Histology and Cell Biology Lecture Notes

Cartilage (Specialized Connective Tissue)

  • Hyaline Cartilage

    • Appearance: glossy, glass-like extracellular matrix
    • Matrix stains light blue to light purple in typical H&E slides.
    • Lacunae (small cavities) each contain 1–2 chondrocytes.
    • No visible fibers under the light microscope; collagen type II is present but too thin to resolve.
    • Locations / Functional Examples:
    • Tip and dorsum of the nose (provides flexible support and maintains shape).
    • Rings of the trachea and primary bronchi (keep airways patent).
    • Articular surfaces at the ends of long bones (forms smooth, low-friction surfaces in synovial joints).
    • Significance: precursor model for endochondral ossification in developing long bones; resists compression due to hydrated proteoglycans.

  • Elastic Cartilage

    • Appearance: identical to hyaline cartilage plus a dense web of dark‐staining elastic fibers.
    • Locations:
    • External ear (auricle/pinna) – maintains shape & flexibility.
    • Epiglottis – allows repeated bending while protecting airway during swallowing.

  • Fibrocartilage

    • Appearance:
    • Rows or stacks of chondrocytes in lacunae.
    • Thick bundles of collagen type I dominate; gives tissue a fibrous, parallel-wavy look.
    • Fewer lacunae compared with hyaline.
    • Locations / Functions:
    • Intervertebral discs – absorbs compressive forces between vertebral bodies.
    • Pubic symphysis – resists pulling and shearing during childbirth & locomotion.
    • Transitional tissue between dense regular connective tissue and hyaline cartilage; strongest cartilage type because collagen I + II reinforce matrix.

Connective Tissue Proper (Loose)

  • Areolar (Loose) Connective Tissue
    • Components: loosely arranged collagen and elastic fibers; abundant ground substance.
    • Cell Types: fibroblasts (produce matrix), mast cells (histamine release), macrophages, adipocytes, plasma cells.
    • Locations:
    • Immediately deep to epithelial linings (lamina propria).
    • Surrounds nerves & blood vessels; packs spaces between organs.
    • Function: universal packing & cushioning material; reservoir for water, salts, inflammatory cells.

Epithelial Tissue Types & Their Hallmark Traits

  • Stratified Squamous Epithelium

    • Structure: multiple layers; apical cells are flattened (squamous); basal layers can appear cuboidal/columnar.
    • Keratinized (skin) vs. non-keratinized (mouth, esophagus).
    • Function: protection against abrasion, pathogens, dehydration.

  • Transitional Epithelium (Urothelium)

    • Structure:
    • Superficial (umbrella) cells are large and rounded when relaxed, but flatten when stretched.
    • Multiple layers; specialized tight junctions prevent urine leakage.
    • Locations: urinary bladder, ureters, proximal urethra.
    • Function: allows repeated cycles of stretching without damage.

  • Pseudostratified Columnar Epithelium

    • Key Point: all cells contact the basement membrane but nuclei are at different heights ➔ appears stratified.
    • Commonly ciliated with goblet cells.
    • Location: trachea & most of the upper respiratory tract (bronchi).
    • Function: mucociliary escalator that traps & moves debris toward pharynx.

  • Simple Squamous Epithelium

    • Single flat cell layer; centrally located nucleus produces "fried-egg" profile.
    • Locations:
    • Alveoli of lungs (gas diffusion).
    • Endothelium of blood / lymph vessels.
    • Thinness enables rapid diffusion & filtration.

  • Simple Cuboidal Epithelium

    • Square-like cells with round central nuclei.
    • Location: kidney tubules, small gland ducts.
    • Functions: absorption, secretion, limited protection.

Bone Microanatomy Terminology

  • Osteocyte
    • Mature bone cell housed in a lacuna; maintains bone matrix.
  • Canaliculi
    • Microscopic channels radiating from lacunae; allow cytoplasmic processes of osteocytes to exchange nutrients/waste via gap junctions.
  • Osteon (Haversian System)
    • Cylindrical structural unit of compact bone.
    • Composed of concentric lamellae around a central canal.
  • Lamellae
    • Rings of mineralized matrix; collagen fibers alternate orientation each lamella for tensile strength.
  • Central (Haversian) Canal
    • Longitudinal canal at osteon center; contains blood vessels, nerves.

Blood Cell Types

  • Red Blood Cells (Erythrocytes)
    • Anucleate, biconcave disks; optimized for O<em>2O<em>2 & CO</em>2CO</em>2 transport via hemoglobin.
  • White Blood Cells (Leukocytes)
    • General function: immune defense; all possess nuclei. Key subtypes:
    • Neutrophils
    • Multilobed nucleus ("polymorphonuclear"); first responders to acute bacterial infection; perform phagocytosis.
    • Lymphocytes
    • Large, round nucleus with scant cytoplasm.
    • Includes TT cells (cell-mediated immunity) and BB cells/plasma cells (antibody production).
  • Platelets (not listed but implied context)
    • Cell fragments from megakaryocytes; critical for blood clotting.

Fat Cells (Adipocytes)

  • Structure: signet-ring appearance; giant lipid droplet pushes cytoplasm & nucleus to periphery.
  • Function: energy storage, insulation, cushioning, endocrine (leptin, adiponectin).
  • Location: subcutaneous tissue, surrounding kidneys, greater omentum, bone marrow (yellow marrow).

Cell Organelles & Their Functions (A&P Refresher)

  • Nucleus
    • Stores genetic material (DNA); coordinates mRNA synthesis, cell cycle regulation.
  • Rough Endoplasmic Reticulum (RER)
    • Surface studded with ribosomes; synthesizes secretory & membrane proteins; begins post-translational modification.
  • Smooth Endoplasmic Reticulum (SER)
    • Synthesizes lipids, phospholipids, steroid hormones; detoxifies drugs; stores Ca2+Ca^{2+} in muscle (sarcoplasmic reticulum).
  • Golgi Apparatus
    • Receives proteins from RER, modifies (glycosylation, phosphorylation), sorts, and packages into vesicles.
  • Mitochondrion
    • Site of aerobic respiration; generates ∼36\sim 36 ATP per glucose via oxidative phosphorylation; contains its own circular DNA.
  • Lysosome
    • Acidic vesicle with hydrolytic enzymes; degrades macromolecules, pathogens, and autophagic cargo.
  • Centrioles
    • Paired cylindrical structures (9 triplets of microtubules); organize mitotic spindle; form basal bodies for cilia/flagella.

Clinical & Real-World Connections

  • Cartilage degeneration (e.g., osteoarthritis) primarily affects hyaline cartilage at articular surfaces; limited self-repair due to avascularity.
  • Herniated disc involves fibrocartilage failure of the annulus fibrosus surrounding a nucleus pulposus.
  • Smoking paralyzes cilia of pseudostratified columnar epithelium, impairing mucociliary clearance and predisposing to chronic bronchitis.
  • Areolar tissue becomes edematous during inflammation because its loose matrix holds excess tissue fluid.
  • Mitochondrial diseases manifest prominently in high-energy tissues (muscle, neurons) because of ATP production defects.