Sofia Ioannidou Steven J Samuelsson and David T Shima

Eyetech Research Center, Eyetech Pharmaceuticals, Inc.

Precise regulation of blood-tissue interchange is critical for proper integration of organ physiology with the cardio-vasculature. Accordingly, microvascular endothelial cells, the primary barrier to free blood-tissue exchange, adopt highly specialized features and behaviors to mediate and monitor the flux of macromolecules and fluids across the vascular wall. For example, the vascular endothelium of the blood-brain and the blood-retinal barriers is fortified by tight junction components that restrict access of fluid and blood components to sensitive neural tissues. The other extreme of endothelial barrier differentiation is that of lymphatic vessels, an endothelium with numerous gaps between cells enabling uptake of extravascular fluid for clearance, transport, and return to the circulation. Finally, organs mediating endocrine, absorptive, or filtrating functions have a demand for abundant but more regulated fluid and macro-molecule exchange and thus are supported by a highly specialized vascular endothelium containing numerous plasma membrane pores, the fenestrae (Figure 1). Derived from the Latin word for window, the term fenestra is used in this context to describe an opening in the vessel wall that connects the luminal and extravascular space. In this chapter, fenes-trae structure, function, and pathophysiological relevance are discussed in detail.

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