MECHANISM OF AGE-DEPENDENT INVOLUTION IN EMBRYONIC CHICK NOTOCHORDS

To study the possible mechanism of the age-dependent involution of the notochord, isolated mesenchyme-free notochords of chick embryos were cultured in vitro and compared with their counterparts in vivo. Two di fferent aspects were evaluated: (1) DNA synthesis measured by [H-3]thy midine incorporation and visualized by autoradiography and (2) cell de ath quantified by counting the number of pyknotic nuclei. The results demonstrate that [H-3]thymidine uptake by notochords shows an age-depe ndent decrease in vitro as well as in vivo. The number of [H-3]thymidi ne-labelled notochord cells, however, is higher in vitro than in vivo. At the same time, there is an age-dependent increase in pyknosis in t he notochord in vivo and in vitro. So, during the aging process, the n umber of both pyknotic nuclei and of [H-3]thymidine-labelled nuclei su ggest a high turnover of notochord cells in vitro. From these results, we can conclude that the process of involution in aging notochord see ms to be controlled by a programmed intrinsic process, which might be influenced partially by the microenvironment in vivo.