Determining the optimal concentration of growth factors is a critical challenge in regenerative medicine that directly impacts the success of tissue engineering and therapeutic interventions. Growth factors are critical molecules for tissue repair and regeneration, yet their clinical application requires precise dosing strategies to maximize therapeutic benefits while minimizing adverse effects. This comprehensive guide explores the science, methodologies, and practical considerations for calculating optimal growth factor concentrations in tissue regeneration applications.

Understanding Growth Factors in Tissue Regeneration

What Are Growth Factors?

Growth factors are molecules capable of stimulating a variety of cellular processes including cell proliferation, migration, differentiation and multicellular morphogenesis during development and tissue healing. These signaling proteins function as powerful biological messengers that orchestrate complex cellular responses essential for tissue development, maintenance, and repair.

Growth factors bind to specific cell surface receptors, activating intracellular pathways like MAPK or PI3K, which leads to gene expression changes that promote proliferation, migration, and differentiation. This cascade of molecular events enables cells to respond appropriately to injury signals and coordinate the regenerative process.

Key Growth Factors in Regenerative Medicine

Several growth factor families play pivotal roles in tissue regeneration, each with distinct functions and applications:

  • Platelet-Derived Growth Factor (PDGF) - Promotes cell proliferation and wound healing
  • Vascular Endothelial Growth Factor (VEGF) - Stimulates angiogenesis and blood vessel formation
  • Transforming Growth Factor-beta (TGF-β) - Regulates cell growth, differentiation, and extracellular matrix production
  • Epidermal Growth Factor (EGF) - Enhances epithelial cell proliferation and wound closure
  • Basic Fibroblast Growth Factor (bFGF) - Supports cell growth and tissue repair
  • Bone Morphogenetic Proteins (BMPs) - Induce bone and cartilage formation
  • Insulin-like Growth Factor (IGF) - Promotes cell survival and tissue growth
  • Hepatocyte Growth Factor (HGF) - Supports tissue regeneration and angiogenesis

PRP (Platelet-Rich Plasma) contains a limited set of platelet-derived growth factors, including PDGF, VEGF, TGF-β, and EGF, making it a commonly used autologous source for regenerative applications.

The Challenge of Clinical Translation

While using growth factors to promote tissue healing has widely shown promising results in pre-clinical settings, their success in the clinic is not a forgone conclusion. Multiple factors contribute to this translational gap.

Translation of growth factors is often limited by their short half-life, rapid diffusion from the delivery site, and low cost-effectiveness. Growth factors typically have short half-lives, lasting only minutes to hours in vivo, which necessitates careful consideration of delivery methods and dosing strategies.

Trying to circumvent those limitations by the use of supraphysiological doses has led to serious side-effects in many cases and therefore innovative technologies are required to improve growth factor-based regenerative strategies. This underscores the critical importance of determining optimal concentrations rather than simply increasing doses.

The Science of Dose-Response Relationships

Understanding Dose-Response Curves

The dose–response relationship describes the magnitude of the response of a biochemical or cell-based assay or an organism, as a function of exposure (or doses) to a stimulus or stressor after a certain exposure time. This fundamental concept provides the framework for determining optimal growth factor concentrations.

A dose–response curve is a coordinate graph relating the magnitude of a dose to the response of a biological system, where the applied dose is generally plotted on the X axis and the response is plotted on the Y axis, with the curve typically being sigmoidal with the steepest portion in the middle.

Key Parameters in Dose-Response Analysis

Several critical parameters are derived from dose-response curves to characterize growth factor activity:

  • EC50 (Half-maximal Effective Concentration) - The concentration producing 50% of the maximum response
  • IC50 (Half-maximal Inhibitory Concentration) - Relevant when growth factors modulate inhibitory pathways
  • Emax (Maximum Effect) - The maximum response achievable at the highest effective dose
  • AUC (Area Under the Curve) - Represents the overall exposure and response relationship
  • Hill Coefficient - Describes the steepness of the dose-response curve

The first point along the graph where a response above zero is reached is usually referred to as a threshold dose, which represents the minimum concentration required to elicit a measurable biological response.

The Importance of Physiological Context

Growth factors act in a complex time-, concentration-, and microenvironment-determined manner, often in conjunction with each other, to control multiple cellular functions and repair processes at the tissue level. This complexity means that optimal concentrations cannot be determined in isolation but must consider the broader biological context.

