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Tissue Review: Connective Tissue and Tissue Types (Lecture Notes)

Connective tissue overview

  • Connective tissue is the most abundant tissue type; all share cells + extracellular matrix (ECM).
  • ECM includes ground substance + protein fibers; spaces between cells are common.
  • Functions vary: framework (skeleton), transport, protection, cushioning, etc.
  • Location and properties:
    • Found beneath almost all body surfaces; not exposed to outside environment.
    • Most connective tissue is vascular, except cartilage and dense regular tissue; all have nerves except cartilage.

Extracellular matrix and ground substance

  • ECM components:
    • Protein fibers: ext{collagen}, ext{elastic}, ext{reticular}
    • Ground substance: the space between cells; can be liquid (e.g., blood plasma), semisolid (e.g., fat), or solid (e.g., bone).
  • Ground substance functions: supports, traps water, enables exchange of substances, influences cell behavior.
  • Collagen: strongest, most abundant protein; provides resistance to pulling with some flexibility; found in bone, cartilage, tendons, ligaments.
  • Reticular fibers: thin, form supportive networks around cells (e.g., around muscle, fat, nerve).
  • Elastic fibers: stretch and recoil due to elastin; abundant in skin, lungs, arteries.

Cell types in connective tissue

  • Fibroblasts: manufacture and secrete collagen, elastic, reticular fibers; major permanent cell.
  • Adipocytes: store fat; adipose tissue.
  • Mast cells: immune cells near blood vessels; release histamine during inflammation.
  • Mesenchymal cells: stem-like cells; may differentiate.
  • Macrophages and white blood cells: wandering cells; respond to injury.

Connective tissue proper: classifications

  • Loose connective tissue: loosely arranged fibers; includes areolar and adipose.
  • Dense connective tissue: many fibers, fewer cells; includes dense regular, dense irregular, elastic.
  • Fluid connective tissue: ECM ground substance is fluid; includes blood and lymph.
  • Supporting connective tissue: cartilage and bone.

Areolar connective tissue

  • Most widely distributed; light and airy packing material.
  • Location: subcutaneous layer; also in dermis.
  • Contains fibroblasts, macrophages, all three fiber types; mast cells present.

Adipose tissue

  • Adipocytes store fat; marshmallow-like appearance.
  • Functions: insulation, padding, energy reserves; surrounds some organs; yellow bone marrow is fat.
  • Nucleus pushed to the side; adipocytes can divide; fat cells shrink with weight loss but number can increase with weight gain.

Reticular connective tissue

  • Forms the stroma: three-dimensional supportive framework in liver, spleen, bone marrow, lymph nodes.
  • Contains reticular cells and reticular fibers (a network).

Dense regular connective tissue

  • Collagen fibers in a parallel pattern; resists pulling forces.
  • Location: tendons (muscle to bone) and ligaments (bone to bone).
  • Avascular; slow to heal due to lack of blood supply.

Dense irregular connective tissue

  • Collagen fibers arranged irregularly; provides strength in multiple directions.
  • Location: dermis of the skin; capsules around organs (e.g., liver, spleen).

Elastic connective tissue

  • Rich in elastic (elastin) fibers; can stretch and recoil.
  • Location: lungs (to allow expansion) and large arteries (e.g., aorta) under high stretch.
  • Note: elastic connective tissue is distinct from elastic cartilage.

Fluid connective tissue

  • Blood and lymph.
  • Blood: plasma (liquid part) with cells (RBCs, WBCs, platelets).
  • RBCs are most abundant; WBCs less common (neutrophils typical in smears).

Cartilage (supporting connective tissue)

  • ECM is rubbery; avascular and aneural; chondrocytes live in lacunae.
  • Types:
    • Hyaline cartilage: most common; weak; found at tips of nose, joints, fetal bone model; lacunae with chondrocytes; collagen fibers largely not visible.
    • Elastic cartilage: contains visible elastic fibers; found in ear, epiglottis; more densely packed chondrocytes.
    • Fibrocartilage: strongest cartilage; found in intervertebral discs and knee menisci; high collagen content; very tough.

Bone

  • The ultimate supportive tissue; rigid ECM with collagen fibers + mineral salts.
  • Cells: osteocytes in lacunae; reside in osteons with central canal, lamellae, and canaliculi (communication channels).
  • Functions: support, protection, muscle attachment, mineral storage, red bone marrow production.
  • Microscopy: compact bone shows repeating osteons with central canals and concentric lamellae around them.

Muscle tissue overview

  • Three types: skeletal, cardiac, smooth.
  • All shorten to generate force; roles vary: movement, posture, circulation, propulsion, heat.
  • Cell respiration produces ATP; byproducts include CO2 and H2O; energy stored as ATP; heat is released.

Skeletal muscle

  • Function: move skeleton, maintain posture, generate heat; voluntary movement; striated; multiple peripheral nuclei; long cylinder cells.
  • Features: striations; multinucleated; nerve supply initiates contraction.

Cardiac muscle

  • Location: heart; pumps blood; striated; branched; usually one nucleus per cell.
  • Special feature: intercalated discs for rapid electrical coupling; involuntary control.
  • Microscopy: striations visible; intercalated discs identifiable as dark bars.

Smooth muscle

  • Location: walls of hollow organs, blood vessels, bronchioles, digestive tract, bladder.
  • Function: propel and move contents; regulate diameter of vessels; involuntary control.
  • Features: non-striated, spindle-shaped cells, single central nucleus.

Nervous tissue

  • Neurons: conduct electrical impulses; large cells with dendrites and axons.
  • Neuroglia (glial cells): support neurons; do not conduct impulses; smaller.

Tissue repair and regeneration

  • Repair capacity ranking (best to worst):
    • Epithelium (stem cells) > Connective tissue (bone good; cartilage poor due to avascularity) > Muscle (skeletal/cardiac poor; smooth moderate) > Nervous tissue (neurons poorly regenerate).
  • Key rule: tissues with blood supply heal faster; avascular tissues heal slowly or not at all.

Quick notes for exam imaging and identification

  • Dermis: top portion contains areolar tissue; deeper portion dense irregular connective tissue.
  • Lab cues: areolar tissue shows all three fiber types; dense regular shows parallel collagen; dense irregular shows random collagen; elastic tissue shows elastic fibers.
  • Cartilage vs bone: cartilage lacunae with chondrocytes; bone lacunae with osteocytes and visible central canal structures.

Vitamin C and connective tissue synthesis (brief)

  • Vitamin C is essential for collagen synthesis; deficiencies can affect connective tissue integrity.