Histology; Microscopy and Ultrastructure of the Cell

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Histology

Why Histology?

Where does Histology fit?

Endpoint?

  • Clinical application
  • VERY topical example
  • Aging immunity may exacerbate COVID-19
    • Arne N. Akbar, Derek W. Gilroy
    • Science 17 Jul 2020:
    • Vol. 369, Issue 6501, pp. 256-257
    • DOI: 10.1126/science.abb0762

Find the Histology:

Catnip

Histology: Foundational Knowledge

Levels of Organisation

Why histology?

  • Knowledge
    • Because all knowledge is worthwhile
    • The more you know, the more you are able to know
  • Know the normal to identify the abnormal
    • Pathology
  • Functions of many organs
    • Lung, Kidney, Liver
    • Informed by cellular organisation
    • Physiology
  • Structure of body informed by cellular organisation
    • Anatomy
  • Inflammation, Repair & Treatment dependent on cellular organisation
    • Pharmacology

What in Histology

Basic framework to work from

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Histology How

Histology is a visual subject.

You have to look at, identify, draw.

Pictures, illustrations, diagrams.

The Cell

Ultrastructure of the cell

The cell

  • Smallest unit living matter
  • Two compartments
  • Nucleus
    • Contains chromatin -> genetic material
  • Cytoplasm
    • Organelles -> metabolically active units
    • Inclusions -> inert accumulations
    • Cytoskeleton

Nucleus

  • Contains genetic material
  • Encoded in DNA of chromosomes
  • Euchromatin
    • lightly stained
    • dispersed chromatin
    • being transcribed
  • Heterochromatin
    • densely stained
    • not being transcribed

Nuclear envelope

  • Two parallel membranes
  • Separated by peri-nuclear space
  • Inner and outer continuous at nuclear pore
  • Nuclear pore interrupt double membrane
  • Outer nuclear membrane
  • Ribosomes attached
  • Continuous with endoplasmic reticulum
  • Inner nuclear membrane
  • Meshwork interwoven filaments
  • Fibrous lamina (nuclear lamina)
  • Anchoring site for interphase chromosomes

Nuclear pore

  • Membrane-bounded channel between nucleus and cytoplasm
  • Outer and inner membrane continuous around pore
  • Provide communication between nucleus and cytoplasm

Nucleolus

  • Well-defined nuclear inclusion
  • Seen in cells actively synthesizing protein
  • Involved in
    • Synthesis rRNA
    • Packaging rRNA into precursors of ribosomes

Cytoplasm organelles

  • Cell membrane
  • Glycocalyx
  • Ribosomes
  • Rough endoplasmic reticulum
  • Smooth endoplasmic reticulum
  • Mitochondria
  • Golgi apparatus
  • Lysosomes
  • Residual body
  • Peroxisome
  • Centrioles

Cytoplasm Cytoskeleton

  • Microtubules
  • Actin microfilaments
  • Myosin filaments
  • Myosin
  • Intermediate filaments
  • Keratin filaments
  • Desmin filaments
  • Vimentin filaments
  • Neurofilaments
  • Glial filaments
  • Microtrabecular lattice

Cytoplasm Inclusions

  • Glycogen
  • Lipid droplets
  • Secretion granules

Cell membrane

  • Surrounds cell
  • Boundary between cell & outside world
  • Phospholipid bilayer with integral proteins
  • Phospholipid
  • Hydrophilic ends face outwards
  • Hydrophobic chains project inwards
  • Protein position varies

Glycocalyx

  • On external surface of plasma membrane
  • Plays role in immunological specificity
  • Contains blood group antigens
  • Has receptor sites
  • Protective mechanical barrier
  • Can contain enzymes

Ribosomes

  • Composed of rRNA & proteins
  • Cluster in groups along mRNA to form polyribosomes
  • Synthesize proteins

Rough endoplasmic reticulum

  • Sheets & cavities bound by membranes
  • Outer surface studded with ribosomes
  • Interior region called cisternae
  • Has receptors where ribosomes bind
  • Common in cells synthesizing proteins for export
    • ie Secretory proteins