Microenvironmental VEGF concentration, not total dose, determines a threshold between normal and aberrant angiogenesis, highlighting that local concentration at the tissue site is more critical than the total amount administered.

Methods for Calculating Optimal Growth Factor Concentrations

In Vitro Experimental Approaches

In vitro studies provide the foundation for determining optimal growth factor concentrations by allowing controlled experimentation with isolated cell populations.

Cell Proliferation Assays

Cell proliferation assays measure the rate of cell division in response to varying growth factor concentrations. Common methods include:

  • MTT and MTS assays for metabolic activity
  • Cell Counting Kit-8 (CCK-8) assays
  • BrdU incorporation for DNA synthesis measurement
  • Direct cell counting using automated systems
  • Flow cytometry for cell cycle analysis

Growth factors were applied at concentrations of 0, 1, 10, and 100 ng/ml in systematic studies to identify optimal dosing ranges. The optimal concentration of both bFGF and EGF to promote cell proliferation and collagen expression in fibroblasts was 10 ng/ml, demonstrating how systematic testing identifies effective concentrations.

Functional Assays

Beyond proliferation, functional assays assess specific cellular responses relevant to tissue regeneration:

  • Migration assays (scratch/wound healing assays, transwell migration)
  • Differentiation markers (gene expression, protein analysis)
  • Extracellular matrix production (collagen synthesis, proteoglycan deposition)
  • Angiogenesis assays (tube formation, sprouting assays)
  • Mineralization assays for bone regeneration

The effects of growth factors were evaluated by measuring the proliferation and collagen secretion of fibroblasts to determine optimal growth factor concentrations, illustrating the importance of assessing multiple functional endpoints.

In Vivo Validation Studies

While in vitro studies provide initial data, in vivo experiments are essential for validating optimal concentrations in the complex physiological environment.

Animal Models

Various animal models are employed to test growth factor concentrations in tissue regeneration:

  • Wound healing models (skin, diabetic ulcers)
  • Bone defect models (critical-size defects, fracture healing)
  • Ischemic tissue models (hind limb ischemia, myocardial infarction)
  • Cartilage regeneration models
  • Nerve regeneration models

A midrange dose of BMP-2 (5 μg) delivered with an electrospun nanofiber mesh and alginate hydrogel was able to promote critical-size femur defect regeneration in the rat, demonstrating effective dosing in bone regeneration applications.

Low doses (0.01– 5 μg/mL) of α2PI1−8-VEGF-A promotes normal angiogenesis, while aberrant vessel formation and vascular hyperpermeability are adverse effects associated with the uncontrolled delivery of VEGF-A, illustrating the critical importance of proper dosing.

Mathematical Modeling and Computational Approaches

Mathematical models provide powerful tools for predicting optimal growth factor concentrations and understanding complex dose-response relationships.

Pharmacokinetic/Pharmacodynamic (PK/PD) Modeling

PK/PD models integrate information about growth factor distribution, metabolism, and biological effects to predict optimal dosing regimens. These models account for:

  • Absorption and distribution kinetics
  • Clearance rates and half-life
  • Receptor binding dynamics
  • Downstream signaling cascade activation
  • Tissue-specific responses

Systems Biology Approaches

Systems biology integrates multiple data sources to create comprehensive models of growth factor signaling networks. These approaches can:

  • Predict synergistic effects of multiple growth factors
  • Identify optimal concentration ratios for combination therapies
  • Account for temporal dynamics in growth factor release
  • Model spatial gradients in tissue engineering scaffolds

Machine Learning and Artificial Intelligence

Advanced computational methods are increasingly applied to optimize growth factor dosing:

  • Neural networks for predicting dose-response relationships
  • Genetic algorithms for optimization
  • Bayesian approaches for incorporating prior knowledge
  • High-throughput screening data analysis

Factors Influencing Optimal Growth Factor Concentrations

Tissue-Specific Considerations

Different tissues have distinct regenerative capacities and requirements that influence optimal growth factor concentrations.

Bone Tissue

Bone regeneration typically requires higher growth factor concentrations due to the dense extracellular matrix and mineralization requirements. BMPs are particularly important, with clinical applications often using microgram to milligram quantities.

