Tissue Repair & Wound Healing

Fracture healing stages: haematoma formation (immediate — blood clot fills fracture gap, providing provisional scaffold and growth factor source), inflammatory phase (days 1-7 — macrophages and platelets release PDGF, TGF-β, BMP, VEGF, recruiting mesenchymal stem cells), soft callus formation (weeks 1-3 — cartilaginous callus bridges the fracture gap through endochondral ossification), hard callus formation (weeks 3-12 — cartilage is mineralised and replaced by woven bone through osteoblast activity), and remodelling (months to years — woven bone is remodelled to lamellar bone along stress lines by coordinated osteoblast/osteoclast activity). PTH analogues' relevance: intermittent PTH stimulates osteoblast differentiation and activity, potentially accelerating hard callus formation and increasing callus bone density. Some clinical trials have investigated teriparatide for fracture healing acceleration, with mixed results. BMPs (bone morphogenetic proteins) are the primary peptide/protein therapeutics approved for bone repair (BMP-2, BMP-7).

Chondrocytes are the sole cell type in mature cartilage, residing in lacunae within the ECM they produce. They maintain the cartilage matrix through balanced synthesis and degradation of type II collagen and proteoglycans (primarily aggrecan). In the growth plate, chondrocytes exist in distinct zones: reserve zone (quiescent stem-like cells), proliferative zone (rapidly dividing, forming columns), pre-hypertrophic zone (beginning to enlarge), hypertrophic zone (maximally enlarged, secreting type X collagen and VEGF, preparing for ossification), and calcification zone (apoptosing, matrix mineralising, replaced by bone). Vosoritide acts on proliferative and pre-hypertrophic chondrocytes: CNP → NPR-B → cGMP → PKG-II → inhibition of FGFR3-activated MAPK pathway → restored chondrocyte proliferation and hypertrophy → increased growth plate height → increased bone elongation. This mechanism is specific to the growth plate — vosoritide does not affect articular chondrocytes or mature bone.

Collagen superfamily: 28 types identified, classified as fibrillar (I, II, III, V, XI — forming rope-like structures), network-forming (IV — basement membranes), fibril-associated (IX, XII, XIV — linking fibrils), anchoring (VII — anchoring basement membrane to underlying tissue), transmembrane (XIII, XVII, XXIII, XXV), and others. Type I collagen: most abundant, forms thick fibres with high tensile strength, predominant in skin (80%), tendons (85%), bone (90%), and ligaments. Triple helix structure: three alpha chains with characteristic Gly-X-Y repeats (where X is often proline, Y is often hydroxyproline) wind around each other in a left-handed helix. Collagen degradation: matrix metalloproteinases (MMPs) — collagenases (MMP-1, MMP-8, MMP-13) make a single cut in the triple helix, followed by gelatinases (MMP-2, MMP-9) degrading the resulting fragments. Collagen turnover in skin: approximately 5-10% per year in adults, declining with age.

Collagen synthesis involves: intracellular steps (gene transcription → mRNA translation on ribosomes → signal peptide cleavage → proline and lysine hydroxylation by prolyl-4-hydroxylase and lysyl hydroxylase (requiring vitamin C/ascorbic acid as cofactor — scurvy results from deficiency) → glycosylation → triple helix formation of three procollagen chains → procollagen secretion) and extracellular steps (N- and C-terminal propeptide cleavage by specific peptidases → tropocollagen self-assembly into fibrils → lysyl oxidase-mediated crosslinking → mature collagen fibres). Growth factors regulating collagen synthesis: TGF-β (most potent stimulator of fibroblast collagen production), PDGF, IGF-1, and FGF. Research peptides investigated for collagen-stimulating properties must demonstrate effects on this pathway at relevant concentrations in appropriate models.

