Chapter 02 Cytology Notes

Microscopy and Imaging

  • Microscopy types and purpose
    • Light microscope (LM)
    • Uses visible light to produce the image; most often used in teaching and clinical labs.
    • Least magnification among common microscope types but useful for live cells and general structure.
    • Transmission electron microscope (TEM)
    • Uses electrons to produce highly detailed internal structures; very high resolution for viewing organelles and ultrastructure.
    • Scanning electron microscope (SEM)
    • Produces 3D-like surface images; excellent for external morphology of cells and textures.
  • Key concept: Magnification vs. resolution
    • Magnification increases apparent size, while resolution is the ability to reveal fine detail and separate adjacent features.
    • LM vs TEM vs SEM differences illustrated in Figures (e.g., Fig. 2.1a,b; Fig. 2.2a,b,c).
  • RBC (red blood cell) imaging across modalities
    • Images of RBCs produced by LM, SEM, and TEM show progressive detail from surface outline to internal features.
    • Example dimensions mentioned for reference:
    • LM and SEM examples show cells around
      ext{10.0 }\mu ext{m} scale (SEM image) and
    • TEM can reveal structures as small as a few hundred nanometers (noted by scale bars in figures).
  • Units and scale notes
    • Micrometer:
      ext{1 micrometer} = 10^{-6} ext{ m} = oldsymbol{1} ext{ } oldsymbol{ imes} oldsymbol{10^{-6}} ext{ m}
    • Naked eye resolution: about
      ext{100 }oldsymbol{BC} ext{m} (100 μm).

Cell Shapes and Sizes

  • Common cell shapes (Fig. 2.3):
    • Squamous: flat and scale-like
    • Cuboidal: as tall as they are wide
    • Columnar: taller than wide
    • Polygonal: irregular, multi-angled
    • Stellate: star-shaped with multiple processes
    • Spheroidal: ball-shaped
    • Discoidal: disc-like
    • Fusiform: spindle-shaped (thicker in middle, tapered ends)
    • Fibrous: elongated and thread-like
  • Cells vary in size and shape depending on function and tissue context.

Cellular Terminology and Basic Units

  • Polar surfaces of a cell

    • Basal surface: oriented toward the basal membrane.
    • Apical surface: faces toward lumen or external environment.
    • Lateral surface: faces adjacent cells.

  • Measurement unit

    • Micrometer (
      oldsymbol{BC}D

    )

    • One-millionth of a meter:
      1 ext{ }oldsymbol{BC} ext{m} = 10^{-6} ext{ m}

  • Basic cell concepts

    • Cell components include:
    • Plasma membrane
    • Cytoplasm
    • Cytoskeleton
    • Organelles
    • Inclusions
    • Cytosol
    • Nucleus is an organelle containing nucleoplasm.

Generalized Cell and Organelles (Fig. 2.5, 2.8)

  • Generalized cell layout (apical, lateral, basal surfaces) with major organelles and structures:
    • Microvilli: plasma membrane extensions increasing surface area; often called brush border.
    • Desmosomes and Hemidesmosomes: cell–cell and cell–basement membrane adhesions respectively.
    • Secretory vesicle, Golgi vesicles, Golgi complex: involved in processing and shipping proteins.
    • Rough endoplasmic reticulum (RER): studded with ribosomes; synthesizes phospholipids and proteins for plasma membrane, secretion, and lysosomes.
    • Smooth endoplasmic reticulum (SER): detoxification in certain cells and steroid synthesis in others.
    • Nucleus, Nucleolus, Nuclear envelope; various components shown in cross-section.
    • Mitochondrion: site of ATP production.
    • Cytoskeletal components: microfilaments, intermediate filaments, microtubules; provide structure and transport routes.
    • Lysosome, peroxisome: organelles involved in digestion and reactive oxygen species management.
    • Centrioles, Centrosome: organization center for microtubules during cell division.
  • Functional coordination
    • The plasma membrane, cytoskeletal framework, and organelles coordinate to maintain cell structure, transport, signaling, and metabolism.

Plasma Membrane Composition and Proteins

  • Membrane composition (Fig. 2.9, 2-11)
    • Phospholipids: ~75% of membrane lipids; form bilayer with hydrophilic heads facing aqueous environments and hydrophobic tails inward.
    • Cholesterol: ~20%; modulates fluidity and stability.
    • Glycolipids: ~5%; contribute to glycocalyx.
  • Membrane proteins types
    • Integral (transmembrane) proteins: span the membrane.
    • Peripheral proteins: associated with membrane faces but do not span the bilayer.
    • Glycoproteins: membrane proteins with carbohydrate chains; part of glycocalyx.
  • Membrane architecture
    • Phospholipid bilayer forms the structural foundation.
    • Glycolipids and glycoproteins extend carbohydrate chains to the extracellular surface, forming glycocalyx.
    • Cytoskeleton-associated proteins help anchor membrane and contribute to cell shape.
  • Functional roles of membrane proteins (Fig. 2.9)
    • Receptor proteins: receive signals from the environment or other cells.
    • Enzymes: catalyze reactions at the membrane surface.
    • Channel proteins: form pores that allow specific ions or molecules to pass.
    • Transport proteins: move substances across the membrane.
    • Cell-identity markers: identify the cell as self or non-self.
    • Cell-adhesion molecules (CAMs): enable cells to attach to each other or to the extracellular matrix.
  • Membrane polarity and transport interfaces
    • Extracellular face and intracellular face of the membrane show distinct protein and lipid arrangements (Fig. 2.6b).

