The survival of retinal ganglion cells and the dendritic development were investigated a) in normal chick embryos, b) in embryos whose primordial optic lobes and adjacent areas were removed (target reduced embryos), and c) in embryos whose optic nerves were transected (target deprived embryos) in order to study the influences of central targets on developing
ganglion cells. The
ganglion cells were stained postmortem with the
carbocyanine dye DiI. Cell body and dendritic field diameters were measured in whole-mounted retinae before and after the period of cell death at embryonic day 10 (E10) and E16. The cell densities within the
ganglion cell layer were counted in
cresyl violet/
thionine stained retinae. The central
retinal projection in target reduced embryos was studied with the anterogradely transported fluorescent marker
rhodamine-B-isothiocyanate (RITC). In normal embryos, the earliest dendritic processes were observed at E6 in the central retina, whereas at E10 elaborate dendritic branching was found across the retina. Different morphological types of
ganglion cells could be identified at E16. In both, target reduced embryos and target deprived embryos, the initial dendritic growth and pattern of ramification were indistinguishable from those of normal embryos up to E10. Cell body diameters, dendritic tree diameters, and cell densities were not significantly different. At the end of the naturally occurring cell death period (E16), the
ganglion cell density was strongly reduced in both experimental groups compared to controls. In particular, when the optic nerve was transected, it resulted in the almost complete degeneration of
ganglion cells. In target reduced embryos, a small population (about 5% of the normal number) of
ganglion cells survived. The proportion of large cells was increased within the total population compared to normal retinae. Displaced
ganglion cells were not affected by partial target removal but strongly affected by transection of the optic nerve. Anterograde labelling from the retina revealed that in target reduced embryos the remaining
ganglion cells innervated non-tectal primary visual nuclei. The present results suggest the following: a) Before the onset of the cell death period, the growth and ramification of
ganglion cell dendrites occur independently of central visual targets. b) In target reduced embryos, a small population of
ganglion cells survives, namely, those cells that project to remaining central areas. Complete disconnection from central targets by transecting the optic nerve leads to the degeneration of almost all
ganglion cells. c) The surviving
ganglion cell population consists mainly of large
ganglion cells.