In normal retinal development, blood vessels arise at the optic nerve head and extend anteriorly to reach the periphery near term. Astrocytes similarly arise from the optic nerve and extend peripherally in advance of the nascent vasculature. In premature infants exposed to supplemental oxygen therapy, vessel growth arrests and a hypercellular ridge may develop at the border between vascularized and avascular retina. Although the ridge plays a critical role in ROP as the site of vascular shunting, and of either disease regression or progression, definitive description of the cellular composition of the ridge is lacking. Spindle-shaped ridge cells have been suggested to be mesenchymal angioblasts, or a heterogeneous population of vascular precursor cells, glia, and possibly pericyte precursors and accessory cells, that serve a transient developmental function.2 Two studies concluded that at least some ridge cells are glial, based on poorly defined microscopic features and GFAP labeling.3,4 We found that nearly all spindle-shaped cells that compose the ridge vanguard are glial: they are predominantly PAX2+ astrocyte precursors and, to a far lesser extent, mature GFAP+ astrocytes. We found no evidence of immature vascular endothelial cells (CD31+/CD34+) within the ridge vanguard. Only a few scattered CD68+ microglia were found in the ridges. Hypercellularity in the ridge vanguard may arise by cell division or by focal accumulation of astrocyte precursors whose radial migration is interrupted at the border of vascularized and avascular retina. The first possibility is supported by enhanced proliferation of retinal astrocyte precursors in low oxygen, while the second is supported by reduced astrocyte migration in hypoxia.5,6 Modest and variable proliferation among ridge cells and the scarcity of astrocyte precursors anterior to the ridge are consistent with both mechanisms. Resolution of this issue requires analysis of proliferation in retinae with late stage 1 and early stage 2 ROP, when the ridge begins to form. Animal models of ROP are of limited value, as they do not exhibit a ridge.