Membrane Transport Mechanisms

  • Filtration (Fig. 2.10a)
    • Capillary blood pressure forces water and small solutes through clefts between cells.
    • Big solutes and blood cells are held back by the clefts (selective barrier).
  • Simple diffusion (Fig. 2.10b)
    • Lipid-soluble solutes diffuse directly through the phospholipid bilayer down their concentration gradient.
    • Water-soluble solutes diffuse through channel proteins (pores) down their concentration gradient.
    • Net movement from high to low concentration.
  • Osmosis (special case of diffusion)
    • Movement of water across a selectively permeable membrane from the more watery (higher water potential) to the less watery side.
  • Facilitated diffusion (Fig. 2.10c)
    • Solute binds to a receptor site on a high-affinity transport protein.
    • Transport protein changes shape to shuttle solute across the membrane down its concentration gradient.
    • Does not require ATP; driven by gradient.
  • Active transport (Fig. 2.10d)
    • Solute binds to receptor site on transport protein; ATP hydrolysis provides energy.
    • Phosphate (P) binds to the protein, inducing a conformational change to move solute against its gradient.
    • Requires cellular energy; can create or maintain concentration gradients.
    • Typical schematic includes ATP → ADP conversion and P transfer to transporter protein.
  • Vesicular (bulk) transport (Fig. 2.11)
    • Pinocytosis (cell drinking): uptake of extracellular fluid and dissolved solutes via pinch-off vesicles.
    • Receptor-mediated endocytosis: selective uptake of specific ligands bound to membrane receptors.
    • Exocytosis: secretory vesicles fuse with the plasma membrane to release contents outside the cell.
    • All involve vesicle formation, trafficking, and fusion to deliver cargo.

Surface Extensions and External Features

  • Microvilli
    • Plasma membrane extensions that increase surface area, enhancing absorption and signaling.
  • Cilia
    • Primary cilium: non-motile, sensory antenna for signaling pathways.
    • Motile cilia: contain an axoneme of microtubules; move substances across cell surfaces.
    • Axoneme structure: central microtubule core with dynein arms (motor proteins) driving bending.
  • Flagella
    • Long axoneme; primarily used to propel sperm cells.
  • Visual references
    • Figures 2.12a, 2.13 show microvilli, cilia, and axoneme structure.

Glycocalyx and Cellular Junctions

  • Glycocalyx
    • Carbohydrate-rich “fuzzy” coating on the extracellular side of the membrane.
    • Functions: protection, cell identity, and binding to tissues.
  • Cellular junctions (Fig. 2.15)
    • Tight junctions: seal neighboring cells to prevent paracellular leakage.
    • Desmosomes: resist mechanical stress by linking cytoskeletons of adjacent cells.
    • Gap junctions: allow direct chemical communication between neighboring cells.

The Cell Interior: Cytosol, Cytoskeleton, and Inclusions

  • Cytosol
    • Fluid portion of the cytoplasm, containing dissolved solutes and ions.
  • Cytoskeleton
    • Structural framework of the cell; determines shape; organizes contents; moves substances and sometimes the cell itself.
    • Three main components:
    • Microfilaments and terminal web (thin filaments): support cell cortex and help with movement.
    • Intermediate filaments: provide tensile strength and structural integrity.
    • Microtubules: provide routes for intracellular transport and form the mitotic spindle.
  • Organelles
    • “Little organs”; metabolically active; compartmentalize cellular contents for specialized functions.
  • Inclusions
    • Not essential for cell survival; stores cellular products or foreign matter.
    • Examples: pigments, fat droplets, granules of glycogen; dust particles, viruses, and intracellular bacteria (nonmetabolic inclusions).

The Cytoskeleton in Detail (Fig. 2.16)

  • Roles of cytoskeletal elements
    • Protein filaments and tubules support cell structure, determine cell shape, organize cellular contents, move substances, and can drive cell movement.
  • Interaction map
    • Microfilaments, microtubules, intermediate filaments interact with organelles like lysosomes, mitochondria, nucleus, centrosome, and motor proteins (e.g., kinesin) to coordinate transport and organization.
  • Visual cues from figures:
    • Secretory vesicle in transport, desmosomes, microvilli, organelle distribution around the cytoskeleton, basal membrane context.