Smooth endoplasmic reticulum

  • Irregular network membrane-bound channels
  • Without ribosomes
  • Branching & anastomosing tubules & vesicles
  • Function depends on cell
    • Steroid hormone synthesis
    • Drug detoxification
    • Metabolism of lipids & cholesterol
    • Calcium ion homeostasis

Lysosomes

  • Membrane-bound dense bodies
  • Contains hydrolytic enzymes
  • Functions in intracellular digestion

Mitochondria

  • Rod-shaped organelles
  • Double-membrane structure
    • Outer membrane surrounds entire organelle
    • Inner membrane folds into interior
  • Inner membrane form cristae
  • Have enzymes for Krebs cycle & fatty acid oxidation
  • Contain own genetic apparatus
    • DNA, mRNA, tRNA & rRNA
  • Produces adenosine triphosphate (ATP)

Golgi apparatus

  • Several disk-shaped cisternae arranged in a stack
  • Two sides on stack
    • Outer convex forming face
      • Associated with small vesicles
      • Usually from rough endoplasmic reticulum
    • Inner concave maturing face
      • Associated with condensing vacuoles
      • Secretory materials condensed into granules
  • Functions
    • Membrane recycling & redistribution
    • Synthesis carbohydrates & proteins
    • Modification of cellular products
    • Concentration & packaging of synthesized material into secretory granules

Residual body

  • Cytoplasmic inclusion
  • Varies in appearance
  • Contains indigestible material

Peroxisome

  • Membrane-bound organelle
  • Stains cytochemically for catalase
  • Catalase synthesizes and destroys hydrogen peroxide
  • Often seen in close association with smooth ER
  • Function
    • Metabolism of
      • hydrogen peroxide
      • cholesterol
      • lipids
    • Breakdown of
      • purines
      • pyrimidines

Centrioles

  • Involved cell division
  • Pair of short rods at right angles to each other
  • Self-replicate prior to cell division
  • Wall of nine tubular triplets like pinwheel
  • Function
    • Form poles of mitotic spindles
    • Form basal bodies for cilia & flagella

Cytoplasm Cytoskeleton

  • Microtubules
  • Actin microfilaments
  • Myosin filaments
  • Myosin
  • Intermediate filaments
  • Keratin filaments
  • Desmin filaments
  • Vimentin filaments
  • Neurofilaments
  • Glial filaments
  • Microtrabecular lattice

Microtubules

  • Straight structures 25 nm x several μm length
  • Labile population
    • Free in cytoplasm
    • Polymerize-depolymerize depending on
    • Temperature, pressure, drugs, etc
  • Stable population
    • Walls of centrioles
    • Axonemes of cilia & flagella
  • Wall 5 nm thick
    • around lumen-like region
    • containing 13 spirally arranged protofilaments
  • Associated with intracellular transport
  • Maintains cell shape
  • Promote movement of cilia, flagella & chromosomes

Actin microfilaments

  • Thin filaments
  • Involved with
    • Ameboid movement
    • Cytoplasmic streaming
    • Contractile ring formation
    • Muscle contraction

Myosin filaments

  • Thick filaments
  • Associated with actin in muscle cells
  • Visible as striations in striated muscle

Intermediate filaments

  • Heterogenous group
  • Includes
  • Keratin
    • Found in epithelial cells
    • Associated with desmosomes
  • Vimentin
  • Desmin
  • Neurofilaments
    • Support long processes of nerve cells
  • Glial filaments
    • Present in nonneuronal cells of central nervous system
    • Astrocytes, oligodendrocytes, microglia

Keratin filaments

Neurofilaments

Glial filaments

Cytoplasm Inclusions

  • Glycogen
    • Storage form of carbohydrate
    • Varying appearance depending on stain
  • Lipid droplets
    • Varying appearance depending on stain
  • Secretion granules
    • Varying appearance
    • Include mucous droplets, hormones, proteins, pigment

