The retinal vasculature consists of a single layer of vascular endothelial cells, arranged to form a lumen, which are ensheathed by pericytes. In the superficial retina, these vessel structures are then ensheathed by astrocytic processes. Each cell layer is separated by a negatively charged basement membrane (Figure 1). In the deeper retina, Müller cells, rather than astrocytes, participate in the formation of the perivascular sheath. Through mechanisms that are not entirely clear, the growth, maturation, and permeability of retinal vessels are dependent on the growth and maturation of pericytes and astrocytes. A significant amount of work has been directed at determining the roles of these supporting cell types in retinal development and homeostasis, and what follows is a brief summary of the current progress. To focus this discussion, we have only included what is known for humans, mice, and rats.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.