Localization of choline acetyltransferase in the developing and adult turtle retinas

Acetylcholine has important epigenetic roles in the developing retina. In t his study, cells that expressed choline acetyltransferase (ChAT), the enzym e that synthesizes acetylcholine, were investigated in embryonic, postnatal , and adult turtle retinas by using immunofluorescence histochemistry. ChAT was present at stage 15 (S15) in cells near the vitreal surface. With the formation of the inner plexiform layer (IPL) at S18, ChAT-immunoreactive (- IR) cells were located in the inner nuclear layer (INL) and the ganglion ce ll layer (GCL). In the INL, presumed starburst amacrine cells were homogeno us in appearance and formed a single row next to the IPL: This pattern was conserved until adulthood. In the GCL, however, there were multiple rows of ChAT-IR cells early in development, and this high density of labeled cells continued during the embryonic stages, until around birth. The high densit y of ChAT-IR cells in the GCL was due in part to a population of cells that expressed ChAT transiently. In postnatal stages and adult retinas, the pre sumed starburst amacrine ChAT-IR cells formed two mirror-like rows of homog enous cells on both borders of the IPL. Two cholinergic dendritic strata th at were continuous with these cells were observed as early as S18, and thei r depths in the IPL were relatively stable throughout development. A third population of ChAT-IR cells was observed toward the middle of the INL aroun d S25 and persisted into adulthood. Finally, cells in the outer nuclear lay er (ONL) were :ChAT-IR during the embryonic stages, were less immunoreactiv e during the postnatal stages, and were not immunoreactive in the adult ret inas. (C) 2000 Wiley-Liss, Inc.