Wound healing involves:

• Regeneration  – restoration of the damaged tissues
• Healing by scarring – repair where complete restoration and recapitulation of normality is not possible

Cell types can be classified into three different groups based on their capacities to divide.

• Labile cells – Keratinocytes, intestinal epithelial cells
• Stable cells – Hepatocytes, renal tubular epithelial cells
• Permanent cells – Neurons, cardiac myocytes

These are cells that are continuously dividing as part of their normal homeostatic function.

Normal function means requires the loss of damaged/aged cells and their replacement by proliferation.

For example, intestinal epithelial cells lost by apoptosis from the lumenal surface are replaced by division of stem/progenitor cells at the base of crypts. The lifespan of an intestinal epithelial cell is ~5 days.

Failure of cell division, for example, induced by chemotherapy or radiation, rapidly leads to symptoms given the short lifespan of epithelial cells.

These cells do not usually divide under homeostatic conditions but can do so after injury.

Renal tubular epithelial cells can be damaged by ischaemia. Nuclei are absent around the complete lumenal circumference, and necrotic epithelial cells are shed into the lumen.

Regenerative epithelial activity is evident rapidly.

Restoration of normal tubular profiles, assuming no further injury, is complete within a few days.

This liver has a profound capacity for regeneration. The Greek myth of Prometheus reflects this capacity, although it is not certain how much was known in ancient times.

Hepatocytes, with contributions from hepatic progenitor cells, divide to replace lost liver parenchyma.

This can be demonstrated experimentally by murine partial hepatectomy. After the removal of 70% of the liver, regeneration restores the liver cell mass almost within a week.

Similar regenerative capacity is evident in humans. This is taken advantage of therapeutically, with partial hepatectomies for localised lesional disease undertaken in the knowledge that regeneration will fully restore the liver.

These are cells that are unable to divide, even after an injury.

Loss of permanent cells leads to healing by scarring, which can be associated with loss of function.

For example, a large healed myocardial infarct can be associated with reduced cardiac capacity and lead to cardiac failure.

In homeostasis, cell numbers must be balanced to account for cell loss and differentiation without uncontrolled proliferation. Cell number must also be carefully controlled after injury.

Uncontrolled expansion of cell number, either by excessive proliferation or failure of normal cell apoptosis is a feature of neoplasia.

Damage to tissue causes changes in the cell-cell and cell-matrix interactions and leads to growth factor p[production. Both influence cellular responses to injury.

Soluble growth factors are released by damaged resident cells or by recruited inflammatory cells.

Growth factors can act upon target cells that have the appropriate receptor in the cell membrane by three routes:

• Autocrine – growth factors are released from a cell into the extracellular environment and bind to receptors on the same cell
• Paracrine – released growth factors act upon adjacent cells in the immediate neighbourhood
• Endocrine – factors are released directly into the blood, acting upon target cells considerable distances from the site of release; in this context, the released factors are called hormones

After a localised injury, most growth factors act through autocrine and paracrine routes.

Growth factors act through specific receptors. For example:

• Epidermal growth factor (EGF) acts through EGF receptors (EGFR).
• Platelet-derived growth factor acts through PDGF receptors (PDGFR).

Ligand-receptor ligation leads to a complex series of intracellular signalling events, specific to the ligand-receptor pair, that often involves phosphorylation and dephosphorylation of intracellular signalling molecules.

One outcome is often the passage of transcription factors into the nucleus of the target cell. Expression of genes pertinent to a response to wounding, for example, those controlling proliferation and migration, is increased.

Outwith the blood, cells do not float free but physically interact with other cells and the extracellular matrix that constitutes the extracellular environment.

Both cell-cell and cell-matrix interactions influence cell behaviour. Loss of edge inhibition after an epithelial wound promotes epithelial proliferation to ‘fill in’ the defect.

Cell-cell and cell-matrix interactions are mediated by cell surface receptors called integrins.

