Maturation promoting factor activation in early amphibian embryos: Temporal and spatial control

The cytoplasmic localisation of factors capable of influencing the behaviou r of nuclei has long been considered a potential mechanism for generating c ell differences during development. Yoshio Masui was instrumental in identi fying two cytoplasmic factors, maturation promoting factor (MPF) and cytost atic factor (CSF), defining the first biological assay for their isolation and characterisation. These biological assays involved the transfer of cyto plasm between amphibian oocytes, MPF being able to promote meiotic maturati on (progression to Mil) and CSF to stabilise the MII state. Masui was subse quently involved in developing a 'cell-free' system with the potential for analysis not just of MPF and CSF, but many aspects of nucleo-cytoplasmic in teraction. Masui and Markert initially showed that MPF activity could be ge nerated in enucleate oocytes following progesterone stimulation, indicating a cytoplasmic origin. Masui subsequently showed that MPF activity was dist ributed unevenly through the egg of Rana pipiens during maturation. In this review we will consider the historical context in which the MPF assays wer e established, then briefly consider some of the molecular components that are now known to influence MPF activation. We will then consider evidence f or the asymmetric activation of MPF and the possibility that the nucleus co ntributes to MPF activation in early embryos. ((C) Elsevier, Paris).