Soft Tissue and Skin

Soft tissue regeneration generally responds to lower growth factor concentrations. ECM-binding variants of VEGF-A, PDGF-BB and BMP-2 significantly increased their therapeutic efficacy compared to the wild-type growth factors in models of skin chronic wound healing and non-union bone defects, when delivered at low doses.

Vascular Tissue

Angiogenesis requires precise VEGF concentrations, as both insufficient and excessive amounts can lead to suboptimal outcomes. The local microenvironmental concentration is particularly critical for proper vessel formation.

Cartilage and Connective Tissue

Cartilage regeneration benefits from TGF-β superfamily members and requires sustained delivery at moderate concentrations to support chondrogenesis while avoiding hypertrophy.

Growth Factor-Specific Properties

Each growth factor has unique biochemical properties that influence optimal dosing strategies.

Stability and Half-Life

Fibroblast growth factor (FGF-1) possesses intrinsically low stability, exhibiting a functional half-life of only 1 h in serum at 37 °C. Growth factors with shorter half-lives may require higher initial concentrations or sustained release systems to maintain therapeutic levels.

Altering the protease-sensitive sites that naturally occurs within GFs can be an efficient method to enhance their activity, with mutations introduced at a known cleavage site in FGF-1 demonstrated to significantly increase the proteolytic resistance of the protein up to 100-fold.

Receptor Binding Affinity

Growth factors with higher receptor binding affinity may achieve therapeutic effects at lower concentrations. Protein engineering approaches can modify binding characteristics to optimize dosing requirements.

Signaling Potency

The signaling properties of GFs can be modified to enhance their regenerative activity, thereby effecting similar or altogether different responses at lower doses. This highlights opportunities for reducing required concentrations through molecular engineering.

Delivery Method and Kinetics

The delivery system profoundly influences the effective concentration of growth factors at the target tissue.

Bolus Injection

Direct injection provides immediate high concentrations but suffers from rapid clearance. This approach may require higher total doses to compensate for rapid diffusion and degradation.

Sustained Release Systems

Controlled release from biomaterials allows lower total doses while maintaining therapeutic concentrations over extended periods. Common systems include:

  • Hydrogels (natural and synthetic polymers)
  • Microspheres and nanoparticles
  • Electrospun scaffolds
  • Injectable depot formulations

When delivered with ECM components, growth factors are protected and released gradually, with their effects persisting for days or weeks due to downstream gene activation and cell recruitment.

ECM-Binding Strategies

Engineering growth factors to target endogenous ECM is a compelling strategy to mimic the physiological delivery of growth factors and optimize their therapeutic effects on morphogenetic processes. This approach can significantly reduce required doses while improving efficacy.

Natural interactions between the ECM and GFs are crucial for tissue healing as many GFs have the ability to bind ECM proteins to some extent, with these interactions often occurring between the heparin-binding domains of ECM proteins and heparin-binding GFs.

Patient-Specific Variables

Individual patient characteristics can significantly influence optimal growth factor concentrations.

Age

Aging affects cellular responsiveness to growth factors, potentially requiring dose adjustments. Older patients may exhibit reduced receptor expression or altered signaling pathway activity.

Comorbidities

Conditions such as diabetes, vascular disease, and immunosuppression can impair tissue regeneration and may necessitate modified growth factor dosing strategies. Diabetic wounds, for example, often show reduced growth factor responsiveness.

Inflammatory Status

The regenerative response to growth factors is influenced by the immune and inflammatory microenvironment, which practically always accompanies tissue repair and regeneration. Macrophage response to BMP-2 and PDGF-BB triggers the release of IL-1β, which encourages inflammation, and because IL-1β inhibits the proregenerative effects of BMP-2 and PDGF-BB, codelivering them with IL-1Ra can enhance bone regeneration.

Genetic Factors

Genetic polymorphisms affecting growth factor receptors, signaling molecules, or metabolic enzymes may influence individual responses to specific concentrations.

Temporal Dynamics

The timing and duration of growth factor exposure critically influence regenerative outcomes.

Sequential Delivery

A combination of growth factors (BMP/VEGF, BMP-2/BMP-7) and their release profiles in different biomaterials has the potential to improve tissue regeneration in vivo. Sequential delivery of different growth factors at specific time points can recapitulate natural healing cascades.

Pulsatile vs. Continuous Exposure

Some cellular responses benefit from pulsatile growth factor exposure, while others require sustained concentrations. The optimal pattern depends on the specific regenerative process and target cells.

