Structure of the Cell: Microscopic Structure of Cellular Compartments

HISTOLOGY AND MICROSCOPY

  • Histology Definitions: Study of cells, tissues, and organs, correlating structure with function.

  • Tools:   - Light Microscope (LM): Most common tool.   - Virtual Microscopy: Digitalized specimens on computer screens.   - Electron Microscope (EM): Includes Transmission (TEM) and Scanning (SEM) for higher resolution.

  • Resolving Power: SEM shows internal surface contours in 3D3D; TEM provides detailed internal sections.

DYES AND STAINING

  • H&E Routine: Hematoxylin and Eosin mixture.

  • Basic Dyes (Hematoxylin):   - Charge: Positive (cationic).   - Targets: Anionic components (phosphate groups in nucleic acids, sulphate in glycosaminoglycans).   - Result: Basophilic (Blue to Purple).   - Examples: Nucleus, nucleolus, rER, cartilage matrix.

  • Acidic Dyes (Eosin):   - Charge: Negative (anionic).   - Targets: Cationic groups (amino groups of proteins).   - Result: Acidophilic (Pink to Red).   - Examples: Cytoplasmic filaments, intracellular membranes, extracellular fibers.

  • TEM Heavy Metals: Lead citrate and uranyl acetate (electron dense appear dark; electron lucent appear light).

EPITHELIAL CLASSIFICATION

  • Simple Epithelium: All cells contact basal lamina.   - Squamous: Rapid exchange (endothelium, mesothelium).   - Cuboidal: Absorption and secretion (kidney tubules, thyroid follicles).   - Columnar: Absorption and secretion (small intestine).

  • Stratified Epithelium: >1 layer; named by surface cell shape.   - Squamous: Protective; keratinized (skin) or non-keratinized (esophagus).

  • Pseudostratified: All cells touch basal lamina, but not all reach the lumen (trachea, epididymis).

  • Transitional (Urothelium): Specialized for urinary bladder stretching.

MEMBRANOUS ORGANELLES

  • Plasma Membrane: Lipid bilayer forming cell boundary (0.008−0.01μm0.008-0.01μm).

  • Rough Endoplasmic Reticulum (rER): Ribosome-associated; site of protein synthesis, folding, and glycosylation. Well-developed in secretory cells (e.g., plasma cells, fibroblasts) and neurons (Nissl bodies).

  • Smooth Endoplasmic Reticulum (sER): Detoxification (liver), lipid/steroid synthesis (Leydig cells), and Ca2+Ca^{2+} regulation (sarcoplasmic reticulum in muscle).

  • Golgi Apparatus: Modifies, sorts, and packages proteins.   - Phases: Cis-face (entry), Medial, Trans-face (exit).   - Vesicle Coats: COP II (Anterograde: TER to Golgi), COP I (Retrograde: Golgi to ER), Clathrin (from trans-Golgi to lysosomes or membrane).

  • Mitochondria: Double membrane; site of ATP production via oxidative phosphorylation. Absent in red blood cells.

  • Endosomes/Lysosomes: Sorting and degradation of macromolecules.

  • Peroxisomes: Degradation of fatty acids and H2O2H_2O_2.

NON-MEMBRANOUS ORGANELLES AND CYTOSKELETON

  • Ribosomes: Composed of rRNA and protein; synthesize proteins.   - Polyribosomes: Synthesize proteins remaining in the cell (e.g., hemoglobin, keratin).

  • Proteasomes: Degrade ubiquitin-tagged proteins.

  • Microtubules: 25 nm25\,nm diameter; composed of α\alpha and β\beta tubulin dimers. Polarized with a growing plus end.

  • Actin Filaments: 6−8 nm6-8\,nm (Thin filaments); G-actin (free) and F-actin (polymerized).

  • Intermediate Filaments: 8−10 nm8-10\,nm; provide tensile strength; tissue-specific (e.g., Keratins in skin, Lamins in nuclear envelope).

  • Centrioles: Paired cylinders in the centrosome; form basal bodies for cilia.

THE NUCLEUS

  • Components: Nuclear envelope, Chromatin, Nucleolus, and Nucleoplasm.

  • Nuclear Envelope: Double membrane with Nuclear Pore Complexes (88 protein subunits). Outer membrane continuous with rER.

  • Nuclear Lamina: Fibrous meshwork of Lamins A, B, and C associated with the inner membrane.

  • Nucleolus: Spherical, non-membranous; site of rRNA synthesis and ribosome assembly. Contains DNA loops from chromosomes 13,14,15,21,2213, 14, 15, 21, 22.

  • Chromatin:   - Euchromatin: Dispersed, pale-staining; transcriptionally active.   - Heterochromatin: Condensed, dark-staining; transcriptionally inactive.

  • Nuclear Count: Usually single; double in hepatocytes; multiple in osteoclasts and skeletal muscle.

CYTOPLASMIC INCLUSIONS

  • Lipofuscin: "Wear and tear" pigment; brownish-gold; seen in aging neurons/muscle cells.

  • Hemosiderin: Iron-containing brown pigment from hemoglobin breakdown.

  • Melanin: Brownish-black pigment in melanocytes.

  • Lipid Droplets: Non-membranous energy storage.

  • Glycogen: Branched polymer for glucose storage (common in liver and muscle).