Cholinergic neuronotrophic factors: III. Developmental increase of trophic activity for chick embryo ciliary ganglion neurons in their intraocular target tissues

Between stages 34 and 40 in the chick embryo, the ciliary ganglion (CG) undergoes a 50% loss of neurons. Such neuronal death is a common feature in neural development and it has been proposed that neurons are dependent for survival on trophic support from their target tissues. Using an in vitro bioassay it was previously shown in this laboratory that trophic activity for CG neurons is highly concentrated in eye structures containing CG target tissues. In the present study we have found that trophic activity in the eye increases markedly between stages 37 and 39, the time when neuronal death in the ciliary ganglion is ending. Thus, a developmental increase in trophic activity within the eye may be involved in determining neuronal survival in the CG. Furthermore, this study provides the first indication that the trophic content of target tissue is itself developmentally regulated.