Cytoprotective mechanisms include: maintenance of cell membrane integrity (through antioxidant defence, membrane repair mechanisms), mitochondrial protection (preserving electron transport chain function, preventing cytochrome c release), anti-apoptotic signalling (activating survival pathways like PI3K/Akt, inhibiting pro-apoptotic factors), reduction of oxidative stress (upregulating antioxidant enzymes — superoxide dismutase, catalase, glutathione peroxidase), and anti-inflammatory effects (reducing cytokine-mediated cell damage). Elamipretide's cytoprotective mechanism: it is a cell-permeable tetrapeptide (D-Arg-Dmt-Lys-Phe-NH2, where Dmt is 2',6'-dimethyltyrosine) that targets the inner mitochondrial membrane, binding to cardiolipin to stabilise cristae structure and optimise electron transport chain complex organisation, improving ATP production and reducing ROS generation. This mitochondrial targeting represents a novel cytoprotective strategy distinct from conventional antioxidant approaches.

EGF (53 amino acids) was the first growth factor discovered (Stanley Cohen, Nobel Prize 1986). It acts through the EGF receptor (EGFR/ErbB1, a receptor tyrosine kinase) expressed on keratinocytes, fibroblasts, and other epithelial cells. EGFR activation → Ras-MAPK and PI3K-Akt pathways → cell proliferation, migration, and survival. In wound healing, EGF stimulates keratinocyte migration and proliferation (driving re-epithelialisation), promotes fibroblast growth factor production, and inhibits gastric acid secretion (relevant to gastrointestinal mucosal healing). Recombinant human EGF is approved in some countries for diabetic foot ulcers (primarily in Cuba and South Korea). EGFR is also a major oncology target (overexpressed in many cancers) — anti-EGFR therapies (cetuximab, erlotinib) are established cancer treatments, illustrating the dual role of growth factor pathways in repair and malignancy.

The ECM is a complex network of: structural proteins (collagens — providing tensile strength; elastin — providing elasticity), glycosaminoglycans (GAGs: hyaluronic acid, chondroitin sulphate, heparan sulphate — hydrated gel that resists compression), proteoglycans (protein-GAG conjugates: aggrecan, decorin, perlecan — providing compressive resistance and growth factor sequestration), adhesive glycoproteins (fibronectin — cell adhesion scaffold during wound healing; laminin — basement membrane component; vitronectin — cell attachment), and matricellular proteins (thrombospondins, tenascins, osteopontin — modulating cell-matrix interactions). The ECM is not merely structural — it actively regulates cell behaviour through: integrin-mediated mechanotransduction, growth factor sequestration and release, and degradation fragment signalling (matrikines). ECM composition varies dramatically between tissues (bone, cartilage, skin, tendon) and changes during wound healing.

Fibroblasts are mesenchymal-derived cells found in connective tissue throughout the body. They exist in two states: quiescent fibroblasts (low metabolic activity, maintaining homeostatic ECM turnover in normal tissue) and activated fibroblasts/myofibroblasts (high metabolic activity, producing large amounts of collagen and ECM during wound healing or fibrotic disease). Activation stimuli: TGF-β (most potent activator), PDGF, mechanical tension, and inflammatory cytokines. Fibroblast heterogeneity: tissue-specific fibroblast populations have distinct gene expression profiles and functional properties (dermal fibroblasts differ from tendon fibroblasts, which differ from synovial fibroblasts). For research peptides claiming tissue repair properties, demonstrating effects on relevant fibroblast populations (not just generic fibroblast cell lines) in physiologically relevant conditions is important for translational validity.

The FGF family comprises 22 members (FGF1-23, with FGF15 being the mouse orthologue of human FGF19) acting through 4 receptor tyrosine kinases (FGFR1-4). In wound healing, FGF-2 (basic FGF) is the most relevant — it stimulates fibroblast proliferation, angiogenesis (working synergistically with VEGF), and keratinocyte migration. FGFR3 is particularly relevant to peptide therapeutics: gain-of-function FGFR3 mutations cause achondroplasia (the most common form of dwarfism) by constitutively activating the MAPK/ERK pathway in growth plate chondrocytes, inhibiting proliferation and hypertrophy. Vosoritide (a CNP analogue) counteracts this by activating NPR-B → cGMP → PKG-II, which inhibits the RAF-MEK-ERK pathway downstream of FGFR3, restoring growth plate function and enabling bone elongation.

