Fibroblasts and myofibroblasts are central to tissue integrity and wound healing. While closely related, they are distinct cell types with specific functions. Fibroblasts serve as the architects of connective tissue, while myofibroblasts are specialized contractors summoned for repair. Understanding the relationship between these cells is fundamental to comprehending how our bodies recover from injury and why that process can sometimes go awry.
The Foundational Fibroblast
Fibroblasts are the most abundant cells within the connective tissue of animals, providing the structural framework for most tissues. These spindle-shaped cells are resident, long-lived components of their environment. Their primary responsibility is the synthesis and organization of the extracellular matrix (ECM), a complex network of proteins that provides structural support to surrounding cells.
The most notable components produced by fibroblasts are collagen and fibronectin. Collagen fibers give tissues their strength and resilience, while fibronectin is an adhesive protein that helps cells attach to the matrix. By managing the ECM, fibroblasts ensure the structural integrity and proper function of tissues throughout the body.
The Specialized Myofibroblast
The myofibroblast is a highly specialized cell that emerges primarily in response to tissue injury. It possesses a hybrid nature, sharing characteristics with both the fibroblast it originates from and a smooth muscle cell. Unlike fibroblasts, myofibroblasts are temporary residents in a tissue.
The defining feature of a myofibroblast is its expression of a protein called alpha-smooth muscle actin (α-SMA). This protein assembles into stress fibers, which are bundles of microfilaments that grant the cell significant contractile power—an ability that ordinary fibroblasts lack. These cells also have a structure that supports intense protein production and secretion, making them effective agents in tissue repair.
The Transformation Process
The conversion of a fibroblast into a myofibroblast is a process known as differentiation or activation, initiated by specific signals from tissue damage. The two main triggers are mechanical stress and chemical messengers. When a wound occurs, the surrounding tissue experiences increased physical tension, which is one of the first cues for resident fibroblasts to change.
This mechanical signal is complemented by chemical factors, most notably Transforming Growth Factor-beta 1 (TGF-β1). Released by cells at the injury site, TGF-β1 instructs fibroblasts to alter their gene expression. In response, the fibroblast produces α-SMA and integrates it into its cytoskeleton, forming the stress fibers that define a myofibroblast capable of exerting strong contractile forces.
Functional Roles in Healing and Fibrosis
In normal wound healing, fibroblasts and myofibroblasts work in a coordinated sequence. Following an injury, fibroblasts migrate to the site and begin depositing a provisional extracellular matrix. The primary role of the resulting myofibroblasts is to contract, pulling the edges of the wound closer together in a process called wound contraction. This action significantly reduces the size of the defect that the body needs to fill.
Once the wound is closed and the new tissue has matured, the repair process concludes with the disappearance of the myofibroblasts through programmed cell death, called apoptosis. If this regulation fails and myofibroblasts persist, their continued activity becomes detrimental. The ongoing contraction and excessive deposition of ECM, particularly collagen, leads to the formation of stiff, dense scar tissue. This pathological state is known as fibrosis.
This process is the underlying cause of various diseases. In the lungs, it can lead to pulmonary fibrosis, where scarring stiffens the lung tissue and impairs breathing. In the liver, chronic injury can result in cirrhosis, where functional liver tissue is replaced by fibrotic scars. On the skin, the overactivity of myofibroblasts can produce hypertrophic or keloid scars.