Polarization of the avian retina. Ocular transplantation studies

The optic fiber layer of the avian retina, as seen in flat mounts, contains morphological markers of vertical and horizontal polarity. Optic vesicles were rotated in chick embryos prior to the appearance of the optic fiber layer. Later, the optic fiber pattern was examined to determin if its polarity was inverted, or upright or if its right-left asymmetry changed. Large grafts showed no tendency toward axial realignment, regardless of the stage of operation. Moderate size grafts became realigned mainly by derotation. In this process the vertical axis of the graft showed a greater tendency to derotate than the horizontal axis. Thus, left-to-right transplants, when performed prior to embryonic stage 12, developed as left retinas with normally aligned vertical axes, regardless of the degree of operative rotation. Small grafts mainly showed cross-axial regulation, as opposed to derotation; the polarity of the horizontal axis became fixed prior to the vertical axis. The equatorial zone of the optic vesicle, which lies about midway between the distal tip of the vesicle and the midline of the embryo, was included in moderate size, but not in small grafts. This zone appears critical in the transfer of morphological information to the developing retina.