Peripheral control of midbrain mitotic activity in the frog

Eyes were removed unilaterally from stage 19-25 embryos and stage I larvae of Rana pipiens. Control and operated animals were fixed 2-27 days later, at stages I through V+. Midbrains were removed and sectioned, and complete counts of all mitoses in both tecta were made, to permit comparisons between tecta receiving optic nerve fibers and those lacking them. Differences between the two sides were insignificant in control animals and in those fixed at stages I and II. In the 39 stage III animals differences approached significance (P less than 0.06) whereas for stages IV and V, with 20 animals in each group, significance was seen (P less than 0.01). Rostral parts of the tectum, richly supplied by optic nerve fibers, showed much greater percentage differences in mitotic activity than did the caudal parts of the tectum, poorly supplied or lacking such fiber input. Overall, areas of ventricular surface in the affected tecta at stages IV and V were reduced very nearly to the same degree as was cell division. The role of the optic nerve fibers in maintaining division rates is made evident, particularly by the reduction of numbers of prospective ependymal cells. That the influence is seen so very early implies that production of prospective neurons, not just glia, has been affected.