Recently, we have reported on the emergence of various retinal cell ty
pes in embryonic rabbit retina transplanted to adult rabbits [5]. When
comparing the relative numbers of the spectrally different cone types
in the transplants to those in the host or age-matched control retina
s, a surprising shift was observed. While in the normal rabbit retina
the middle-wavelength-sensitive (M) cones are considerably more abunda
nt than the short-wave-sensitive (S) cones, the S/M cone ratio was fou
nd to be the opposite in the graft. The number of rosettes containing
only S-cones in high density was found to be considerably higher than
that of M-cone rich rosettes. The number of S-cones also exceeded that
of the M-cones in each rosette that contained both cell types. Our re
sults were obtained from the systematic immunocytochemical analysis of
15 different transplants derived from transplantations of embryonic r
abbit retinas into adult hosts of the same species. The emergence and
proportion of the two cone types were followed between 14 and 63 days
after transplantation (between 29 and 78 postconceptional days of the
donor tissue). Sections from various parts of the transplants were rea
cted with the monoclonal antibodies COS-1 and OS-2, specific for the m
iddle- and short-wavelength-sensitive cones, respectively. The explana
tion for the reverse cone ratio in these transplants is not known yet,
however, the observed phenomenon may indicate differences between the
specification of the two basic cone types.