Granulation tissue is the hallmark of the proliferative phase — a provisional tissue composed of: new capillaries (providing the pink/red colour and bleeding on gentle touching), fibroblasts (producing ECM), inflammatory cells (macrophages, lymphocytes), and a loose ECM (rich in fibronectin, hyaluronic acid, and type III collagen). The name derives from its granular appearance (each 'granule' is a capillary loop with surrounding stroma). Healthy granulation tissue is beefy red, moist, and bleeds easily — signs of adequate blood supply and cellular activity. Unhealthy granulation: pale (anaemic, poor blood supply), excessive/hypergranulation (rising above wound edges, preventing epithelialisation), or absent (chronic wound stalled in inflammation). Assessment of granulation tissue quality and quantity is a key clinical indicator of wound healing progress.

Growth factors are polypeptides that regulate cell proliferation, differentiation, migration, and survival by binding to specific cell surface receptors (predominantly receptor tyrosine kinases). Major families relevant to peptide therapeutics: EGF family (EGF, TGF-α — keratinocyte proliferation, wound re-epithelialisation), FGF family (22 members — fibroblast proliferation, angiogenesis, bone development; FGFR3 is the achondroplasia target), PDGF family (fibroblast recruitment, proliferation — becaplermin is recombinant PDGF-BB approved for diabetic ulcers), VEGF family (angiogenesis — critical for wound healing and tumour vascularisation), TGF-β superfamily (TGF-β1/2/3, BMPs, activins — fibrosis, bone formation, immune regulation), IGF family (IGF-1, IGF-2 — growth, anabolism), and NGF/BDNF/GDNF (neurotrophic support). Growth factors typically act through autocrine (on self), paracrine (on nearby cells), and endocrine (via bloodstream) signalling modes.

The inflammatory phase is essential for debridement and infection control. Neutrophils arrive within hours (recruited by complement fragments, bacterial products, and platelet chemokines), peaking at 24-48h. They phagocytose bacteria and debris, release reactive oxygen species and proteases, and die within 24-48h (forming part of wound exudate). Macrophages arrive by day 2-3 and orchestrate the inflammatory-to-proliferative transition by: clearing neutrophil debris (efferocytosis), producing growth factors (TGF-β, PDGF, VEGF, FGF), and phenotype-switching from pro-inflammatory (M1) to pro-reparative (M2). Excessive or prolonged inflammation impairs healing — chronic wounds (diabetic ulcers, venous ulcers) are often stalled in the inflammatory phase. Anti-inflammatory peptides are investigated for rebalancing this phase without completely suppressing the protective immune response.

Ligament healing biology parallels tendon healing but with important differences: the ACL (anterior cruciate ligament) heals poorly after rupture due to its intra-articular location (surrounded by synovial fluid that dilutes haematoma growth factors and prevents organised clot formation), while extra-articular ligaments like the MCL (medial collateral ligament) heal more successfully because they are surrounded by vascularised tissue that supports a healing response. MCL healing produces functionally adequate scar tissue in 6-12 weeks with conservative management; ACL rupture typically requires surgical reconstruction. Ligament healing research models: MCL transection in rodents (common model because it heals predictably), ACL transection (less commonly used for healing studies — more often for instability models). Any peptide claiming ligament healing properties should be evaluated in the specific ligament model relevant to the clinical application.

