Review Lecture Nucleocytoplasmic interactions in morphogenesis

Abstract

Many classical experiments have clearly established the importance of cytoplasmic organization during the very early stages of development: for instance, removal of the ‘polar lobe’ a t the trefoil stage of cleavage results, in molluscs, in various defects in the larv a ; mild centrifugation of a fertilized amphibian egg induces marked microcephaly, while inversion of its normal polarity, by keeping the egg upside down, produces Siamese twins. The dorsoventral organization of the amphibian egg can be established at will by the experimenter by simply forcing the egg to rotate in a given plane. As a result of this rotation, the dorsal side becomes visible by the appearance of the so-called grey crescent. If the cortex (about 1 jum thick) of this dorsal side is removed, the egg will cleave and form a blastula, but it will not develop further. If, on the contrary, the dorsal cortex of a fertilized egg is grafted on the ventral side of another fertilized egg, a double embryo will form (Curtis 1960). In all these experiments, nothing is apparently done to the egg nucleus, while minor alterations of the cytoplasm have farreaching consequences for further development. On the other hand, many experiments in which the nucleus has been intentionally injured (in order to produce mutations, aneuploidy, haploidy, hybridization, etc.) definitely show that nuclear integrity is required in order to obtain full and normal development. This effect of the nucleus is hardly conspicuous until the initial period of cleavage is over. But morphogenetic movements, neural induction, organogenesis, tissue and cell differentiation all require the presence of normal nuclei.