Integrins exist as heterodimers composed of an α and β subunit. The large number of different α and β subunits gives considerable complexity and variety to integrins heterodimers.

Different heterodimers interact with the spectrum of extracellular matrix (ECM) proteins. Integrin-ECM interactions lead to changes in cell behaviour, including cell migration.

1. The immediate response to an injury is bleeding into the wound from damaged blood vessels.
2. The would is plugged by a fibrin-rich clot formed by the clotting cascade. Epithelial proliferation begins almost immediately.
3. The rapid recruitment of neutrophils and growth factor release triggers angiogenesis, forming granulation tissue. The wound begins to re-epithelialise.
4. Granulation tissue is replaced over 5-10 days by new ECM produced by recruited fibroblasts as the epithelial surface heals.
5. Vessel growth competes, and the ECM begins to mature.
6. Over time, fibroblast numbers diminish and the ECM fully matures.

Granulation tissue is a pathological term for the early appearances at the base of an ulcer or wound.

Macroscopically, it is the slightly grainy brown layer at the base of a skin wound before a scab is formed.

Histologically, it is composed of newly formed capillaries and neutrophils set within newly deposited ECM.

Angiogenesis is the formation of new vessels during adulthood. This may occur as part of a physiological process, for example, endometrial growth during the menstrual cycle, during wound healing, or during a pathological process such as tumour growth.

Bone-marrow-derived precursors may contribute towards angiogenesis, but the process of sprouting from new vessels is best described.

• Immediately after injury, released nitric oxide (NO) causes vasodilation and vascular endothelial growth factor (VEGF) causes increased vascular permeability
• Matrix-metalloproteinase mediated degradation of the endothelial basement membrane allows migration of endothelial cells expressing VEGF receptors towards the wound
• Vascular branching and elongation, followed by lumen formation, generate new vessels. Anastamosis of vessels creates new capillary networks and flow
• Pericytes, cells that wrap around blood vessels to provide support and maintain function, are recruited to new vessels via PDGF and angiopoietin activity
• The new synthesis of basement membrane ECM stabilises the new vessels

Fibroblasts are mesenchymal cells resident within tissues. They sit within and are responsible for the production of, extracellular matrix.

Fibroblasts are non-polarised (the same in all directions, in contrast to epithelial cells, for example, that have a lumenal and a basal surface) and highly migratory.

In response to growth factors released by injured and recruited cells, they proliferate (for example, in response to platelet-derived growth factor) and increase production of ECM components (for example, in response to tissue growth factor (TGF) β).

They are also capable of producing factors that inhibit ECM degradation, further enhancing ECM accumulation.

Extracellular matrix consists of collagenous and non-collagenous components. Collagens can be fibrillar, structurally robust types such as collagen I and III, or basement-membrane types such as collagen IV.

Fibrillar collagens in scars, deposited after chronic damage can be visualised with a picrosirius red stain.

Non-collagenous ECM components include elastin, laminins, proteglycans and glycoproteins.

Extracellular matrix components can be degraded by matrix metalloproteases (MMPs). MMPs are produced by many cell types, including macrophages and fibroblasts.

MMPs that degrade the normal basement membrane type components can contribute to injury in some contexts, but are also necessary for ‘loosening’ endothelial cells to allow sprouting in angiogenesis.

MMPs that degrade fibrillar collagens are capable of remodelling large areas of scarring after any injurious stimulus has ceased.

MMP activity is controlled by their rate of production, and by native inhibitors called tissue inhibitors of metalloproteases (TIMPs). The overall MMP-TIMP balance, along with the rate of ECM production, determines whether there is net ECM accumulation or degradation.

Although scars formed after injury can be remodelled, continued injury as part of a disease, usually inflammatory, leads to end-stage scarring that could not be remodelled if were the injury were to cease.

In the liver, this is called cirrhosis.

Equivalent continued injury to the kidney leads to the replacement of all normal renal structures, tubulointerstitium and glomeruli, with dense scars.