Pericytes are cells closely associated with the smallest blood vessels, primarily capillaries and post-capillary venules. They act as supporting partners to the endothelial cells that form the inner lining of these vessels. Pericytes play an important role in maintaining the health and function of the microvasculature. Their positioning allows them to interact directly with the vascular wall, contributing to various physiological processes important for tissue health.
Where Pericytes Reside
Pericytes are embedded within the basement membrane, an extracellular layer surrounding capillaries and post-capillary venules. They are often described as “mural cells” because they reside directly on the vessel wall. This association allows for direct communication between pericytes and the vessel lining.
These cells exhibit a distinctive morphology, characterized by an irregular cell body from which multiple elongated processes extend. These processes wrap around the circumference of the capillary, resembling arms or fingers embracing the vessel. This “hugging” or “wrapping” arrangement is important for their various functions, providing structural support and facilitating direct cell-to-cell signaling with the endothelial cells. The extent of this wrapping can vary, with some pericytes covering a significant portion of the capillary surface while others have more limited contact.
Essential Roles of Pericytes
Pericytes contribute to important processes that ensure the functioning of the microvascular system. Their main responsibility involves maintaining vascular stability and integrity. They achieve this by forming physical connections with endothelial cells, reinforcing the vessel wall, and contributing to tight junctions that regulate permeability.
Pericytes regulate local blood flow in capillary networks. They possess contractile proteins, allowing them to constrict or relax capillaries. This ability influences the diameter of tiny vessels, modulating the delivery of oxygen and nutrients to specific tissue areas based on metabolic demand.
An important function of pericytes is their involvement in maintaining the blood-brain barrier (BBB). In the brain, pericytes are components of the neurovascular unit. They contribute to the formation and stability of tight junctions between brain endothelial cells, which control the passage of substances from the bloodstream into the central nervous system.
Pericytes participate in angiogenesis, the process of forming new blood vessels from existing ones. They guide and stabilize newly formed endothelial tubes during vascular development and repair. Their presence is necessary for the maturation and stabilization of new capillaries.
Beyond their structural and regulatory roles, pericytes contribute to the immune response and modulate inflammation within the microvasculature. They release signaling molecules that influence immune and endothelial cells, helping to control inflammation following injury or infection.
Pericytes exhibit stem cell-like properties. They can differentiate into various cell types, contributing to tissue regeneration and wound healing processes.
Pericytes and Human Health
Dysfunction or loss of pericytes has implications for human health, contributing to the progression of various diseases. In neurological disorders, compromised pericytes are recognized for their role in blood-brain barrier (BBB) breakdown. This BBB dysfunction is implicated in neurodegenerative conditions like Alzheimer’s and Parkinson’s diseases, where barrier integrity is compromised, allowing harmful substances to enter the brain.
The retina, a highly vascularized tissue, is vulnerable to pericyte damage. In diabetic retinopathy, a leading cause of blindness, pericyte loss is an early event. This loss weakens retinal capillaries, leading to microaneurysms, hemorrhage, and leakage.
Pericytes play a role in cancer. While important for normal angiogenesis, their involvement in tumor angiogenesis can promote tumor growth by supplying oxygen and nutrients to cancer cells. Targeting pericytes or their interaction with endothelial cells is being explored as a strategy for anti-cancer therapies to starve tumors of their blood supply.
In conditions like stroke, pericytes contribute to both initial damage and subsequent repair processes. After an ischemic stroke, pericytes can detach from the vessel wall, exacerbating BBB breakdown and contributing to brain edema. They also participate in the post-stroke repair phase, helping to restore vascular integrity and promote neurovascular recovery. Understanding these dual roles is important for developing therapies to improve stroke outcomes.