Regeneration depends on the tissue involved, its capacity for cell proliferation, and preservation of the supporting tissue structure.
Regenerative capacity of different tissues
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In intestinal and skin epithelia, injured cells are rapidly replaced by proliferation of residual cells and differentiation of tissue stem cells, provided the underlying basement membrane remains intact.
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Parenchymal organs can regenerate when their mature cells retain proliferative capacity. Apart from the liver, this process is usually limited.
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The pancreas, adrenal, thyroid, and lung have some regenerative capacity.
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After surgical removal of one kidney, the remaining kidney responds with both hypertrophy and hyperplasia of proximal duct cells. The mechanisms underlying this response are not well defined.
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The liver has an extraordinary capacity for regeneration.
Structural requirements for restoration
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Normal tissue architecture can be restored only when the residual tissue remains structurally intact, as after partial surgical resection of the liver.
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When infection or inflammation damages the entire tissue, including its supporting framework, regeneration is incomplete and accompanied by scarring.
Growth factors acting on epithelial cells
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Epidermal growth factor (EGF)* is produced by activated macrophages, salivary glands, keratinocytes, and many other cells. It stimulates keratinocyte and fibroblast proliferation, keratinocyte migration, and granulation tissue formation.
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Transforming growth factor-a (TGF-a)* is produced by activated macrophages, keratinocytes, and many other cell types. It stimulates proliferation of hepatocytes and other epithelial cells.
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Hepatocyte growth factor (HGF), also called scatter factor, is produced by fibroblasts, liver stromal cells, and endothelial cells. It enhances proliferation of hepatocytes and other epithelial cells and increases cell motility.
Growth factors acting on vessels and connective tissue
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Vascular endothelial growth factor (VEGF)* is produced by mesenchymal cells. It stimulates endothelial cell proliferation and increases vascular permeability.
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Platelet-derived growth factor (PDGF)* is produced by platelets, macrophages, endothelial cells, smooth muscle cells, and keratinocytes.
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PDGF attracts neutrophils, macrophages, fibroblasts, and smooth muscle cells. It also activates and stimulates proliferation of fibroblasts, endothelial cells, and other cells, and stimulates extracellular matrix protein synthesis.