Fibroblasts

Fibroblasts are mesenchymal cells that are essential in maintaining tissue homeostasis, and play a significant role in the development of many diseases, including cancer. They produce the building blocks of the extracellular matrix (ECM) – specifically, fibrous proteins and ground substance – which support all tissues, including the parenchyma.¹ Fibroblast heterogeneity is well documented, and the functional specialization of these cells depends on their organ of origin and location within the body. Differences in fibroblast phenotypes have even been seen across different sites of the same organ.¹

Formation

Fibroblasts are not terminally differentiated, and retain the potential to mature into various subtypes.¹ They originate from epiblasts in the mesenchyme via type 1 epithelial-to-mesenchymal transition (EMT) during gastrulation.² Further differentiation gives rise to the mesendoderm, and subsequently the endoderm and mesoderm, the latter forming the true mesenchyme.² Resident (mature) fibroblasts develop from the mesenchyme, along with other cells for connective tissue, bones, cartilage, and blood and lymphatic circulatory systems.² The mesoderm also gives rise to adipocytes, epithelial cells, endothelial cells, perivascular cells, and bone marrow-derived fibroblasts and fibrocytes, which, together with mature fibroblasts, can generate fibroblast-like cells in response to injury.²

The developmental heterogeneity of fibroblasts is best understood in the mammalian dermis, though these principles are thought to be shared with other tissues, including the intestine, lung, heart, and muscle.³ Differences are believed to exist from a combination of intrinsic factors – including transcriptional regulatory networks and epigenetic processes – and extrinsic influences, such as cell-cell contact, secreted signalling factors, and interactions with ECM elements,³ which can be different depending on where the cell is found within the body.

Function

The primary function of mature fibroblasts is ECM production and maintenance. They produce and secrete all components of the ECM, including structural and adhesive proteins, and ground substance. Structural proteins have distinct properties and can increase ECM rigidity – for example, with collagen type 1 – or plasticity, with elastin.¹ Adhesive proteins, such as fibronectin and laminin, connect cells and the ECM, while ground substance allows nutrients to travel into tissues beyond the reach of blood vessels, as well as acting as a pathway for intercellular communication.¹

Their function makes fibroblasts vital to wound healing in response to tissue injury, where they also play a role in inflammation and immune cell recruitment. Fibroblasts respond to and synthesize cytokines and chemokines to help guide the inflammatory response.⁴ They can also differentiate into myofibroblasts, which produce myosin and alpha smooth muscle actin that increase ECM production.^1,4 Uncontrolled proliferation of myofibroblasts or fibroblasts is the pathological feature of fibrosis, which is responsible for multiple diseases across a variety of organs, and plays a role in most cases of organ failure.¹ Many cardiac diseases, for example, are associated with fibrosis, where excessive accumulation of ECM leads to adverse rebuilding of cardiac tissue, ultimately affecting function, and increasing the potential for cardiac failure.^5,6 Further examples of fibrosis affecting other organs or tissues include idiopathic pulmonary fibrosis, liver cirrhosis, kidney fibrosis, and systemic sclerosis.^1,6 Fibroblasts also contribute to blood clotting – by producing urokinase plasminogen activators and their inhibitors – and interact with endothelial cells to facilitate angiogenesis.¹

Another profound impact fibroblasts have on human health is their role in cancer progression. Cancer-associated fibroblasts (CAFs) synthesize and remodel the ECM of tumors, produce growth factors, and enhance angiogenesis, as well as influencing drug efficacy and therapy responsiveness.⁷ Indeed, their impact on the ECM can increase tissue stiffness – which triggers pro-survival and pro-proliferation signaling in cancer cells – and mechanical stress, promoting more aggressive cancer phenotypes and reducing the effectiveness of drug delivery.⁷

Emerging research

CAFs are of great interest for the development of novel cancer therapies, and some studies have documented that modifying CAF number or function is linked to outcome.⁷ Mechanisms to block cellular communication arising from CAFs – such as TGFβ or CXCL12 signalling – are being explored, as are methods to reprogram CAFs to develop an antitumorigenic phenotype, or even transform them into ‘normal’ fibroblasts.⁷

Treatments for fibrosis are also being explored, with one approach investigating specific inhibition of pathologic fibroblast subpopulations while preserving homeostatic function.^3,8 Inactivation of the transcription factor PU.1, for example, which plays a major role in fibroblast polarization and fibrogenesis, has been shown to reprogram fibrotic fibroblasts into ‘normal’ types, leading to fibrotic regression.⁸ Small molecule approaches, such as αvβ1 integrin inhibitors, which are highly expressed on activated fibroblasts,⁹ are also an active area of research, as are cytokine-mediated anti-fibrotic strategies.⁸ Further research into the vast heterogeneity of fibroblasts and their contribution to diseases will help accelerate the development of these therapies.

Tools to study fibroblasts

Fluorescence-activated cell sorting (FACS) has been successfully used to identify fibroblast lineages in mural dorsal skin,¹⁰ and characterize mouse lung fibroblast subpopulations.¹¹ However, cell surface marker expression in fibroblasts has been shown to shift when transitioning to cell culture, and mitogenic stimuli in cell culture may also have an impact on cellular identity.³ Advancements in single cell multiomics analysis are helping to overcome these issues, and have already been applied to fibrosis research.^3,8

Cell markers

Cell markers for fibroblasts are dependent on their subtype and tissue location. CD44, CD49b, CD87, CD95, and Ly-6C, for example, have been identified as cell surface markers for CAFs in a mouse model,¹² while PDGFRα, MEFSK4, DDR2, CD90, and Sca1 are known markers for cardiac fibroblasts.¹³ The identification of fibroblasts through surface marker detection can be difficult as markers aren’t usually solely expressed by these cells.

Additional markers for fibroblasts^{3,6,7,8,10,11,12}