Organelles: The Functional Units Within the Cell (Fig. 2.18–2.19e)

  • Nucleus
    • Largest organelle; contains the cell’s chromosomes; genetic control center.
    • Functions include production of ribosomes.
    • Key components: nuclear envelope, nuclear pores, nucleoplasm, chromosomes, nucleoli.
  • Endoplasmic reticulum (ER)
    • “Little network within the cytoplasm” with cisterns.
    • Rough ER (RER): studded with ribosomes; synthesizes phospholipids and proteins for the plasma membrane; proteins for secretion and lysosomes.
    • Smooth ER (SER): detoxification in cells; synthesizes steroids in steroid-producing cells.
  • Ribosomes
    • Locations: cytosol, rough ER, nuclear envelope, nucleoli, mitochondria.
    • Function: translate RNA into proteins; read mRNA and assemble amino acids into polypeptide chains.
  • Golgi complex
    • Consists of cisterns; receives transport vesicles from the RER; forms Golgi vesicles containing packaged proteins.
    • Functions include lysosome formation, directing proteins to the plasma membrane or secretion via secretory vesicles.
  • Proteasomes
    • Cylindrical complexes that degrade and recycle damaged or unneeded proteins; degrade ~80% of a cell’s proteins.
  • Lysosomes
    • Contain enzymes in a single-unit membrane; clean up cell via autophagy (degrading organelles) and apoptosis (programmed cell death).
  • Peroxisomes
    • Similar to lysosomes; oxidize fatty acids and other organic molecules; produce hydrogen peroxide and degrade it with catalase; abundant in liver and kidneys.
  • Mitochondria
    • Powerhouses of the cell; specialized for aerobic respiration and ATP production.
    • Structure: outer and inner membranes; cristae; mitochondrial matrix; contain mitochondrial DNA (mtDNA).
  • Centrioles
    • Composed of microtubules in a 9-triplet arrangement.
    • Centrosome: cytoplasm region that contains the perpendicular pair of centrioles; important for organizing spindle during mitosis.
    • Basal body: foundation for cilia and flagella.
  • Inclusions (nonessential structures)
    • Pigments, lipid or glycogen granules, and stored products; foreign materials like dust or bacteria can also accumulate as inclusions.

The Cell Cycle: Growth, Replication, and Division (Figs. 2.39–2.43)

  • Major phases
    • Interphase: growth, metabolic activity, and DNA replication.
    • G1 (First gap phase): growth and normal metabolic roles.
    • S (Synthesis phase): DNA replication.
    • G2 (Second gap phase): growth and preparation for mitosis; DNA proofreading.
    • Mitotic phase (M): division of nuclear material and cytoplasm.
    • Prophase: chromatin condenses; nuclear envelope breaks down; nucleolus disappears; spindle fibers form and attach to kinetochores.
    • Metaphase: chromosomes align at the cell center; asters attach to plasma membrane.
    • Anaphase: centromeres split; sister chromatids pulled to opposite poles.
    • Telophase: chromatids arrive at poles; chromosomes decondense; new nuclear envelope forms; nucleoli reform; mitotic spindle vanishes.
    • Cytokinesis: division of cytoplasm; cleavage furrow forms; cell splits into two identical daughter cells.
  • Visual reference: Fig. 2.23 illustrates stages of mitosis with labeled structures (centrioles, chromatids, kinetochores, mitotic spindle).

Stem Cells and Developmental Potentials

  • Stem cells defined
    • Immature cells capable of developing into one or more mature, specialized cell types; possess developmental plasticity.
  • Adult stem (AS) cells
    • Present in most body organs; responsible for normal turnover and maintenance of tissue.
    • Multipotent example: bone marrow cells (can differentiate into several related cell types).
  • Embryonic stem (ES) cells
    • Derived from early embryo (up to ~150 cells in embryo stage); pluripotent, meaning they can differentiate into many cell types.
    • Considered excess supply from in vitro fertilization contexts.

Connections and Relevance

  • Foundational principles
    • Structure-function relationships are evident: membrane composition affects fluidity and transport, organelle distribution supports metabolism and protein processing, cytoskeletal networks determine shape and transport pathways.
    • Transport mechanisms (diffusion, osmosis, facilitated diffusion, active transport, vesicular transport) underpin nutrient uptake, waste removal, and signaling.
  • Real-world relevance
    • Understanding plasma membrane dynamics is essential for pharmacology (drug transport), pathophysiology (membrane dysfunction, junction disorders), and cell biology techniques (microscopy).
    • Mitochondrial function and dynamics relate to energy metabolism and diseases; lysosomal and proteasomal pathways are central to cellular quality control and cancer biology.
  • Ethical/philosophical notes
    • Stem cell plasticity and the use of ES cells involve ongoing ethical discussions about embryo use; iPS cell technology offers alternatives by reprogramming adult cells.
  • Key symbols and equations used in this unit
    • Membrane composition (by fraction):
    • ext{Phospholipids} = 0.75 of membrane
    • ext{Cholesterol} = 0.20 of membrane
    • ext{Glycolipids} = 0.05 of membrane
    • Microscopic scale units:
    • 1 ext{ μm} = 10^{-6} ext{ m}$$
    • Typical cellular processes involve energy, gradients, and vesicular trafficking described qualitatively alongside the quantitative framework above.