[Retinal ganglion cell topography during prenatal development]

A fundamental feature of the human visual system is nonuniform distribution of ganglion cells across the retinal surface. In adults, the density of ganglion cells peaks around the fovea and declines sharply towards the periphery, whereas in fetal retina it does not. In the fetal retina, the retinotopic map is dominated by nonuniform expansion and/or ganglion cell death. We examined how the numbers and distributions change according to the horizontal eccentricity from the foveola, using toluidine blue stained vertical sections of the human fetal retina (gestational age 32W and 40W). The vertical section of the fovea showed almost similar density at 32 weeks, 40 weeks and in adults. This fact suggested that the fundamental structure of the fovea is already established at 32W. In the nasal retina, the cellular density was higher than that in the temporal half at 32W, then markedly decreased at 40W and in adults. We assume that in the nasal retina, it is impossible to account for the cell decrease only with nonuniform expansion and that even after 32W ganglion cell death occurs. On the other hand, in the mid-temporal retina, the cellular density was almost similar at 32W, 40W and in adults. It suggested that in the mid-temporal retina, the topography is already established at 32W. In the peripheral temporal retina, the cellular density was relatively high at 32W and decreased at 40W and in adults. In this area, the topography is still changing. We assume that in the fetal retina, retinotopic map is dominated by several factors and the dominant factor differs according to stage and retinal area.