Study on the cytoplasmic and inductive factors for the excitable membrane differentiation in the cleavage-arrested and isolated blastomeres of the early embryo.

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As previously reported, when the cell-cleavage of the Halocynthia embryo of the protochordate is arrested at the early developmental stage, such as 1- to 8-cell, and the arrested embryo is cultured further until the normal hatching time, the membrane of a large and multinuclear blastomere expresses one of four types of excitability, such as neural, epidermal, muscular and non-excitable types. This suggests that all nuclei in the blastomere apparently select only one cell type for their expression though the blastomere has multiple fates during normal development. In this research project, this selection is presumed to be one of the important mechanisms for cellular differentiation in general. Following two kinds of experiments were performed. Firstly, the cell autonomous differentiation of the isolated and cleavage-arrested blastomeres of the 8-cell was studied. Secondly the differentiation under the interaction with neighboring cells was studied by contacting two types of the blastomere during development. The anterior a42 and posterior b42 ectodermal blastomeres of the 8-cell evoked a long-lasting Ca action potential and antiepidermal monoclonal antibody 2C5 was bound to their cytoplasm, suggesting the epidermal type differentiation when they were cultured in isolated condition. In contrast, the a42's produced Na spikes in 70% of all examined cases when they were cultured under close contact with the A41 blastomere. In this situation the 2C5 was not bound to the cytoplasm, suggesting the differentiation other than the epidermal type. Considering that in the case of vertebrate embryo's the axial mesoderm including the presumptive notochordal region can induce the neural plate, in this protochordate, an ancestoral form of the vertebrate, the a42 including presumptive brain vesicle region was inferred to be induced by the A41 including the presumptive notochordal region and to differentiate into neural type.