Patterning of neuronal locus specificities in retinal ganglion cells after partial extirpation of the embryonic eye

A variety of transplantation procedures were used to analyze the determinants of locus specificity pattern in eye fragments prepared at stage 31/32. When right eye fragments were rotated through 0–360° in situ, or grafted to a stage 31/32 or stage 27/28 host, or grown in vitro 48 hours prior to reimplantation, the orientation of the retinotectal map always matched the anatomical orientation of the rounded‐up fragment, while the internal organization of the map was characteristic for the original fragment type: normal or double‐nasal in NF eye grafts, normal or double‐temporal in TF eye grafts, normal or double‐ventral in VF eye grafts. Fragments from stage 31/32 left eyes showed similar behavior, but mapped from the right orbit with one axis inverted as occurs after contralateral transplantation of whole left eyes. When one rounded‐up fragment and one normal eye (or one rounded‐up nasal fragment and one rounded‐up temporal fragment) were placed side‐by‐side in the same orbit and allowed to “superinnervate” the same tectum, both retinae mapped across the entire tectum with no unshared tectal regions. Finally, in fragments prepared from previously‐rotated (at stage 25/26) eyes, the organization and orientation of the map conformed to the new retinal axes, often in direct conflict with the anatomical polarity of the eye. The results are considered in terms of (1) the histogenesis of the retina in eye fragments; (2) the stability of the program for patterning of locus specificities, which is set down when the optic cup undergoes axial specification at stages 28–31; (3) the factors controlling selective modification of this program and their localization in eye fragments; and (4) the extent to which the range of locus specificities, deployed across the ganglion cell population of rounded‐up fragments, approximates that found in the normal unoperated eye.