MMPs are a family of 23+ zinc-dependent endopeptidases classified by substrate preference: collagenases (MMP-1, -8, -13 — cleave fibrillar collagens I/II/III at a specific Gly-Ile/Leu bond 3/4 from the N-terminus), gelatinases (MMP-2, -9 — degrade denatured collagen/gelatin, type IV collagen in basement membranes), stromelysins (MMP-3, -10, -11 — broad substrate range), matrilysins (MMP-7, -26 — small, no hemopexin domain), membrane-type MMPs (MMP-14/MT1-MMP — cell-surface anchored, activates MMP-2), and others. MMP activity is regulated at: transcription (induced by cytokines, growth factors, mechanical stress), activation (secreted as inactive zymogens, activated by proteolytic cleavage), and inhibition (by TIMPs). In wound healing, MMPs are essential for ECM remodelling and cell migration; in pathology, excessive MMP activity causes tissue destruction (chronic wounds, arthritis, tumour invasion). The MMP/TIMP balance is a key determinant of wound healing quality.

MSCs (also called mesenchymal stromal cells) are defined by the ISCT criteria: plastic-adherent under standard culture conditions, express CD73, CD90, CD105 (and lack CD45, CD34, CD14, CD11b, CD19, CD79a, HLA-DR), and can differentiate into osteoblasts, adipocytes, and chondroblasts in vitro. MSCs reside in: bone marrow (most studied), adipose tissue (most abundant, easily harvested), umbilical cord/Wharton's jelly, dental pulp, and periosteum. Beyond differentiation capacity, MSCs exert paracrine effects: secreting anti-inflammatory cytokines (IL-10, TGF-β), growth factors (VEGF, HGF, IGF-1), and extracellular vesicles (containing mRNA, miRNA, proteins). These paracrine effects underlie most of MSCs' therapeutic potential — they modulate immune responses and promote tissue repair without necessarily differentiating into new tissue cells. MSC-based therapies for tissue repair are an active area of clinical research, intersecting with peptide growth factor biology.

Myofibroblasts express alpha-smooth muscle actin (α-SMA) — the definitive marker — and generate contractile force through stress fibre formation. Differentiation from fibroblasts requires: TGF-β1 signalling (through Smad2/3 pathway), mechanical tension (provided by provisional ECM), and the splice variant ED-A fibronectin. Myofibroblasts are essential for wound contraction (closing wound defects) but persistent myofibroblast activity leads to fibrosis (excessive scarring in skin, liver cirrhosis, pulmonary fibrosis, cardiac fibrosis). Normal resolution: when wound healing is complete, myofibroblasts undergo apoptosis — failure of this apoptosis underlies fibrotic diseases. Anti-fibrotic strategies aim to either prevent myofibroblast differentiation or promote their apoptosis. Understanding myofibroblast biology is relevant to evaluating peptide compounds claimed to modulate wound healing or fibrosis.

CNS regeneration barriers: myelin-associated inhibitors (Nogo, MAG, OMgp — expressed on oligodendrocytes, binding NgR receptor on axons to actively inhibit regrowth), glial scar formation (reactive astrocytes produce chondroitin sulphate proteoglycans that physically and chemically block axon extension), limited adult neurogenesis (new neurons generated primarily in hippocampal subgranular zone and subventricular zone — very limited compared to embryonic neurogenesis), and loss of intrinsic growth capacity (adult neurons have reduced expression of regeneration-associated genes compared to embryonic or peripheral neurons). Peripheral nervous system regeneration is more successful because: Schwann cells support axon regrowth (providing growth factor trophic support and clearing myelin debris), the basal lamina provides a guidance tube, and peripheral neurons maintain intrinsic growth competence. Trofinetide's mechanism (modulating neuroinflammation and synaptic function in Rett syndrome) represents a neuroprotective rather than neuroregenerative approach.

