The Timing, Sequence and Spatial Pattern of Cell Genesis in the Rat Retina

Abstract

Abstract: : Purpose: To understand the mechanisms of cell fate determination in the vertebrate retina, the time course over which the major cell types are generated needs to be established. However, a thorough retinal cell "birthdating" study has not been performed for the common laboratory rat. We investigated the timing and spatial pattern of cell genesis in the rat using ₃H-Thymidine (₃H-Thy). Methods: A single injection of ₃H-Thy was administered to pregnant rats or their progeny between embryonic day (E) 8 and postnatal day (P) 13. The animals were allowed to survive to maturity whereupon radial and en face sections of the retina were cut. Radioactive labeled cells were visualized by autoradiography. Results: Rat retinal cell genesis occurs between E9 to P11, with 50% of cells generated at approximately P1. The first cells generated were all found in the retinal ganglion cell layer and range in size from 9-15µm in diameter. Previous studies have shown that this size range includes small to medium-sized retinal ganglion cells and few displaced amacrine cells. The sequence of cell genesis was determined by interpolating the age at which 5%, 50% and 95% of the total population of each cell type was generated. With few exceptions, cell types reached each of these developmental landmarks in the order: retinal ganglion cells, horizontal cells, cones, amacrine cells, rods, bipolar cells and Müller glia. For each of these cell types, the first cells generated are located in central retina and the last in peripheral retina. Within the sequence of cell genesis two phases could be detected based on differences in kinetics and central-peripheral gradients. Phase 1 consists of ganglion cells, horizontal cells and cones; Phase 2 of amacrine cells, rods, bipolar cells, and Müller glia. Phase 1 cells are produced more rapidly than Phase 2 and the central-peripheral gradient is less tight. Conclusion: Retinal cells in the rat are generated in a sequence very similar to the one found in a primate retina where retinogenesis spans 100 days. In most respects these data are also similar to those obtained from other vertebrate species. To the extent that the sequence reflects underlying mechanisms of cell fate determination, they appear to be highly conserved.

Keywords: 564 retinal development