Table 1.

Brief description of physiologic and pathologic roles of pericytes.

Function	Comments/reference
Physiological
Blood brain barrier	The blood brain barrier serves to prevent the passive transport of cells, proteins and bioactive compounds from the systemic blood to the vasculature of the CNS (Abbott, Patabendige, Dolman, Yusof, & Begley, 2010). Studies have demonstrated that the brain has the highest coverage of pericytes and that they play a critical role in the maturation and maintenance of the barrier. Deficiency of pericytes results in increased permeability to water and low and high molecular weight compounds (Armulik et al., 2010).
Vascular permeability	Increased vascular permeability is seen in response to a number of pathological states including sepsis, trauma, tumors and microangiopathy where pericytes have been observed to migrate from the vessels wall resulting in increased permeability (Nag, Kapadia, & Stewart, 2011).
Vasoconstriction	The role that pericytes play in capillary constriction was one of the first described functions of pericytes and has been observed in vitro (Díaz-Flores et al., 2009), and in vivo (Fernández-Klett, Offenhauser, Dirnagl, Priller, & Lindauer, 2010). This contraction facilitates variations in regional blood flow and contributes to the control of blood pressure.
Stem cell	Many studies have demonstrated that pericytes represent a population of MSC progenitor cells that express MSC markers in vivo, and are capable of multilineage differentiation in vitro (Crisan et al., 2008). Subsequent in vivo studies have demonstrated the role that pericytes play in the regeneration of white adipose tissue (Tang et al., 2008), odontoblasts (Feng et al., 2011) and skeletal muscle (Dellavalle et al., 2011) among other tissues.
Pathological
Fibrosis	The myofibroblast is a critical cell in fibrosis and is responsible for the deposition of pathological extracellular matrix. The pericyte has been demonstrated to be the progenitor of the myofibroblast in many solid organs including liver, lung, heart (Kramann et al., 2015) and skin (Dulauroy, Di Carlo, Langa, Eberl, & Peduto, 2012).
Neurodegeneration	Loss of pericytes from blood vessels in the brain has been shown to lead to increased vascular permeability and promotion of neurodegeneration in a murine model of aging.
Cancer	A significant proportion of a tumor’s bulk is the stromal component of which pericytes form a large part. The stroma has key roles in tumor growth and progression and therefore pericytes represent a potential target for anti-cancer therapies (Pietras & Ostman, 2010). The exact origin and role of pericytes in tumor biology remains relatively poorly understood.
Pericyte derived tumors	A group of related tumors demonstrates pericytes marker expression and may be derived from modified pericytes (Shen et al., 2015a; Shen et al., 2015b).
Diabetic retinopathy	Diabetic retinopathy is a common complication experienced by up to a third of adult diabetics. Pericyte death is the first in a chain of events that leads to basement membrane thickening, capillary leakage, vessel occlusion and the subsequent proliferative retinopathy (Barber, Gardner, & Abcouwer, 2011).