NO is produced by three nitric oxide synthase isoforms: nNOS/NOS1 (neuronal — neuromodulation), iNOS/NOS2 (inducible — produced by macrophages during inflammation, killing pathogens via high NO concentrations), and eNOS/NOS3 (endothelial — producing low-level NO for vasodilation and endothelial homeostasis). The reaction: L-arginine + O2 → L-citrulline + NO (requiring BH4, NADPH as cofactors). NO activates soluble guanylyl cyclase → cGMP → PKG → smooth muscle relaxation (vasodilation). In wound healing, NO modulates: inflammation (regulating macrophage and neutrophil activity), angiogenesis (stimulating VEGF expression and endothelial cell migration), collagen deposition (stimulating fibroblast collagen synthesis at low concentrations), and antimicrobial defence (high concentrations from iNOS kill bacteria). Several research peptides are proposed to work through NO pathway modulation. Vosoritide's downstream signalling also involves cGMP (via particulate guanylyl cyclase NPR-B rather than soluble guanylyl cyclase).

Osteoblasts differentiate from mesenchymal stem cells through a pathway regulated by Runx2 (master transcription factor for osteoblast commitment), Osterix, and Wnt/β-catenin signalling. Mature osteoblasts synthesise and secrete type I collagen (forming the organic bone matrix/osteoid), alkaline phosphatase (involved in mineralisation), and osteocalcin (a bone-specific protein). Approximately 60-80% of osteoblasts die by apoptosis after completing matrix deposition; the remainder become osteocytes (embedded in mineralised matrix, sensing mechanical load) or bone lining cells (quiescent surface cells). PTH analogue mechanism: intermittent PTH(1-34) exposure (daily injection producing a brief pulse) activates PTH1R on osteoblasts → Gαs/cAMP/PKA and Wnt/β-catenin signalling → increased osteoblast number (promoting differentiation, inhibiting apoptosis) → increased bone formation rate. This anabolic window (osteoblast stimulation exceeding osteoclast stimulation) lasts for approximately 18-24 months of treatment.

Osteoclasts are large, multinucleated cells derived from monocyte/macrophage precursors through RANKL-RANK-OPG signalling: osteoblasts express RANKL (receptor activator of NF-κB ligand) which binds RANK on osteoclast precursors → NF-κB and MAPK activation → differentiation and fusion into mature osteoclasts. OPG (osteoprotegerin), a decoy receptor for RANKL produced by osteoblasts, inhibits osteoclastogenesis. Mature osteoclasts resorb bone by: creating a sealed resorption lacuna (ruffled border), secreting HCl (dissolving mineral) and cathepsin K (degrading organic matrix). Calcitonin-salmon directly inhibits osteoclast activity by binding calcitonin receptors on osteoclasts → cAMP elevation → disruption of ruffled border → cessation of bone resorption. This acute anti-resorptive effect explains calcitonin's use in hypercalcaemia emergencies. PTH paradoxically activates osteoclasts (via osteoblast RANKL production) with continuous exposure but predominantly activates osteoblasts with intermittent exposure.

PDGF is a dimeric glycoprotein composed of A and B chains (PDGF-AA, PDGF-AB, PDGF-BB, plus PDGF-CC and PDGF-DD). Released from platelet alpha-granules during clotting (and also produced by macrophages, endothelial cells, and fibroblasts), PDGF is among the earliest growth factors to arrive at a wound site. It acts through receptor tyrosine kinases PDGFRα and PDGFRβ. Key wound healing effects: potent chemoattractant for fibroblasts and smooth muscle cells, stimulates fibroblast proliferation and collagen production, promotes angiogenesis, and recruits macrophages. Becaplermin (Regranex) is recombinant human PDGF-BB — the only growth factor approved by the FDA for wound healing (specifically for diabetic foot ulcers), demonstrating the therapeutic potential of growth factor-based peptide/protein therapeutics in tissue repair.

