Development of choline acetyltransferase-immunoreactive neurons in normal and intracranially transplanted retinas in rats.

Retinas from embryonic day 14 (E14) Sprague-Dawley rats were transplanted to the tectum of newborn (P0) recipient rats, and the distribution pattern of choline acetyltransferase immunoreactivity (ChAT-I) in developing transplants was studied and compared with those observed in the retinas of normal developing rats. In normal retinas, ChAT-I cells were first identified in restricted regions in the ganglion cell layer (GCL) at P4, but were found to cover the entire GCL by P6. A second population of ChAT-I cells was detected in the inner nuclear layer (INL) at P8, and they were observed in most parts of the INL on P10 when two immunoreactive sublaminae began to appear in the inner plexiform layer (IPL). The adult pattern of having two distinct populations of ChAT-I cells, organized in mirror symmetrical fashion in the inner retinal layers was basically established by P12. The time course of development and overall distribution pattern of ChAT-I cells in developing retinal transplants on the whole were very similar to those observed in normal retinas. The first identification of these cells and the establishment of their final distribution pattern were made at stages corresponding to P4 and P12 of normal developing retinas respectively. However, ChAT-I somata were located in the INL at a much earlier stage compared with their counterparts in the normal retina, and a transient population of immunoreactive cells with their processes extending to retinal layers other than the IPL was observed in some transplants from P6 to P10. These features were not observed in normal developing retinas. These results suggest that the development of cholinergic neurons, especially the expression of their characteristic antigen and their final distribution pattern is largely determined by programmes which are intrinsic to the original retinal tissue, despite some minor deviation or variation in the developmental process which may occur under certain abnormal conditions.