Positional specificity tests in co-cultures of retinal and tectal explants.

In previous studies we demonstrated that HRP-labeled ganglion-cell axons arising from fetal mouse retinal explants can distinguish appropriate (tectum) from inappropriate (spinal cord) CNS target regions in vitro. Retinal fibers preferentially invade the tectum where they ramify, develop terminal arbors, and make functional connections. The present study attempts to determine if fibers from a half-retina prefer the 'appropriate' half-region of a tectal explant. In one series, nasal or temporal halves were placed near the medial edge of the tectum, to test turning toward or away from the appropriate (posterior or anterior) half-tectal region. Several co-cultures showed apparent preferences, especially those with many ingrowing retinal fibers; but in other co-cultures the data were equivocal. A second paradigm placed the half-retinas near anterior or posterior tectal edges, and simply scored presence vs absence of retinal fiber ramifications within the nearest tectal half-region. This identified entry of retinal fibers into appropriate vs inappropriate half-tectal regions in 8 out of 8 blind scorings. These data encourage further, more critical analyses of retinotectal co-cultures to explore the basis of the specific neuritic connections which form in situ.