Development of the outer retina in the mouse

Mice represent a valuable species for studies of development and disease. With the availability of transgenic models for retinal degeneration in this species, information regarding development and structure of mouse retina has become increasingly important. Of special interest is the differentiation and synaptogenesis of photoreceptors since these cells are predominantly involved in hereditary retinal degenerations. Thus, some of the keys to future clinical management of these retinal diseases may lie in understanding the molecular mechanisms of outer retinal development. In this study, we describe the expression of markers for photoreceptors (recoverin), horizontal cells (calbindin), bipolar cells (protein kinase C; PKC) and cytoskeletal elements pivotal to axonogenesis (beta-tubulin and actin) during perinatal development of mouse retina. Immunocytochemical localization of recoverin, calbindin, PKC and beta-tubulin was monitored in developing mouse retina (embryonic day (E) 18.5 to postnatal day (PN) 14), whereas f-actin was localized by Phalloidin binding. Recoverin immunoreactive cells, presumably the photoreceptors, were observed embryonically (E 18.5) and their number increased until PN 14. Neurite projections from the immunoreactive cells towards the outer plexiform layer (OPL) were noted at PN 0 and these processes reached the OPL at PN 7 coincident with histological evidence for the differentiation of the OPL. Outer segments, all the cell bodies in the ONL, as well as the OPL were immunoreactive to recoverin at PN 14. Calbindin immunoreactive horizontal cells were also present in E 18.5 retinas. These cells became progressively displaced proximally as the ONL developed. A calbindin immunoreactive plexus was seen in the OPL at PN 7. PKC immunoreactive bipolar cells developed postnatally, becoming distinguished at PN 7. Both beta-tubulin and actin immunoreactive cells were present in the IPL as early as E 18.5; however, appearance of processes labeled with these markers in the OPL was delayed until PN 7, concurrent with the first appearance of photoreceptor neurites, development of the horizontal cell plexus, and development of synaptophysin immunoreactivity at this location. These results provide a developmental timeframe for the expression of recoverin, calbindin, synaptophysin, beta-tubulin and actin. Our findings suggest that the time between PN 3 and PN 7 represents a critical period during which elements of the OPL are assembled.