During the development of the retina, platelet-derived growth factor released by retinal neurons stimulates the proliferation of astrocytes, which release vascular endothelial growth factor (VEGF) to stimulate blood vessel growth. On p. ,West and co-workers describe how the developing retinal vessels provide feedback signals that trigger astrocyte differentiation, thus downregulating VEGF production and preventing runaway angiogenesis. In newborn mice raised in a high-oxygen atmosphere, retinal blood vessel development is blocked,resulting in retinal hypoxia. In this situation, report the researchers,retinal astrocytes continue to proliferate and fail to differentiate,indicating that retinal blood vessels usually limit their own formation by signalling to the astrocytes. Culturing astrocytes in a low-oxygen atmosphere reproduces these effects on astrocyte behaviour, suggesting that blood-borne oxygen might be the signal that indirectly prevents the over-elaboration of the vascular network. Similar feedback mechanisms may also govern vascularisation elsewhere in the nervous system.
