Regeneration and repair: types of physiologic renewal and posttraumatic repair in different cells and organs
Following a traumatic incident, there are two possible outcomes, cellular regeneration or fibrosis (scarring). These two components are defined as Tissue Healing.
Regeneration - Not all tissues can regenerate, there are three classes of cells differentiated based on their regenerative capacities:
- Labile cells (stem cell) regenerate throughout life e.g. skin, mucosal lining
- Stable cells (stem cells and differentiated cells) which replicate at low levels. E.g. Hepatocytes
- Permanent cells (differentiated cells with few stem cells) are not able to regenerate. E.g. neurons and cardiac muscle
Fibrosis (and remodelling) - replaces tissue with connective fibrous tissue.
- Fibrosis is predominantly mediated by TGF-beta (Transforming Growth Factor beta), Fibroblast growth factor, platelet-derived growth factor and vascular endothelial growth factor.
- These factors form Granulation tissue is formed with active fibroblasts and capillary proliferation which lays down collagen type 3
- This produces a scar made of collagen type 1, with wound contraction mediated by myofibroblasts
H&E, healing biopsy site at 11 days: granulation tissue, with small new capillaries, plump activated fibroblasts in a loose pink matrix, scattered lymphocytes and red cells.
Image: “Histopathology of biopsy site at 11 days, high magnification” by Mikael Häggström, M.D. Author info - Reusing images - Conflicts of interest: None Mikael Häggström, M.D. Consent note, CC0, via Wikimedia Commons.
Types of wound healing
- Category 1
- Also known as healing by first intention, occurs within hours as a result of a surgical incision where there is minimal cellular damage and the wound is sewn together.
- Category 2
- Where wound edges are not sewn together, can be called delayed primary. Phagocytosis of contaminated tissues occurs along with epithelization, collagen deposition and maturation.
- Category 3
- Also known as healing by secondary intention, in this case there is a more intense inflammatory process and a larger quantity of granulation tissue is formed, with a more significant scar.
Example: a clean surgical incision closed with sutures heals by first intention, leaving a thin linear scar; a large pressure ulcer or an infected, gaping wound left open heals by second intention, filling from its base with granulation tissue and contracting markedly, leaving a broad scar.