📝 STANDARD DESCRIPTION:
Cell = the structural & functional unit of life (Schleiden + Schwann, 1838-39).
3 main parts:
1. Plasma Membrane — phospholipid bilayer + proteins (fluid mosaic model).
2. Cytoplasm — cytosol + organelles + inclusions.
3. Nucleus — control centre with DNA.
Major Organelles:
• Mitochondria — 'powerhouse'; ATP via oxidative phosphorylation.
• Rough ER — protein synthesis (with ribosomes).
• Smooth ER — lipid synthesis, detoxification.
• Golgi apparatus — packaging & modification of proteins.
• Lysosomes — digestive enzymes (intracellular digestion).
• Peroxisomes — fatty acid β-oxidation, H₂O₂ detox.
• Ribosomes — protein synthesis (free or RER-bound).
• Centrioles — organize spindle in mitosis.
• Cytoskeleton — microfilaments (actin), intermediate filaments, microtubules.
📚 DETAILED DESCRIPTION:
CYTOLOGY — THE CELL UNDER THE MICROSCOPE
The cell is the smallest living unit. Veterinary histology uses light microscopy (LM) for routine work and electron microscopy (EM) for ultrastructure. The standard stain is Haematoxylin & Eosin (H&E): H stains nuclei BLUE-PURPLE (basophilic), E stains cytoplasm PINK (eosinophilic).
1. PLASMA MEMBRANE (Plasmalemma):
• Phospholipid bilayer (~7-10 nm) with embedded proteins (Singer-Nicolson 'fluid mosaic model').
• Hydrophilic phosphate heads outside, hydrophobic fatty tails inside.
• Cholesterol stiffens the bilayer.
• Membrane proteins: integral (transmembrane channels, pumps, receptors), peripheral (signaling).
• Glycocalyx: sugar coat on outer face — cell recognition, blood groups.
• Functions: selective permeability, signaling, cell-cell adhesion, transport (passive: diffusion, facilitated; active: pumps, endocytosis, exocytosis).
2. CYTOPLASM:
• Cytosol — gel-like fluid (~70% water + ions + enzymes + metabolites).
• Organelles (membrane-bound + non-membrane-bound) — see below.
• Inclusions — non-living deposits: glycogen, lipid droplets, melanin, hemosiderin.
3. NUCLEUS:
• Largest organelle; usually central, spherical/oval.
• Nuclear envelope — double membrane with nuclear pores; continuous with RER.
• Chromatin — DNA + histones; appears as euchromatin (light, active) + heterochromatin (dark, inactive).
• Nucleolus — dense region; site of rRNA synthesis + ribosome assembly.
• Carries the genetic blueprint.
4. MITOCHONDRIA — 'Cellular Power Plants':
• Double-membrane organelle (~0.5-1 μm).
• Outer membrane smooth, inner forms cristae → site of ETC + ATP synthase.
• Matrix contains mitochondrial DNA (circular, maternally inherited), enzymes of Krebs cycle.
• Numerous in metabolically active cells (cardiac muscle, liver, renal tubule).
5. ENDOPLASMIC RETICULUM (ER):
• Rough ER (RER) — studded with ribosomes; protein synthesis (especially secretory + membrane proteins). Abundant in pancreatic acinar cells, plasma cells.
• Smooth ER (SER) — no ribosomes; lipid + steroid synthesis (gonads, adrenal cortex), Ca²⁺ storage (muscle = sarcoplasmic reticulum), drug detoxification (liver).
6. GOLGI APPARATUS:
• Stack of flattened cisternae near nucleus.
• Cis face (receiving) ↔ Trans face (shipping).
• Modifies, packages, sorts proteins from RER → secretory vesicles or lysosomes.
• Active in goblet cells, plasma cells.
7. LYSOSOMES:
• Membrane sacs of acid hydrolases (40+ enzymes).
• 'Cellular stomach' — intracellular digestion of phagocytosed particles + worn organelles (autophagy).
• Active in macrophages, neutrophils.
8. PEROXISOMES:
• Smaller than lysosomes; contain catalase + oxidases.
• β-oxidation of long-chain fatty acids; detoxify H₂O₂.
• Abundant in liver and kidney.
9. RIBOSOMES:
• Made of rRNA + protein; 60S + 40S subunits = 80S in eukaryotes.
• Free in cytosol = synthesise cytosolic proteins.
• Bound to RER = synthesise secretory/membrane proteins.
10. CENTRIOLES & CENTROSOME:
• Pair of cylinders (9 triplets of microtubules) at right angles, near nucleus.
• Organise the mitotic spindle in cell division.
• Form basal bodies of cilia/flagella.
11. CYTOSKELETON:
• Microfilaments (actin, ~7 nm) — cell shape, motility, contraction.
• Intermediate filaments (~10 nm) — mechanical strength; type varies with cell (keratin in epithelia, vimentin in connective tissue, neurofilaments in neurons, GFAP in astrocytes — basis of immunohistochemistry).
• Microtubules (tubulin, ~25 nm) — intracellular transport tracks, mitotic spindle, cilia/flagella core.
12. CILIA & FLAGELLA:
• Microtubule-based projections (9+2 axoneme).
• Cilia (short, many) — move fluid (e.g., trachea); a single primary cilium acts as a sensor in many cells.
• Flagellum (long, single) — sperm tail.
// COMPARATIVE ANALYSIS
| ALL CELLS |
Universal organelles — same plan in ox, horse, dog, fowl. Differences are quantitative (more mitochondria in equine cardiac muscle, more SER in steroidogenic cells). |
| AVIAN RBC |
Bird red blood cells RETAIN their nucleus (mammalian RBCs are anucleate). Important for haematology slides. |
CLINICAL RELEVANCE
Lysosomal Storage Diseases: Hereditary deficiency of one lysosomal enzyme → substrate accumulates → cells balloon. e.g. Mannosidosis in Angus calves.
Mitochondrial Myopathies: Defective mtDNA → 'ragged red fibres' on muscle biopsy.