Key proliferative phase events: fibroplasia (fibroblast migration into the wound along the fibrin scaffold, proliferation, and collagen synthesis — peak collagen deposition around day 7-14), angiogenesis (VEGF-driven sprouting of new capillaries from existing vessels, providing oxygen and nutrients to the healing tissue — the pink granulation tissue appearance reflects this rich capillary network), and re-epithelialisation (keratinocyte migration from wound edges and skin appendages, proliferation, and restoration of the epithelial barrier — growth factors EGF and KGF drive this process). The balance of matrix synthesis and degradation (MMPs vs TIMPs) determines wound quality. Growth factors including PDGF (FDA-approved as becaplermin gel for diabetic foot ulcers), FGF, EGF, and VEGF are the primary drivers. Research peptides targeting the proliferative phase typically aim to enhance fibroblast activity, collagen synthesis, or angiogenesis.

Remodelling involves coordinated collagen synthesis, degradation, and reorganisation. Collagenases (MMP-1, MMP-8, MMP-13) cleave intact collagen, while gelatinases (MMP-2, MMP-9) degrade the resulting fragments. The MMP/TIMP balance determines net collagen turnover. Type III collagen (predominant in granulation tissue) is progressively replaced by type I collagen (organised along mechanical stress lines through mechano-transduction by fibroblasts). Wound tensile strength increases from approximately 3% of normal at 1 week to 20% at 3 weeks to maximum 70-80% at 12+ months — wounds never fully regain original tissue strength. Hypertrophic scars result from excessive collagen deposition (elevated TGF-β, reduced MMP activity), while keloids represent pathological scarring that extends beyond original wound boundaries. Understanding remodelling biology is relevant to evaluating peptides claimed to improve scar quality or accelerate tissue maturation.

Scar tissue differs from normal tissue in: collagen organisation (parallel, densely packed fibres rather than basket-weave pattern of normal skin), collagen composition (initially type III, transitioning to type I but with different crosslinking patterns), absence of skin appendages (no hair follicles, sweat glands, or sebaceous glands in mature scar), reduced elasticity (less elastin), altered vascularity (initial hypervascularisation followed by regression), and reduced tensile strength (70-80% of normal maximum). Pathological scarring: hypertrophic scars (raised, red, confined to wound boundaries — regress over 1-2 years) and keloids (raised, extending beyond wound boundaries, do not spontaneously regress — genetically predisposed, more common in darker skin types). Anti-fibrotic/anti-scarring peptide research aims to modulate TGF-β signalling, MMP/TIMP balance, or myofibroblast apoptosis to promote regenerative rather than fibrotic healing.

Stem cell types: embryonic stem cells (pluripotent — can differentiate into any cell type, ethically complex), adult/tissue stem cells (multipotent — restricted to cell types within their tissue: haematopoietic stem cells → blood cells, mesenchymal stem cells → bone/cartilage/fat/muscle, neural stem cells → neurons/glia), and induced pluripotent stem cells (iPSCs — adult cells reprogrammed to pluripotency). Motixafortide's mechanism: CXCR4 antagonism. The CXCL12/CXCR4 axis is the primary retention mechanism for haematopoietic stem cells (HSCs) in bone marrow niches — CXCL12 (stromal cell-derived factor 1, SDF-1) expressed by bone marrow stromal cells binds CXCR4 on HSCs, anchoring them in the marrow. Motixafortide blocks this interaction, releasing HSCs into the peripheral blood for collection by apheresis. It is used with G-CSF for stem cell mobilisation prior to autologous transplantation in multiple myeloma patients.

Tendon healing proceeds through: inflammatory phase (0-7 days: neutrophils then macrophages infiltrate, cytokines recruit fibroblasts and progenitor cells), proliferative phase (1-6 weeks: tenocytes/fibroblasts produce type III collagen, granulation tissue forms, neovascularisation occurs), and remodelling phase (6 weeks - 12+ months: type III collagen progressively replaced by type I, collagen fibres align along the axis of mechanical loading, crosslinking increases tensile strength). Key limitation: tendon repair tissue is scar-like (disorganised collagen) rather than regenerative (recreating original tendon architecture), resulting in biomechanically inferior tissue. Healing is slow because: tendons have limited vascularity (blood supply from paratenon, muscle-tendon junction, and bone insertion is sparse in the tendon mid-substance), tenocytes have low metabolic activity, and mechanical loading is required for collagen alignment but can disrupt early repair. Research peptides for tendon healing must demonstrate effects in this specific tissue context, not just generic wound healing models.