Advanced Strategies for Optimizing Growth Factor Concentrations

Combination Therapy Approaches

Using multiple growth factors simultaneously can achieve synergistic effects and allow lower individual concentrations.

Synergistic Combinations

The optimized concentration of combined bFGF and EGF promoted the proliferation of fibroblasts, which are the main cells in repair of pelvic ligaments, contributing to efficient construction of tissue with seed cells for tissue engineering. This demonstrates how combining growth factors can enhance outcomes.

Multiple GF release plays an essential role in the recruitment, proliferation, and functional activities of MSCs during the early phases of wound repair and the promotion of tissue regeneration.

Determining Optimal Ratios

When using multiple growth factors, determining optimal concentration ratios is critical. Systematic testing of different ratios using factorial experimental designs can identify synergistic combinations.

Protein Engineering for Dose Reduction

Modifying growth factor structure can enhance activity and reduce required concentrations.

Enhanced Stability Variants

Engineering more stable growth factor variants extends their functional half-life and reduces the total dose needed for therapeutic effect.

Super-Affinity Variants

PIGF-2123–144, a placental growth factor-2-derived ECM-binding domain, promiscuously binds multiple ECM proteins with high affinity, and when fused to VEGF-A, PDGF-BB, and BMP-2, the engineered variants showed the ability to bind several ECM proteins with much higher affinity (super-affinity) compared to their wild-type counterparts, contributing to improved therapeutic efficacy in murine models of chronic wounds and bone regeneration.

Biomaterial-Based Optimization

Advanced biomaterials can optimize growth factor presentation and concentration at the cellular level.

Affinity-Based Release

Biomaterials designed with specific binding domains can control growth factor release kinetics based on cellular demand, maintaining optimal local concentrations.

Cell-Responsive Systems

Smart biomaterials that respond to cellular signals (enzymes, pH changes, mechanical forces) can provide on-demand growth factor release, automatically adjusting concentrations based on tissue needs.

Endogenous Growth Factor Activation

Rather than delivering exogenous growth factors, some strategies focus on activating or protecting endogenous growth factors.

Platelet-Rich Plasma (PRP)

The PRP gel provides more similarity to the natural wound healing process involving multiple growth factors in their biologically determined ratios, and acts as a tissue sealant and sustained delivery system for accelerating bone repair, promoting fibroblast proliferation, and increasing tissue vascularity.

Protease Inhibition

Protecting growth factors from degradation can maintain effective concentrations without increasing the administered dose. This is particularly relevant in chronic wounds where excessive protease activity degrades therapeutic proteins.

Clinical Considerations and Safety

Regulatory Perspectives

Therapies based on recombinant growth factors are still hindered by limitations that include ineffectiveness at low doses and serious side effects at high doses, which has led the U.S. Food and Drug Administration to release boxed warning for some growth factors such as bone morphogenetic protein-2 (BMP-2) and platelet-derived growth factor–BB (PDGF-BB).

Regulatory agencies require comprehensive dose-ranging studies demonstrating both efficacy and safety across the therapeutic window. Key requirements include:

  • Dose-escalation studies in preclinical models
  • Identification of the no-observed-adverse-effect level (NOAEL)
  • Determination of the therapeutic index
  • Long-term safety monitoring
  • Evaluation of off-target effects

Adverse Effects of Suboptimal Dosing

Underdosing Consequences

Insufficient growth factor concentrations may result in:

  • Inadequate tissue regeneration
  • Delayed healing
  • Incomplete functional recovery
  • Wasted resources and patient burden

Overdosing Risks

Excessive growth factor concentrations can cause:

  • Aberrant tissue formation (excessive scarring, heterotopic ossification)
  • Uncontrolled cell proliferation
  • Abnormal angiogenesis
  • Inflammatory responses
  • Potential oncogenic effects with prolonged exposure

One of the major side effects of BMP-2 in clinical use is rampant inflammation, highlighting the importance of proper dosing to minimize adverse reactions.

Cost-Effectiveness Considerations

BMP-2 and PDGF-BB have raised major concerns regarding safety and cost-effectiveness for multiple clinical applications, likely due to the use of high doses coupled with suboptimal delivery systems.

