Autoradiographic analysis of effects of 5-bromodeoxyuridine on neurogenesis in the chick embryo spinal cord.

Bromodeoxyuridine (BUdR) can reversibly inhibit the terminal differentiation of embryonic cell types when they are grown in culture. The goal of these experiments was to see if BUdR could interfere with the terminal differentiation of neurones in the intact chick embryo, a possibility disputed in different reports. Therefore, 0.02 mg BUdR plus 15 microCi [3H]BUdR was injected into the albumen of incubating eggs at stage 14-16 of development. Controls received an equimolar amount of [3H]thymidine ([3H]TdR). The doses and mode of administration are known to result in availability times for the nucleosides that are longer than one cell generation cycle and therefore similar to the pharmacokinetic conditions possible in vitro. The time of treatment is known to correspond to the period of most rapid neurone production in the chick spinal cord. By autoradiography of semi-thin sections the fates of the cells that had incorporated the nucleosides could be followed. BUdR was taken up by the S phase population of the spinal cord neuroepithelium (NE). In the first 10 h after treatment the BUdR treated NE behaved the same as the control. From then until 24 h after treatment, NE cells underwent necrosis as a result of BUdR and neurone production was almost completely suppressed. Between 24 and 48 h after treatment the BUdR-treated NE produced neurones at a faster rate than the TdR-treated controls. However, this effort at compensation was not entirely effective and by 6 days after treatment the BUdR-treated embryos had an absolute reduction in motor neurone number. Motor neurones with BUdR incorporated in their DNA that survived until after neurogenesis was completed were strikingly more lightly labelled than those in controls treated with [3H]TdR. This suggests that the survivors had incorporated less BUdR than those that had died. It was concluded that the neuronal deficit resulting from BUdR treatment was not the result of an inhibition of terminal differentiation, but rather of cytotoxicity.