms glands
  • connetcive = support structurally and functionally
  • muscle = contarctile cells for movement
  • nerve = receives and transmits information and controls activities
• epithelium:

  • avascular
  • cover exterior body surfaces
  • body tubes that communicate with exterior
  • secretory portion of parenchyma
• epithelium functions:

  • protection from abrasion and injury
  • transport of molecules across sheets
  • secrteion of mucinogen, hormones and enzymes
  • absorption of material from lumen
  • selective permeability = control of substances
  • detection of sensations = taste buds
• epithelial junction complexes:

  • tight junctions
  • gap junctions
  • desmosomes
• tight junctions:

  • impermeable
  • hold cells together by very close lateral surfaces
  • edges joined by kiss sites = tight seal
  • claudins (proteins) fuse
  • water tight blocking everything
  • in bladder, kidney and intestines
• desmosomes:

  • adhering junction that spot rivet cells
  • function is to anchor cells together subject to stretching
  • attachment = cadherins
  • water and ions can flow in between
  • most abundant in skin, heart and uterus
• gap junctions:

  • communicating junctions
  • made up of connexons
  • form a tunnel that permits movement of charge-carrying ions
  • movement of ions - transmit electrical activity
  • found in cardiac muscle and neurons
• epithelial cells:

  • has apical, basal and lateral location
  • lateral only place for junctions
  • basal surface attached to basement membrane
• classification of epithelial tissues based on:

  • number of cell layers = simple (1) or stratified (multiple)
  • shape of surface cells = squamous (flat), cuboidal (cube) or columnar (tall and thin)
  • keratinised = lost nucleus as packaged with keratin
  • transitional = only in urinary system as reflex and distend as dome shaped, changes depending on functional state
• structure-function correlation:

  • simple = diffusion, absorption and secretion = regulated transport of cells/molecules/gases
  • stratified = protection and barrier to water and disease
• structural surface modifications:

  • microvilli = actin = increased SA for absorption and transport
  • sterocilia = very long making protrusions for absorption and sensitivity for inner ear signals
  • cilia = microtubules = motile which transports sceretions and primary which acts as sensory antennae
• epithelial cell renewal:

  • self renew
  • rate of cell turnover is very specific
  • replacement cells produced by mitotic division of adult stem cells in niches
• connective tissue:

  • provides structural and metabolic support
  • can also be called supporting tissue
  • fluid connective = blood and lymphatic
  • supporting = cartilage
• general structure:

  • mesenchymal stem cells = replicate to connetcive tissue following injury
  • fibroblasts = maintain extracellular material
  • adipocytes = storage and metabolism of lipids
  • cells with defense and immune function
• connetcive tissue different:

  • cell poor and matrix rich
  • diverse cellular component
  • no basement membrane
  • vascularised
  • never exposed to external environment
• connective tissue function:

  • structural framework
  • protect organs
  • insulate body
  • defend against pathogens
• classification of connective tissue:

  • loose = areolar = least specialised but most common, acts as space occupying tissue, absorbing shock and acts as reservoir for salts and water
  • dense irregular = mesh of fibres providing physical support found in dermis of skin
  • dense regular = bundles of fibres providing tensile strength and found in ligaments and tendons
• specialised forms of connective tissue:

  • adipose
  • cartilage
  • blood
  • bone
• collagen fibres:

  • main fibre of connective tissue
  • 29 forms
  • present as filaments, fibrils and meshworks
• unilocular adipose tissue:

  • white
  • located in deep skin, arranged in sheets amongst supporting tissue = fibrofatty tissue
  • functions = energy store, thermal insulation, cushioning and hormonal role
  • seen as little empty droplets
• multilocular adipose tissue:

  • brown
  • prominent in foetus
  • numerous small lip is droplets
  • function to metabolise fat during neonatal period to procure heat and maintain temperature
  • high mitochondrial content
• basement membrane:

  • boundary between epithelial tissue and underlying connective tissue
  • composed of 3 layers
  • function = structural support, contorl of epithelial growth, diffusion
• extracellualr matric of connective tissues composed of:

  • ground substance
  • fibres = collagen and elastin
  • structural glycoproteins
• 4 groups of contractile cells:

  • muscle
  • myofibroblasts
  • myoepithelial
  • pericytes
• myofibroblasts:

  • spindle shaped
  • can secrete collagen to firm scaffold = scar
  • contractile properties
  • inactive porpialuion of cells
  • proliferate following tissue damage
• myoepithelial cells:

  • found in exocrine glands
  • located between glandular epithelium and basement membrane
  • expel secretions and prevent end piece distension
  • stellate morphology = contractile meshwork
• pericytes:

  • stellate cells found circumferentially around capillaries
  • contain actin and myosin
  • relatively undifferentiated can develop into fibroblasts or smooth muscle cell following injury
• muscle cell classsification:

  • skeletal
  • cardiac
  • smooth
• skeletal muscle:

  • voluntary
  • striated
  • responsible for movement of skeleton
  • connective tissues surrounfs muscle fibres fasciculi = mechanically transmit forces generated by contraction of muscles
  • 3 levels: muscle = myofibril= myofilament
• skeletal muscle fibre types:

  • type I = slow twitch = aerobic metabolism = red = slow fatigue = narrow diameter
  • type IIa = fast twitch = anaerobic metabolism = red = moderate fatigue = intermediate diameter
  • type IIb = fast twitch = anaerobic metabolism = white = rapid fatigue wide diameter
• sarcomere:

  • z line to z line
  • thick filament = myosin
  • thin filament = actin = I band
  • myosin and actin = H zone
• sliding filament theory:

  • when muscle contracts, the thin filament slide past thick
  • sarcomere shortens
• actin:

  • troponin
  • tropomyosin
  • nebulin = helps align actin
• myosin:

  • head
  • titin = elasticity and stabilises myosin
• cardiac muscle:

  • involuntary
  • straited
  • myocardium
  • branched fibres joined by intercalated discs
  • centrally located nuclei
• cardiac muscle function:

  • rhythmic contraction of heart
  • influenced by external automatic and hormonal stimuli
• cardiac muscle diggers form skeletal:

  • cardiac muscle = shorter with nucleus
  • fibres appear forked end to end with intercalated discs to facilitate anchorage and conductivity
  • absence of stem cells so no regeneration