Pericytes are star-shaped cells found throughout the body, intimately associated with the microvasculature. These cells wrap around the endothelial cells that form the inner lining of capillaries and postcapillary venules. This close proximity allows them to communicate with neighboring cells. Pericytes are fundamental in maintaining the healthy function of various bodily systems, contributing to tissue stability and overall homeostasis.
Pericytes in Blood Vessel Health
Pericytes are deeply involved in maintaining the integrity and function of blood vessels, especially within the brain. They are a component of the blood-brain barrier (BBB), a highly selective border that controls the passage of substances from the bloodstream into the brain. Pericytes contribute to the BBB’s tight seal by interacting with endothelial cells, influencing the expression of genes that regulate barrier properties and guiding the organization of astrocyte end-feet.
The regulation of blood flow is another function performed by pericytes. They possess contractile properties, allowing them to adjust the diameter of capillaries. This ability enables pericytes to influence local blood supply, ensuring that tissues receive adequate oxygen and nutrients. In the brain, pericytes can actively relax or contract capillaries to adjust cerebral blood flow in response to neuronal activity, a process known as neurovascular coupling.
Beyond regulating existing vessels, pericytes also play a role in the formation and stabilization of new blood vessels, a process called angiogenesis. They are recruited to newly formed endothelial tubes, providing structural support and promoting their maturation. This interaction helps to stabilize developing vessels and prevents leakage, which is crucial for proper tissue development and repair after injury.
Pericytes Beyond Blood Vessels
Pericytes exhibit functions extending beyond their direct involvement with blood vessel integrity and blood flow. They possess stem cell-like properties, contributing to the regeneration of various tissues. In skeletal muscle, for instance, pericytes contribute to muscle repair following injury.
There are at least two pericyte subpopulations in skeletal muscle, with distinct regenerative roles. Type-2 pericytes, identifiable by specific markers, actively participate in forming new muscle tissue after damage. Conversely, type-1 pericytes contribute to fat accumulation within the muscle.
Pericytes also have roles in other organs. In the kidney, they contribute to the regulation of blood flow within the renal cortex and medulla, influencing filtration processes. In the retina, these cells are present in high numbers and are involved in maintaining the blood-retinal barrier, which is similar to the blood-brain barrier. In the lungs, pericytes contribute to capillary homeostasis and can differentiate into other cell types in response to injury, supporting tissue maintenance.
Pericytes and Disease
Dysfunction or loss of pericytes can contribute to various diseases. In neurodegenerative conditions like Alzheimer’s disease and stroke, impaired pericyte function is linked to the breakdown of the blood-brain barrier. This breakdown can lead to the leakage of harmful substances into the brain, reduced cerebral blood flow, and subsequent neuronal damage, contributing to cognitive decline and other neurological impairments.
Pericytes also have a complex role in cancer. They can either support or hinder tumor growth and metastasis. In some cancers, pericytes can detach from blood vessels and transform into stromal fibroblasts, cells that promote tumor invasion and spread. Conversely, maintaining proper pericyte coverage of tumor vessels can help stabilize them, limiting the ability of cancer cells to escape into the bloodstream and metastasize.
Diabetic retinopathy, a common eye condition in people with diabetes, is strongly associated with pericyte loss. This loss is an early event in the disease, preceding the formation of leaky blood vessels and microaneurysms in the retina. The reduction in pericyte numbers compromises the stability of retinal capillaries, leading to impaired vision and, in severe cases, blindness.