Tenocytes (tendon fibroblasts) are elongated, spindle-shaped cells aligned between collagen fibre bundles. They communicate through gap junctions and a network of cellular processes extending between collagen fascicles. Tenocyte functions: synthesising tendon ECM (primarily type I collagen, with small amounts of type III, V, XII, XIV collagen, plus proteoglycans and elastin), sensing mechanical load (through integrin-mediated mechanotransduction), and maintaining matrix homeostasis (balancing synthesis and MMP-mediated degradation). Tenocyte metabolic rate is low (reflecting the low vascularity of tendon tissue), which contributes to slow healing. Loading is essential for tenocyte function — immobilisation causes matrix degradation, while appropriate mechanical loading stimulates collagen synthesis (the basis for physiotherapy in tendon rehabilitation). Research peptides targeting tendon healing must demonstrate effects on tenocyte biology — stimulating collagen synthesis, promoting appropriate matrix organisation, and enhancing the cellular response to mechanical loading.

Four TIMPs (TIMP-1, -2, -3, -4) inhibit MMPs by binding in a 1:1 stoichiometric complex to the MMP active site zinc. Each TIMP has a somewhat different MMP inhibition profile: TIMP-1 preferentially inhibits MMP-9 and most soluble MMPs; TIMP-2 inhibits MMP-2 (but also paradoxically activates pro-MMP-2 in complex with MT1-MMP at low concentrations); TIMP-3 is the broadest spectrum inhibitor (also inhibits ADAMs — a disintegrin and metalloproteinases); TIMP-4 is predominantly cardiac. Beyond MMP inhibition, TIMPs have growth factor-like activities: TIMP-1 promotes cell proliferation, TIMP-2 inhibits endothelial cell proliferation (anti-angiogenic). In chronic non-healing wounds, the MMP/TIMP ratio is elevated (excessive MMP activity degrades newly deposited matrix faster than it can accumulate); in fibrotic conditions, the ratio is reduced (insufficient MMP activity allows excessive collagen accumulation).

The TGF-β superfamily includes TGF-β1/2/3, BMPs, activins, and inhibins. TGF-β signals through serine/threonine kinase receptors (TGFβRI/ALK5 and TGFβRII) → Smad2/3 phosphorylation → Smad4 complex → nuclear translocation → gene transcription. TGF-β has context-dependent effects: pro-fibrotic (stimulating fibroblast→myofibroblast differentiation, collagen synthesis, TIMP production) and anti-inflammatory (suppressing immune cell activation). In wound healing, TGF-β1 is the predominant isoform driving scar formation; TGF-β3 is associated with scarless foetal healing. The TGF-β/Smad pathway is the primary driver of pathological fibrosis in multiple organs. Anti-fibrotic peptide research targets TGF-β signalling modulation to promote regenerative rather than scarring healing. Palovarotene (a retinoic acid receptor gamma agonist for FOP) works through a pathway that intersects with BMP/TGF-β superfamily signalling.

Type I collagen is a heterotrimer composed of two alpha-1(I) chains and one alpha-2(I) chain, encoded by COL1A1 and COL1A2 genes. Molecular weight: approximately 300 kDa per triple helix, approximately 1.5nm diameter × 300nm length. Mechanical properties: ultimate tensile strength approximately 50-100 MPa (comparable to aluminium wire). Biomarkers of Type I collagen metabolism: formation markers — PINP (procollagen type I N-terminal propeptide, cleaved during extracellular processing) and PICP (C-terminal propeptide); degradation markers — CTX-I (C-terminal crosslinked telopeptide, released during osteoclastic bone resorption) and NTX-I (N-terminal crosslinked telopeptide). These biomarkers are used in osteoporosis trials to monitor bone turnover: PTH analogues (teriparatide, abaloparatide) increase PINP (formation) before increasing CTX-I (resorption), confirming the anabolic window of intermittent PTH therapy.