Optimizing growth factor concentrations has significant economic implications:

  • Reducing the amount of expensive recombinant proteins required
  • Minimizing adverse events and associated treatment costs
  • Improving treatment success rates and reducing revision procedures
  • Enabling broader access to regenerative therapies

Practical Guidelines for Determining Optimal Concentrations

Step-by-Step Approach

Step 1: Literature Review and Preliminary Range Identification

Begin by conducting a comprehensive literature review to identify:

  • Previously tested concentration ranges for your specific growth factor and application
  • Physiological concentrations in relevant tissues
  • Concentrations used in approved clinical products
  • Reported effective and toxic doses

Step 2: In Vitro Dose-Response Studies

Conduct systematic in vitro experiments:

  • Test a wide concentration range (typically spanning 3-4 orders of magnitude)
  • Use relevant cell types for your application
  • Assess multiple endpoints (proliferation, differentiation, migration, matrix production)
  • Include appropriate controls and replicates
  • Generate complete dose-response curves

Step 3: Delivery System Integration

Evaluate how your delivery system affects growth factor bioavailability:

  • Measure release kinetics from your chosen biomaterial
  • Determine the relationship between loaded and released concentrations
  • Assess growth factor stability in your delivery system
  • Optimize loading efficiency and release profiles

Step 4: In Vivo Validation

Validate optimal concentrations in relevant animal models:

  • Start with concentrations identified as optimal in vitro
  • Include dose-ranging groups
  • Monitor both efficacy and safety endpoints
  • Assess tissue-level responses and systemic effects
  • Conduct histological and functional analyses

Step 5: Refinement and Optimization

Iteratively refine concentrations based on experimental results:

  • Narrow the concentration range around optimal values
  • Test combination therapies if applicable
  • Evaluate temporal release patterns
  • Assess patient-specific factors in relevant models

Quality Control and Standardization

Ensuring reproducible results requires rigorous quality control:

  • Use certified growth factor preparations with documented activity
  • Verify growth factor concentration by appropriate assays (ELISA, Western blot)
  • Maintain consistent storage and handling procedures
  • Document lot-to-lot variability
  • Include positive and negative controls in all experiments
  • Use standardized protocols across experiments

Emerging Technologies and Future Directions

High-Throughput Screening Platforms

Advanced screening technologies enable rapid evaluation of multiple growth factor concentrations and combinations:

  • Microfluidic devices for gradient generation
  • Automated liquid handling systems
  • High-content imaging for multi-parameter analysis
  • Organ-on-chip platforms for physiologically relevant testing

Personalized Medicine Approaches

Future strategies may tailor growth factor concentrations to individual patients:

  • Patient-derived cell testing to predict responsiveness
  • Genetic profiling to identify optimal dosing
  • Biomarker-guided dose adjustment
  • Real-time monitoring of tissue responses

Smart Delivery Systems

Next-generation biomaterials will provide unprecedented control over growth factor concentrations:

  • Feedback-controlled release systems
  • Externally triggered delivery (light, ultrasound, magnetic fields)
  • Self-regulating systems responsive to tissue healing status
  • Nanotechnology-based precision delivery

Gene Therapy and Cell-Based Delivery

Alternative approaches to exogenous growth factor delivery include:

  • Gene therapy for sustained endogenous production
  • Engineered cells secreting optimal growth factor levels
  • CRISPR-based regulation of growth factor expression
  • Exosome-mediated delivery of growth factor signals

Case Studies: Optimal Concentrations in Specific Applications

Bone Regeneration with BMP-2

BMP-2 has been extensively studied for bone regeneration, with optimal concentrations varying based on delivery method and defect size. Clinical applications have used doses ranging from micrograms to milligrams, though concerns about high-dose side effects have driven research toward lower, more controlled delivery approaches.

Wound Healing with PDGF

PDGF-BB is FDA-approved for diabetic foot ulcers at a concentration of 0.01% (100 μg/g). This concentration was determined through extensive clinical trials demonstrating efficacy without significant adverse effects.

Angiogenesis with VEGF

VEGF dosing for therapeutic angiogenesis requires careful optimization, as the concentration window between insufficient and excessive angiogenesis is relatively narrow. Successful approaches often use sustained low-dose delivery rather than bolus administration.

Cartilage Repair with TGF-β

TGF-β family members support chondrogenesis at nanogram to low microgram concentrations. Optimal dosing depends on the specific TGF-β isoform and the stage of cartilage development being targeted.

