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

The cytoplasmic localisation of factors capable of influencing the behaviour of nuclei has long been considered a potential mechanisms for generating cell differences during development. Yoshio Masui was instrumental in identifying two cytoplasmic factors, maturation promoting factor (MPF) and cytostatic factor (CSF), defining the first biological assay for their isolation and characterisation. These biological assays involved the transfer of cytoplasm between amphibian oocytes, MPF being able to promote meiotic maturation (progression to MII) and CSF to stabilise the MII state. Masui was subsequently involved in developing a 'cell-free' system with the potential for analysis not just of MPF and CSF, but many aspects of nucleo-cytoplasmic interaction. Masui and Markert initially showed that MPF activity could be generated in enucleate oocytes following progesterone stimulation, indicating a cytoplasmic origin. Masui subsequently showed that MPF activity was distributed unevenly through the egg of Rana pipiens during maturation. In this review we will consider the historical context in which the MPF assays were established, then briefly consider some of the molecular components that are now known to influence MPF activation. We will then consider evidence for the asymmetric activation of MPF and the possibility that the nucleus contributes to MPF activation in early embryos.