At E17.5, however, the number of Zic2+ RGCs and the number of Islet1/2+ cells do not differ in pigmented and albino retina at E17.5 (Figure?3D, right). The timing of ventrotemporal retinal ganglion cell genesis is altered in albino retina The difference in the number of Islet1/2+ cells at E15.5 followed by an equalization of numbers at E17.5 in pigmented and albino VT retinae (Figure?3D) suggests a disparate pattern of RGC production in pigmented and albino VT retinae: fewer cells are produced before E15.5 and more cells are produced after E15.5 in the albino retina compared with the pigmented retina. in the albino ventrotemporal (VT) ZC3H13 retina compared with the pigmented VT retina, as we previously reported. However, the reduction in Zic2+ RGCs in the albino is not accompanied by a compensatory increase in Zic2-negative (Zic2?) RGCs, resulting in fewer RGCs in the VT retina at this time point. At E17.5, however, the number of RGCs in the VT region is similar in pigmented and albino retinae, implicating a shift in the timing of RGC production in the albino. Short-term birthdating assays reveal a delay in RGC production in the albino VT retina between E13 and E15. Specifically, fewer Zic2+ RGCs are born at E13 and more Zic2? RGCs are born at E15. Consistent with an increase in the production of Zic2? RGCs born at later ages, more RGCs at E17.5 express the contralateral marker, Islet2, in the albino VT retina compared with the pigmented retina. Conclusions A delay in neurogenesis in the albino retina is linked to the alteration of RGC subtype specification and consequently leads to the reduced ipsilateral projection that characterizes albinism. 100?m. In the albino retina at E15.5, there are fewer Zic2+ RGCs within a given sector (Figure?2C) and when summed across sectors (by 35.06%, P?0.001). In contrast, the number of Zic2? RGCs is similar in pigmented and albino VT retinae (Figure?2D), suggesting that only RGCs specified to express Zic2 are affected in the albino VT retina at this age. Because the reduction of Zic2+ RGCs is not accompanied by an increase in Zic2? RGCs, the total number of Islet1/2+ (postmitotic) cells in the VT region of the albino retina is decreased in the more central sectors (Figure?2F) and when summed across sectors (reduced by 21.32%, P?0.05). To discount the possibility that fewer RGCs are found Guadecitabine sodium in the albino retina due to differences in the size of the retina, the area of the retina was averaged in the same coronal sections used for counting RGCs and was similar in pigmented and albino mice (Figure?2E), consistent with area measurements in retinal whole mounts (Figure?1D). To ascertain whether the reduction of Islet1/2+ cells is specific to the Zic2-expressing region in VT retina, Islet1/2+ cells in the dorsotemporal (DT) region of the retina (Figure?2H) were counted in the same representative sections Guadecitabine sodium used for VT analysis described above. Zic2 is not expressed in the dorsal retina. The number of Islet1/2+ cells in the DT retina was not significantly different in pigmented and albino retinae (Figure?2G). These results suggest that the decrease in RGCs at E15.5 is specific to Zic2+ RGCs and to the VT retina. At embryonic day 17.5, retinal ganglion cell number is similar in pigmented and albino ventrotemporal retinae Previous studies have shown that, at adult ages, the number of RGCs in the retinal periphery of pigmented and albino mice is approximately the same [23,26], suggesting Guadecitabine sodium that the reduction in RGCs that we observed at E15.5 does not persist to adulthood and that RGC numbers eventually equalize. To test this, we compared the numbers of Zic2+ and Islet1/2+ cells in VT retina of pigmented and Guadecitabine sodium albino mice at E17.5 (Figure?3A). Herrera and colleagues  showed that Zic2 is downregulated in RGCs after their axons have crossed the chiasm and expressed only in the most peripheral (immature) RGCs, implying that the majority of RGCs that express Zic2 at E15.5 do not express Zic2 at E17.5. In contrast to our observations in E15.5 retina, we found that the number of Zic2+ RGCs in the albino retina at E17. 5 is not significantly different compared with the pigmented retina in all sectors, although there tend to be fewer Zic2+ RGCs in the albino retina (Figure?3B). Further, the number of Islet1/2+ cells in VT retina does not differ in pigmented and albino mice at E17.5 (Figure?3C). To confirm that all Islet1/2+ cells at E17.5 were RGCs as opposed to other cell types, as has been suggested in previous reports , Islet1/2 was co-immunostained.