Week 3: Tissue Repair Flashcards
Tissue Repair
- Removal of exudate and cellular/tissue debris
- Replacement of cells and tissues lost
2 types of Reactions:
1. Regeneration: The cells replacing those lost in inflammation are IDENTICAL to those lost
Occurs by the proliferation of uninjured residual cells and/or maturation of tissue stem cells.
- CT deposition/scar formation: The cells replacing those lost in inflammation consist of DIFFERENT cells to those lost.
Occurs by replacement by connective (fibrous) tissue i.e. scarring.
Not uncommonly, BOTH regeneration and CT deposition/scar formation contribute, in varying degrees, to tissue repair.
Regeneration vs. CT vs. both?
- Proliferative Potential of the Cell type affected
- Is there damage to the ECM?
Regeneration only: labile/stable cells
CT only: permanent cells
Proliferative Potential
Ability to regenerate (cell cycle phase)
Labile Cells
Proliferate throughout life.
Epidermis, epithelial lining body cavities, blood cells.
Stable Cells
Low normal level of replication, but are able to divide in response to stimuli!
Epithelia of most parenchymal organs (liver, kidney, pancreas)
Considered to be in G0 phase, but can be stimulated to re-enter the cell cycle.
Permanent Cells
Can NEVER divide again
Cardiac muscle, neurons, skeletal muscle
LEFT THE CELL CYCLE FOREVER
ONLY CT deposition/scarring
ECM Damage
Labile and stable cells ONLY
IF the ECM is damaged and cells proliferate in a haphazard way –> disorganized and non-functional tissues
If injury does NOT damage ECM: regeneration only (hepatectomy, hep A)
If injury DOES damage ECM: CT and regeneration (chronic hep B/C)
CT Deposition/Scar Formation
When healing cannot be accomplished by regeneration alone, it occurs by replacement of injured cells with connective tissue (i.e. scarring) or by some combination of regeneration and CT replacement/scarring.
Example: Cutaneous wound healing
Cutaneous Wound Healing
Example of CT Deposition/Scar formation
- Hemostatic Plug/crust formation = quick and immediate closure to the wound (coagulated blood dries and hardens) **ONLY IF blood is available and it can dry
- Inflammatory phase = leukocytes (neutrophils -> macrophages); removal of dead tissue via sloughing, liquefaction, phagocytosis
- Proliferative phase = granulation tissue 5 days after injury; scaffold upon which the final scar will form (pink granules that bleed). Reepithelialization
- Remodeling/Cicatrization = loose connective tissue converted to stable fibrous scar (2-3 weeks after injury); never regain initial strength (70-80%)
Granulation Tissue
New tissue containing proliferating cellular & extracellular components replaces the tissue lost
- Proliferating fibroblasts
- Proliferating endothelial/vascular cells (in a process called angiogenesis)
- Loose connective tissue
- Many macrophages
Insensitive to pain (no nerves), resistant to infection
Re-epithelialization
- Regression of vessels of granulation tissue
- Regression of macrophages
- Proliferation of fibroblasts
- Collagen deposition
Remodeling/Cicatrization
Conversion of loose CT in the granulation tissue to a stable fibrous scar
Begins 2-3 weeks after the injury and may continue for many months
Most wounds involving skin, fascia or tendon never regain initial strength of tissue derived (70-80% of tensile strength achieved at 3 months).
Loss of strength is related to the type of collagen laid.
Adult skin – collagen type I
Granulation tissue – collagen type III
Cicatrization – replacement of type III by type I
Not as much type I compared to the original
First Intention (Primary Repair/Primary Union)
When edges of the wound are close together (small granulation tissue)
Second Intention (Secondary Repair)
Considerable loss of tissue; edges of the wound are far apart. Edges cannot be brought together; requires a lot more granulation tissue.
Bigger, more visible scar.
Complications of Cutaneous Wound Healing
- Dehiscence
- Excessive Scar Formation
- Hypertrophic Scar
- Keloid
- Contractures