Cell surface modifications

  • Cell surfaces of epithelia has specific modifications, relating to the function of epithelia
  • Three surfaces show modifications
  • Lateral surface
    • Junctional complexes
    • Gap junctions
    • Lateral interdigitations
  • Basal surface
    • Basal lamina
    • Hemidesmosomes
    • Basal plasma membrane infoldings
  • Apical surface
    • Microvilli
    • Stereocilia
    • Cilia

Epithelia

Lines internal -

Covers external -

Surfaces of the body

Three surface areas

  • Apical surface
  • Face the lumen or external environment
  • Lateral surface
  • Where cells are in contact with each other
  • Basal surface
  • Bottom edge of cell adjacent to basement membrane

Lateral surface

  • Junctional complexes
  • Gap junctions
  • Lateral interdigitations

Junctional complexes

  • Three components
  • Zonula occludens (tight junction)
    • Outer part of adjacent plasma membranes fuse
    • Around entire apical perimeter of cells
    • Prevents entrance or exit into intercellular space
  • Zonula adherens
    • Plasma membranes separated by 10 - 20 nm
    • Reinforced on cytoplasmic surface by actin filaments
    • Adhesive and structurally supportive
  • Macula adherens
    • Focal disk shaped adhesive junction
    • Plasma membrane separated by 15 - 30 nm
    • Intermediate filaments loop from cytoplasma
    • Dense material in intercellular space

Gap junctions

  • Not limited to epithelial cells
  • Also present in CNS, cardiac muscle & smooth muscle
  • Opposing plasma membranes separated by 2 nm gap
  • Gap is bridged by connexons
  • Permit passage of ions & small molecules
  • Intercellular communication

Lateral interdigitations

  • Finger-like projections interlocking adjacent cells

Basal surface

  • Basal lamina
  • Hemidesmosomes
  • Basal plasma membrane infoldings

Basal lamina

  • Acellular support structure
  • 20 - 100 nm thick
  • Visible by electron microscope
  • Produced by epithelium resting on it
  • Consists of two zones
    • Lamina rara - low density, next to plasma membrane
    • Lamina dense - dense filament network, next to CT
    • Together = basement membrane of LM

Hemidesmosomes

  • Appearance of one-half of desmosome
  • Present along basal surface of some cells
  • Attaches cell to underlying basal lamina

Basal plasma membrane infoldings

  • Seen in ion-transporting epithelia
  • Deep invaginations
  • Compartmentalize mitochondria
  • Ion pumps in plasmalemma in close contact with energy supply (ATP from mitochondria)

Apical surface

  • Microvilli
  • Stereocilia
  • Cilia

Microvilli

  • Finger-like projections of epithelium
  • Approximately 1 µm in length
  • Increase absorptive surface
  • Brush border of kidney proximal tubule cells
  • Striated border of intestinal absorptive cells
  • Actin filaments in core

Stereocilia

  • Very long microvilli
  • Not true cilia
  • Epididymus & vas deferens

Cilia

  • Active motile specializations of epithelia that transports substances along their surface
  • 5 - 10 µm in length
  • Covered by plasma membrane
  • Contains axoneme
  • Nine pairs double tubules around two single microtubules (9+2 pattern)
  • At base cylindrical basal body with 9+0 pattern

Cell cycle

  • Consists of two main states and four phases
  • Interphase
    • Gap 1: cell increase in size, prepare for DNA synthesis
    • Synthesis: DNA replication
    • Gap 2: continue growth; prepare for division
  • Cell division
    • Mitosis: growth stop; division into daughter cells
  • Resting = Gap 0
    • Cell left the cycle and stopped dividing

Cell division

  • Mitosis
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
    • Replicated chromosomes separated into 2 new nuclei
    • 2 identical diploid cells
  • Meiosis
    • Cell division used to produce sperm or egg cells
    • Two rounds of division
    • 4 haploid cells

Mesenchymal stem cells undergoing mitosis.

Mitosis

Evilonan; Wikipedia; CC-BY-SA 4.0

Chromosomes and SARS-CoV-2

The End