Development of cholinergic traits in the quail ciliary ganglion: expression of choline acetyltransferase-like immunoreactivity

The avian ciliary ganglion is a parasympathetic ganglion derived from the neural crest whose neurons provide cholinergic innervation to the eye. Here, we describe the time course of appearance and the morphology of cholinergic cells in the ciliary ganglion, as assessed by antibodies against choline acetyltransferase. Choline acetyltransferase immunoreactivity was first observed in 5.5-day-old quail embryos, 1 day after condensation of the ciliary ganglion. Both the intensity of choline acetyltransferase immunoreactivity and size of the choline acetyltransferase-immunoreactive cells increased with ganglionic age. By 12 days, a second population of choline acetyltransferase-immunoreactive cells, possibly corresponding to choroid neurons, was observed whose cells were smaller and less intensely stained than earlier differentiating choline acetyltransferase-immunoreactive cells. The percentage of choline acetyltransferase-immunoreactive cells was initially high, constituting approximately 50% of the total cell population. As a function of time, the proportion of cholinergic cells decreased, probably due to proliferation of non-neuronal cells and naturally-occurring cell death. Our results confirm the existence of two morphologically distinct populations of cholinergic neurons in the avian ciliary ganglion and demonstrate that these neuronal subpopulations express choline acetyltransferase immunoreactivity at different times in development. Because choroid neurons innervate their targets later than ciliary neurons, this finding is consistent with the hypothesis that target interactions regulate expression of choline acetyltransferase.