Role of messenger ribonucleic acid in embryonic development

Tissue-specific changes in nuclear RNA content during early development of Triturus vulgaris

During early development of Triturus vulgaris, as a measure for nuclear activity in neuroectoderm, mesoderm and endoderm, nuclear RNA content was determined by cytochemical methods. In the first stages of gastrulation, that is to say during the early phase of neural induction, the RNA content of the inducing system is considerably higher than in the reacting system. Then, with a phase-shift of about 10 h, the RNA content of the neuroectoderm increases quickly also. In the following stages the nuclear RNA content of both regions is reduced. A second continuous increase in the RNA amount coincides with the formation of the neural tube. In the mesoderm, enhancement of RNA content correlates with cytodifferentiation of the chorda. In all stages the RNA content of the endoderm is higher than in the other tissues and it becomes successively diminished from the early gastrula to the tailbud stage.

Control of cell division and cell differentiation by deoxynucleotides in the early embryo of Xenopus laevis

It is proposed that the deoxyriboside triphosphates present in the egg of Xenopus laevis support the synchronous cell divisions occurring during the earliest phase of embryonic development and that, as long as synchrony prevails, the cells are prevented from undergoing differentiation. This hypothesis has been tested by injecting deoxyribonucleotides into fertilized eggs. The following effects were observed: 1) the duration of synchrony is prolonged, 2) the morphological development is suppressed, an effect which is greatest when four nucleotides are injected together and 3) the synthesis of all kinds of RNA is inhibited, including the mRNA required for differentiation to occur.

Nucleocytoplasmic interactions in morphogenesis

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.