Pericytes are specialized cells that wrap around the body’s smallest blood vessels, the capillaries and venules. Found in every tissue with blood vessels, they are a component of the microcirculation system. These cells are embedded within the basement membrane that also surrounds the endothelial cells lining these vessels. This close association is important for the health and operation of the circulatory system at a microscopic level.
The Structure and Location of Pericytes
Pericytes possess a distinct structure related to their placement within the microvasculature. They have a prominent, round cell body housing the nucleus, which distinguishes them from the flat nuclei of neighboring endothelial cells. From this central body, pericytes extend long, finger-like projections that wrap around the capillary wall. This arrangement allows for extensive contact with the vessel.
These projections enable pericytes to communicate directly with endothelial cells through physical connections known as gap junctions, which permit the exchange of ions and small molecules. They are located at the interface between blood vessels and surrounding tissue, positioning them to interact with signals from both the bloodstream and the organ. The density of pericytes varies between organs, with higher concentrations in tissues like the central nervous system, reflecting specialized roles.
Key Functions in the Body
Pericytes regulate blood flow through the smallest vessels. By contracting or relaxing their cytoplasmic extensions, these cells can change the diameter of the capillaries they surround. This action finely tunes the amount of blood reaching specific areas of tissue, ensuring oxygen and nutrient delivery matches the metabolic needs of the cells. This function is important in organs with high and fluctuating energy demands, such as the brain.
Pericytes also maintain the structural stability of blood vessels. They support the endothelial cells and contribute to the architecture of the microvasculature. Through direct contact and signaling molecules, pericytes guide the maturation and stabilization of newly formed vessels. Without adequate pericyte coverage, capillaries can become weak and prone to leakage, compromising the vascular network.
A specialized function of pericytes is maintaining the blood-brain barrier (BBB). In the central nervous system, pericytes work with endothelial cells and astrocytes to form this protective barrier. The BBB strictly controls the passage of substances from the bloodstream into the brain, protecting it from harmful toxins and pathogens. Pericytes help reinforce the tight junctions between endothelial cells, acting as gatekeepers that regulate vascular permeability.
Role in Tissue Repair and Regeneration
In response to injury, pericytes act like stem cells. They can differentiate, or transform, into other cell types needed for tissue repair. For instance, following an injury, pericytes can become fibroblasts, which produce collagen and other components of the extracellular matrix to mend damaged tissue.
Pericytes contribute to angiogenesis, the formation of new blood vessels. This process is necessary for wound healing, as new vessels deliver oxygen, nutrients, and immune cells to the site of injury. Pericytes participate in the development and stabilization of these new capillaries, ensuring they mature into functional vessels. They are recruited to the site of injury and help guide the growth of endothelial cells.
This regenerative capacity, however, can have a downside. When the healing process is not properly regulated, pericyte activity can contribute to fibrosis, the formation of excessive scar tissue. In some organs, pericytes can differentiate into myofibroblasts, cells that produce the dense, fibrous tissue that characterizes a scar. This excessive scarring can impair the function of the affected organ.
Connection to Disease Processes
Pericyte dysfunction can lead to various diseases. The loss or dysfunction of these cells is linked to a breakdown in the stability of small blood vessels. This is evident in diabetic retinopathy, a complication of diabetes where pericyte death leads to leaky and damaged blood vessels in the retina, which can cause vision loss.
In neurodegenerative conditions such as Alzheimer’s disease, the failure of pericytes is associated with a breakdown of the blood-brain barrier. Pericyte degeneration, especially in the hippocampus, correlates with increased permeability of the BBB, allowing harmful substances from the blood to enter the brain. This vascular damage is believed to contribute to the neuronal injury and cognitive decline seen in Alzheimer’s patients.
Pericytes also play a complex role in cancer. Tumors require a dedicated blood supply to grow and spread, a process fueled by angiogenesis. Cancer cells can co-opt the body’s vessel-forming processes, recruiting pericytes to help build new blood vessels that feed the tumor. The presence of pericytes on tumor vessels can influence tumor growth, metastasis, and the effectiveness of certain cancer therapies.