Early chick neuroretinal responses following direct exposure to methotrexate.

Explants of 4.5-day-old chick embryonic neuroretinas with mesenchyme were exposed to Methotrexate (MTX) in medium 199 with embryo extract. Proliferative responses of the cultured neuroretinas were followed radioautographically by administration of 3H-thymidine to the cultures. The DNA synthetic, mitotic and pyknotic responses of the ventricular cells of the neuroretina were followed over a 16-hour period. The responses observed suggested that MTX caused a synchronization of the ventricular cells in the pre-mitotic phases with no direct inhibition of mitosis. Furthermore, prolonged exposure to MTX resulted in the accumulation of labeled pyknotic cells, indicating a decline in the regenerative capacity of the proliferative ventricular cells.