Platelet extravasation in the microvasculature: an under-appreciated role for platelets

In recent years, evidence has accumulated highlighting the presence and role of platelet extravasation - wherein platelets accumulate in tissue parenchyma - at a variety of sites throughout the body. While platelets are traditionally known for their roles in hemostasis and thrombosis, it is evident that platelets are potent mediators of inflammation across an array of physiological and pathological contexts. While anucleate and small in size, platelets contain a rich diversity of molecules in their granules capable of modulating cell proliferation, tissue repair, and a host of immune responses. There is growing evidence that platelet extravasation out of the vascular space and into tissue parenchyma not only occurs in a host of pathological conditions, but it may also play a role in inflammation and disease progression. This review examines the evidence of platelet extravasation in a number of microvascular beds, including the skin, tumor, cortical, corneal, lung, and liver microvasculature, discussing mechanisms of extravasation and roles platelets play in these contexts. Understanding the dynamics and functional relevance of platelet extravasation may provide insight into novel therapeutic targets and lines of scientific inquiry into diseases and conditions affecting the microvasculature.