Astrocyte-endothelial cell relationships during human retinal vascular development

PURPOSE. To evaluate evidence for the presence of vascular precursor cells (angioblasts) and astrocyte precursor cells (APCs) in the developing human retina and determine their relationship. METHODS. Pax-2/GFAP/CD-34 triple-label immunohistochemistry was applied to four retinas aged 12, 14, 16, and 20 weeks of gestation (WG) to label APCs, astrocytes, and patent blood vessels. APCs are Pax-2⁺/GFAP^-, whereas astrocytes are Pax-2 ⁺/GFAP⁺. Adenosine diphosphatase (ADPase) enzyme histochemistry, which identifies endothelial cells and vascular precursors, was applied to human retinas aged 12, 16, 17, and 19 WG. Nissl stain, a nonspecific cell soma marker, was applied to 14.5-, 18-, and 21-WG retinas. Established blood vessels were visualized with CD34 and ADPase. RESULTS. Topographical analysis of the distribution of Nissl-stained spindle cells and ADPase ⁺ vascular cells showed that these two populations have similar distributions at corresponding ages. ADPase+ vascular precursor cells preceded the leading edge of patent vessels by more than 1 millimeter. In contrast, Pax-2⁺/GFAP^- APCs preceded the leading edge of CD34 ⁺ blood vessels by a very small margin, and committed astrocytes (Pax-2⁺/GFAP⁺) were associated with formed vessels and nerve fiber bundles. Two populations of ADPase⁺ cells were evident, a spindle-shaped population located superficially and a deeper spherical population. The outer limits of these populations remain static with maturation. CONCLUSIONS. A combination of Pax-2/GFAP/CD34 immunohistochemistry, Nissl staining, and ADPase histochemistry showed that the vascular precursor cells (angioblasts), identified using ADPase and Nissl, represent a population distinct from Pax-2⁺/GFAP^- APCs in the human retina. These results lead to the conclusion that formation of the initial human retinal vasculature takes place through vasculogenesis from the prior invasion of vascular precursor cells.