A pericyte, not to be confused with the homonym parasite, is a sort of hybrid human cell, part nervous and part vascular. It is a type of nerve cell that is primarily found encasing and protecting the vascular blood vessels of the brain. One of its most important functions is as an agent of the so-called “blood-brain barrier.” These cells prevent some blood-born toxic chemicals and foreign organisms from being released into sensitive brain tissue. Thy also have other, equally important functions, and are believed to be critical keys to understanding the cause and perhaps cure of some brain diseases.
The term is derived from the Greek peri for “around,” and cyte for “cell.” A cell is the smallest self-contained unit of life. Multi-cellular organisms, such as humans, are composed of many different types of cells organized as systems, co-dependent with other systems to sustain their lives. The defining characteristic of the pericyte is that they tightly wrap around the outer wall of the smallest blood vessels, called capillaries. They are especially prevalent in the brain, but are found on blood vessels that nourish the body’s many other nerve cells with oxygen and nutrients.
A pericyte grips its tubular vessel by means of finger-like projections. This shape also allows the cells to interlock with each other. The type of cells that form the wall of blood vessels is called endothelial cells. Although there is a thin layer of connective tissue between them called a basement membrane, pericytes are in direct contact with the endothelial cells. The primary medium of cell-to-cell communication are signaling chemicals called integrins.
The walls of blood vessels are permeable, designed to allow oxygen and selectively large molecules such as sugars to pass through them. All of the cells of the human body, including those of the brain, need this supply. Unfortunately, blood is also the transport of choice for substances and organisms potentially harmful to the delicate, stable chemistry of the brain. Its network of blood vessels requires an extra measure of selectivity in what is allowed to escape its vascular walls. This has come to be popularly called the blood-brain barrier.
A pericyte that encases the brain’s blood vessels is the first line of defense in this barrier. They can contract, both constricting the flow of blood within the capillary as well as closing the gaps in its wall through which large chemical molecules would otherwise pass. In a behavior normally associated with predatory immune cells in human blood, these versatile cells have the additional ability to engulf and digest foreign particles, such as dead cells. The endothelial cells of blood vessels, similarly to skin, are regenerative and pericytes are furthermore believed to be involved in chemically controlling their re-growth. They also exhibit embryonic qualities, able to morph into other entirely different cell types as needed, such as smooth muscle cells, also called mural cells.
Abnormalities have been linked to some diseases. Hemangiopericytoma, for example, is a rare form of cancer caused by an excess of pericytes strangling abnormally formed blood vessels. Diabetic retinopathy, the common complication of the disease diabetes resulting in loss of vision, has been found to be caused by the insufficiency of pericytes to protect the capillaries in the human eye. Many degenerative diseases of the brain, such as Alzheimer’s and Multiple Sclerosis, are believed to principally start from loss of pericytes and the resulting breach in the blood-brain barrier.