Challenges and Limitations

Translational Gaps

Significant challenges exist in translating optimal concentrations from preclinical to clinical settings:

  • Species differences in growth factor responsiveness
  • Scaling issues from small to large defects
  • Differences between acute experimental models and chronic clinical conditions
  • Variability in human patient populations

Technical Limitations

Current methodologies face several constraints:

  • Difficulty measuring local growth factor concentrations in vivo
  • Limited ability to monitor real-time tissue responses
  • Challenges in creating truly physiological in vitro models
  • Complexity of multi-factor interactions

Economic and Practical Barriers

Real-world implementation faces obstacles:

  • High cost of recombinant growth factors
  • Manufacturing and quality control challenges
  • Regulatory requirements for dose optimization studies
  • Limited reimbursement for some applications

Best Practices and Recommendations

For Researchers

  • Conduct comprehensive dose-response studies rather than testing single concentrations
  • Use physiologically relevant cell culture conditions and 3D models when possible
  • Consider the delivery system as an integral part of dose optimization
  • Evaluate both efficacy and safety endpoints across the concentration range
  • Report complete experimental details to enable reproducibility
  • Consider temporal dynamics and not just steady-state concentrations

For Clinicians

  • Follow evidence-based dosing guidelines from clinical trials
  • Monitor patients for both therapeutic response and adverse effects
  • Consider patient-specific factors that may influence optimal dosing
  • Participate in registries and post-market surveillance to contribute to dosing knowledge
  • Stay informed about new delivery technologies that may alter optimal concentrations

For Industry

  • Invest in dose-optimization studies early in product development
  • Develop delivery systems that enable lower, safer doses
  • Conduct thorough pharmacokinetic and pharmacodynamic studies
  • Provide clear dosing guidance based on robust clinical data
  • Support post-market studies to refine dosing recommendations

Conclusion

Calculating optimal growth factor concentrations for tissue regeneration is a complex, multifaceted challenge that requires integration of biological understanding, experimental rigor, and clinical insight. While a number of GFs have been demonstrated to be efficient at high doses in multiple biomedical applications, safety and precision medical treatment have driven research towards the development of novel delivery systems that enable dose reduction and optimize the concentrations of the right GFs at the right time.

The field has evolved from simple dose-escalation approaches to sophisticated strategies incorporating protein engineering, advanced biomaterials, and computational modeling. To pave the way toward safe and cost-effective growth factor-based therapies, several strategies to mimic natural ECM functions have been explored, with the goal of achieving local and sustainable delivery of bioactive growth factors, and thus allowing the reduction of therapeutic doses.

Success in determining optimal concentrations requires systematic experimental approaches, beginning with in vitro dose-response studies and progressing through in vivo validation in relevant models. The delivery method is not merely a vehicle but an integral component that profoundly influences the effective concentration at the tissue level. Patient-specific factors, tissue type, inflammatory environment, and temporal dynamics all contribute to the optimal dosing strategy.

Recapitulating the concentrations and spatial and temporal distributions of bioactive factors during tissue development and healing processes, accounting for the effects of cellular heterogeneity and their carriers on target cells, could serve as engineering design criteria to specifically guide the release of GF from the delivery system, which should not only release the appropriate GFs at accurate doses and kinetics but also offer insights on multiple GFs and their native microenvironments during tissue healing in vivo.

As regenerative medicine continues to advance, emerging technologies including high-throughput screening, personalized medicine approaches, and smart biomaterials promise to further refine our ability to determine and deliver optimal growth factor concentrations. The integration of these approaches with a deeper understanding of growth factor biology will ultimately enable safer, more effective, and more accessible regenerative therapies.

For those working in this field, whether in research, clinical practice, or industry, the key is to recognize that optimal concentration is not a single number but rather a carefully balanced parameter influenced by multiple biological, technical, and patient-specific factors. By applying rigorous scientific methods, considering the broader biological context, and remaining attentive to both efficacy and safety, we can continue to improve outcomes in tissue regeneration and advance the promise of regenerative medicine.

For more information on tissue engineering and regenerative medicine, visit the National Institute of Biomedical Imaging and Bioengineering. Additional resources on growth factor biology can be found at the Nature Research Growth Factors portal. For clinical trial information on growth factor therapies, consult ClinicalTrials.gov.