Cell Biology Basics

Most commonly used stain in routine histology examination.
H & E provides the best differentiation between tissue components.
Hematoxylin (H) is a basic stain that binds to acidic components of a cell (e.g., nuclei) and appears blue.
Eosin (E) is an acidic stain that binds to basic components of a cell (e.g., cytoplasmic proteins) and appears pink.

Cells can be spherical, rod-shaped, flat, concave, curved, rectangular, oval, and more.

Cytoplasm
- Colloidal solution called the cytoplasmic matrix or cytosol.
- Contains suspended bodies, giving it a granular appearance.
- Cytosol contains enzymes, mRNA, tRNA, enzymes, oxygen, CO2, ions, salts, and waste products.
Nucleus
- The nucleus is a prominent organelle found in all cells.
- Contains genetic material (DNA).

These are living structures essential for cell vitality and function.
Membranous Cell Organoids:
- Cell membrane (plasmalemma): Outermost layer of the cell.
- Endoplasmic reticulum: Network of interconnected membranes involved in protein synthesis and lipid metabolism.
- Golgi apparatus: Stacked membrane-bound sacs involved in packaging and modifying proteins.
- Mitochondria: Powerhouse of the cell, responsible for ATP production.
- Lysosomes: Vesicles containing enzymes for digestion and breakdown of cellular debris.
- Coated vesicles: Small, membrane-bound sacs involved in transporting materials within the cell.
- Endosomes: Involved in the sorting and delivery of materials internalized by endocytosis.
- Peroxisomes: Contain enzymes that break down fatty acids and detoxify harmful substances.
Non-membranous Cell Organoids:
- Ribosomes and polyribosomes: Involved in protein synthesis.
- Microtubules: Hollow, cylindrical structures involved in cell shape, movement, and organelle transport.
- Filaments: Protein fibers that provide structural support and facilitate movement.

By Light Microscopy (L/M):
- Appears as a thin, delicate line.
- 7.5-10 nm in thickness.
- Too thin to be seen with L/M.
By Electron Microscopy (E/M):
- Trilaminar or unite membrane structure.
- Two electron-dense layers separated by a lucent intermediate layer.
- Covered by the cell coat (glycocalyx) on its outer surface.

Lipids:
- Primarily composed of phospholipid molecules.
- Phospholipids form a bilayer – two rows of molecules.
- Each phospholipid molecule has a polar (hydrophilic) phosphate head and a non-polar (hydrophobic) tail (two fatty acid chains).
- Hydrophilic heads face the watery environment inside and outside the cell.
- Hydrophobic tails face each other in the middle of the membrane.
Cholesterol:
- Present among phospholipid fatty acids.
- Stabilizes the membrane and regulates its fluidity.
Proteins:
- Make up 50% of the membrane.
- Two types: Integral (intrinsic) and peripheral (extrinsic).
- Integral (intrinsic) proteins:
  - Span the membrane from one side to the other.
  - Some are partly embedded in the hydrophilic region of the lipid bilayer.
  - Require detergent to be removed.
- Peripheral (extrinsic) proteins:
  - Bound to one of the membrane surfaces, especially the cytoplasmic side.
  - Can be easily removed with a salt solution.
Glycoproteins and Glycolipids:
- Project from the outer surface of the membrane to form the cell coat (glycocalyx).
- Glycoproteins: Oligosaccharide chain + protein molecule.
- Glycolipids: Oligosaccharide chain + phospholipid molecule.

Selective barrier (semipermeable):
- Keeps the ion content of the cytoplasm constant, which differs from the extracellular fluid.
Membrane transport and selective permeability:
- Controls the movement of substances in and out of the cell.
Communication:
- Some integral membrane proteins act as receptors that recognize signaling molecules from other cells.
Vesicular transport (bulk transport):
- Endocytosis: A process by which the cell takes in substances.
  - Phagocytosis: Engulfing large particles.
  - Pinocytosis: Engulfing fluids and dissolved substances.
- Exocytosis: A process by which the cell releases substances.
Receptor-mediated endocytosis (highly selective transport):
- Specific molecules (ligands) bind to receptors on the cell surface.
- Receptors cluster in coated pits.
- The coated pit invaginates and pinches off into the cytoplasm as a coated vesicle.
- The coated vesicle carries the ligand and its receptor into the cell.
- The clathrin molecules detach and are recycled back to the cell membrane for new coated pit formation.
- This process is important for the uptake of LDL (low-density lipoprotein) and other molecules.

Portions of the cell membrane are incorporated into endocytotic vesicles during endocytosis.
During exocytosis, membrane is returned to the cell surface.
The process of membrane movement and recycling is called membrane trafficking.
This is important for maintaining the health and integrity of the cell by preserving membrane area.