Growth Factors

Definition

Polypeptide signalling molecules that activate intracellular pathways to drive cell proliferation. Produced mainly by activated macrophages at injury sites (also epithelial/stromal cells). Bind ECM proteins, which concentrates and displays them locally. Integrin–ECM contact also contributes proliferative signal alongside soluble growth factors.

Which cells respond

• Continuously dividing (labile): marrow, skin/GI epithelium — regenerate readily if stem cell pool intact.
• Quiescent but capable (stable): liver, kidney, pancreas, endothelium, fibroblasts, smooth muscle — respond to growth factors, esp. in wound healing.
• Terminally differentiated (permanent): most neurons, cardiac muscle — cannot respond, heal by scar only.

Major growth factors — table

Role in repair

Liver regeneration: IL-6 (from Kupffer cells) primes hepatocytes → HGF + TGF-α drive proliferation → up to 90% regeneration from remnant after hepatectomy.

Wound healing: epithelial cells migrate to cover wound → endothelial cells proliferate (angiogenesis) → fibroblasts migrate/proliferate, lay down collagen. Fibroblasts + new vessels + ECM = granulation tissue.

Regeneration needs intact residual architecture + proliferation-capable cells; otherwise → scar formation instead.

Clinical relevance

Explains why liver/epidermis regenerate but heart/brain only scar — depends on whether target cells can respond to growth factors at all. VEGF/PDGF/FGF dysregulation → tumour angiogenesis, excess fibrosis. ECM is an active regulator of growth factor delivery, not a passive scaffold.

Definition

Growth factors are polypeptide signalling molecules that drive cell proliferation by activating intracellular signalling pathways, ultimately stimulating a target cell to divide. They are produced predominantly by macrophages activated at a site of tissue injury, though epithelial and stromal cells nearby also contribute. Many growth factors bind directly to proteins of the extracellular matrix, which concentrates them at the site of injury and displays them at high local concentration to the cells that need to respond. In addition to direct growth factor signalling, cells also receive proliferative signals through integrins binding extracellular matrix proteins, so matrix contact itself contributes to the proliferative response alongside soluble growth factors.

Which Cells Can Respond

A cell’s capacity to respond to growth factor signalling depends on its proliferative category.

• Continuously dividing tissues — haematopoietic cells of the bone marrow, and surface epithelia such as the basal layers of the skin and the columnar epithelium of the gastrointestinal tract — regenerate readily as long as the pool of tissue stem cells is preserved.
• Quiescent but capable tissues — the parenchyma of most solid organs (liver, kidney, pancreas), together with endothelial cells, fibroblasts, and smooth muscle cells — are normally resting in the G0 phase of the cell cycle but proliferate in response to growth factors, a response of particular importance in wound healing.
• Terminally differentiated, non-proliferative tissues — the great majority of neurons and cardiac muscle cells — cannot regenerate no matter what growth factor signal they receive, so injury to these tissues is repaired by scarring rather than restitution.

Major Growth Factors

Role in Tissue Regeneration and Repair

Growth factor signalling underlies both major forms of tissue repair.

• Regeneration, in which surviving cells proliferate under growth factor and matrix signalling to restore the tissue essentially to its original state, is possible only where the residual tissue architecture remains structurally intact and the constituent cells retain proliferative capacity.
• Liver regeneration is the clearest illustration of coordinated growth factor action: after partial hepatectomy, cytokines such as interleukin-6, produced mainly by Kupffer cells, first prime hepatocytes to become responsive to growth factor signalling; growth factors such as HGF and TGF-α then act on these primed hepatocytes to drive their proliferation, allowing regeneration of up to ninety percent of the liver from the surviving remnant.
• Wound healing depends on a coordinated sequence of growth factor-driven events: epithelial cells respond to locally produced growth factors and migrate to cover the wound; endothelial and other vascular cells proliferate to form new vessels (angiogenesis); and fibroblasts proliferate and migrate into the wound to lay down the collagen that forms the eventual scar. The combination of proliferating fibroblasts, new blood vessels, and extracellular matrix produced during this phase constitutes granulation tissue.
• Where the injury is too severe or the tissue architecture too disrupted for regeneration, growth factor-driven fibroblast proliferation instead produces scar formation, replacing the damaged tissue with fibrous connective tissue rather than restoring the original cell population.

Clinical Relevance

• Recognising which growth factors act on which cell types explains why some tissues heal by near-perfect regeneration (liver, epidermis) while others of comparable clinical importance can only scar (myocardium, brain) — the difference lies in whether the target cells retain the capacity to respond to growth factor signalling at all.
• The same growth factors that drive normal wound healing and liver regeneration — particularly VEGF, PDGF, and the FGFs — are implicated in pathological processes when their signalling becomes dysregulated, including tumour angiogenesis and excessive fibrosis.
• Because growth factors are concentrated at injury sites through binding to extracellular matrix proteins, the local matrix environment is not a passive scaffold but an active participant in regulating how much and how long a proliferative signal is delivered to nearby cells.

Diagram 1: Growth Factor Sources and Targets in Wound Healing

Draw a healing skin wound in cross-section, with the wound bed at the centre and labelled cell types positioned around and within it, connected by short arrows to the structures they influence.

• Draw a macrophage at the base of the wound with arrows radiating outward, labelled with the growth factors it releases (PDGF, FGF, TGF-β) and the target each acts on (fibroblasts, endothelium).
• Draw platelets at the wound surface releasing PDGF and TGF-β toward nearby fibroblasts.
• Draw budding capillary sprouts labelled “VEGF-driven angiogenesis.”
• Draw a fibroblast laying down collagen fibres, labelled “TGF-β and PDGF stimulate ECM synthesis.”
• Draw epithelial cells migrating across the wound surface from the wound edge, labelled “EGF/TGF-α-driven migration.”

Labels required

• Each cell type (macrophage, platelet, fibroblast, endothelial cell, epithelial cell) must be individually labelled with the specific growth factor(s) acting on or produced by it — a generic “growth factors present” label is insufficient.
• Granulation tissue must be labelled explicitly as the combination of proliferating fibroblasts, new blood vessels, and extracellular matrix.

Errors commonly made

• Attaching a growth factor to the wrong cell type or function, such as assigning VEGF to fibroblast proliferation instead of endothelial proliferation.
• Omitting the macrophage as the central source of multiple growth factors — it is easy to draw platelets and forget that macrophages are the dominant growth factor source overall.

Diagram 2: Which Tissues Can Regenerate (three-column chart)

Draw three columns headed “Labile,” “Stable,” and “Permanent,” matching the same categories used for hyperplasia, each listing its tissues and a regeneration verdict beneath.

Labile: bone marrow, skin/GI epithelium — verdict: “regenerates readily, growth-factor responsive.”

Stable: liver, kidney, pancreas, endothelium, fibroblasts, smooth muscle — verdict: “regenerates when growth-factor stimulated.”

Permanent: most neurons, cardiac muscle — verdict: “cannot regenerate — heals by scar only.”

Labels required

• The verdict line under each column must use language distinguishing “readily,” “when stimulated,” and “cannot” — this diagram tests the same three-tier distinction as the hyperplasia topic, applied specifically to growth-factor responsiveness.

Errors commonly made

• Treating growth factor availability as sufficient on its own to guarantee regeneration — permanent cells do not regenerate no matter how much growth factor is present, and this diagram should make that limitation explicit.