Stromal cells are a diverse group of cells recognized for their supportive role in the human body. They are widespread and foundational for maintaining the health and function of various tissues. These cells provide a structural backdrop and interact with other cell types, influencing numerous biological processes.
What Stromal Cells Are
Stromal cells are a diverse group of connective tissue cells that provide the underlying framework, or stroma, for organs and tissues. They are distinct from parenchymal cells, which are the functional cells of an organ, such as liver cells or kidney cells. Stromal cells effectively serve as the “soil” in which other functional cells grow and thrive, providing physical support and a conducive environment.
These cells are characterized by their ability to produce and organize the extracellular matrix (ECM), a complex network of macromolecules like collagen, proteoglycans, and elastin. The ECM provides structural integrity to tissues and also influences cell behavior through mechanical and biochemical cues. Stromal cells also secrete a variety of soluble factors, including growth factors and cytokines, which signal to neighboring cells and modulate their activities.
Many stromal cells, particularly mesenchymal stromal cells (MSCs), are multipotent, meaning they can differentiate into various cell types. This multipotency allows them to contribute to tissue repair and regeneration by giving rise to specialized cells like bone, cartilage, or fat cells under specific conditions.
Where Stromal Cells Reside
Stromal cells are found extensively throughout the body, forming the connective tissue in nearly every organ and tissue. They are abundant in the bone marrow, where they support the development of blood cells. Adipose, or fat, tissue also contains a significant population of stromal cells, which are involved in its structure and function.
These cells are present in the skin, contributing to its regeneration and repair, and in muscles, where they aid in tissue maintenance. Various organs such as the liver, kidneys, lymph nodes, prostate, and ovaries also host stromal cells that support the primary functions of their respective parenchymal cells.
The Many Roles of Stromal Cells
Stromal cells perform a wide array of functions that are indispensable for tissue health and overall bodily processes. One of their primary roles is providing structural support by synthesizing the extracellular matrix. This matrix acts as a scaffold, giving tissues their shape and mechanical properties, and influencing how cells within the tissue organize and interact.
Beyond structural support, stromal cells engage in extensive signaling and communication with other cell types. They release growth factors, cytokines, and other molecules that regulate the growth, differentiation, and behavior of neighboring cells, including immune cells and epithelial cells. This complex molecular dialogue helps maintain tissue homeostasis and orchestrate cellular responses to various stimuli.
Stromal cells also play a role in immune regulation, modulating inflammatory responses throughout the body. They can influence the activity of immune cells, either promoting or suppressing inflammation, which is important for resolving infections and preventing excessive tissue damage. Their interactions with immune cells help shape the local immune environment within tissues.
These cells are actively involved in tissue repair and regeneration, acting as responders to injury. They contribute to wound healing by producing factors that stimulate the proliferation of new cells and by forming scar tissue when necessary. Their ability to create a supportive microenvironment aids the body’s natural repair mechanisms.
Stromal cells additionally facilitate nutrient and waste exchange, contributing to the transport of substances to and from parenchymal cells within tissues.
Key Examples of Stromal Cells
Fibroblasts are among the most common stromal cells, found throughout connective tissues. Their primary function involves synthesizing collagen and other components of the extracellular matrix, which provides mechanical strength and structural integrity to tissues. Fibroblasts also play an active role in wound healing, adapting their phenotype to synthesize building blocks for repairing damaged tissue.
Mesenchymal Stem Cells (MSCs), also known as mesenchymal stromal cells, represent a multipotent population with the capacity to differentiate into various cell types, including bone, cartilage, fat, and muscle cells. MSCs are found in many tissues, such as bone marrow and adipose tissue, and contribute to tissue regeneration and immune modulation through their secretory functions. They respond to injury by secreting bioactive molecules that reduce inflammation and promote healing.
Adipocytes, commonly known as fat cells, are primarily recognized for energy storage but also function as stromal components within adipose tissue. They store lipids and release hormones and signaling molecules that influence metabolism and inflammation throughout the body. Adipocytes interact with other stromal cells and immune cells, contributing to the overall tissue microenvironment.
Pericytes are cells that wrap around the endothelial cells lining small blood vessels, including capillaries and venules. They contribute to the stability and integrity of blood vessels, regulating blood flow and maintaining the blood-brain barrier. Pericytes also participate in tissue repair and angiogenesis (the formation of new blood vessels) by producing growth factors and cytokines.