Type II collagen is a homotrimer of three alpha-1(II) chains, forming the primary structural component of hyaline cartilage (50-80% of cartilage dry weight). It provides tensile strength that complements the compressive resistance of proteoglycans (aggrecan). In osteoarthritis, type II collagen is degraded by collagenases (MMP-13 is the primary pathological collagenase in cartilage), with degradation fragments (C2C, CTX-II) serving as biomarkers of cartilage destruction. Vosoritide's mechanism (promoting growth plate chondrocyte proliferation and hypertrophy) depends on the collagen II-rich cartilage matrix providing the structural scaffold for longitudinal bone growth. Type II collagen is also of interest in the dietary supplement and peptide research spaces, where hydrolysed collagen II supplements are marketed for joint health — though clinical evidence for specific peptide sequences is variable.

Type III collagen is a homotrimer of three alpha-1(III) chains that forms fine reticular fibres. It is particularly abundant in: embryonic/foetal skin, blood vessel walls (tunica intima and media), intestinal wall, uterus, and early wound healing tissue (granulation tissue contains predominantly type III collagen before remodelling). During wound maturation, type III collagen is gradually replaced by type I collagen — this transition increases tissue tensile strength from approximately 20% of normal at 3 weeks to 70-80% of normal by 12 months (mature scars never fully recover original strength). The type III/type I collagen ratio is a marker of wound maturity and scar quality — higher type III proportion indicates immature or poorly remodelled tissue. Mutations in COL3A1 (type III collagen gene) cause Ehlers-Danlos syndrome vascular type, characterised by fragile blood vessels and organs.

The VEGF family includes VEGF-A (the primary member), VEGF-B, VEGF-C, VEGF-D, and PlGF (placental growth factor), acting through receptor tyrosine kinases VEGFR-1 (Flt-1), VEGFR-2 (KDR/Flk-1, primary signalling receptor), and VEGFR-3 (lymphangiogenesis). VEGF-A is the master regulator of angiogenesis: hypoxia activates HIF-1α → VEGF-A transcription → endothelial cell proliferation, migration, and tube formation. In wound healing, VEGF-driven angiogenesis is essential for granulation tissue formation. In tumour biology, VEGF enables tumour neovascularisation — NETs are often highly vascular, which facilitates somatostatin receptor-targeted imaging and therapy. Anti-VEGF therapies (bevacizumab) are cornerstone cancer treatments. In the research peptide space, compounds claiming pro-angiogenic properties for tissue repair must demonstrate VEGF pathway activation in relevant models.

The four overlapping phases: (1) Haemostasis (seconds to hours): platelet activation, coagulation cascade, fibrin clot formation — the clot provides temporary wound closure and a scaffold for incoming cells, and platelet degranulation releases growth factors (PDGF, TGF-β, EGF) initiating the repair cascade. (2) Inflammation (hours to days 5-7): neutrophil infiltration (peak 24-48h, phagocytose bacteria and debris), followed by monocyte/macrophage infiltration (peak day 3-5, phagocytosis + cytokine/growth factor secretion — macrophages are critical orchestrators of the transition from inflammation to proliferation). (3) Proliferation (days 3-21): fibroblast migration and proliferation, collagen synthesis, angiogenesis (VEGF-driven), granulation tissue formation, and re-epithelialisation (keratinocyte migration from wound edges). (4) Remodelling (weeks to 1-2 years): collagen crosslinking, type III→type I collagen transition, wound contraction (myofibroblasts), and scar maturation. Each phase offers potential intervention points for wound healing peptides.