During ascidian embryogenesis, about 42 unicellular striated muscle cells are formed in the tail of the tadpole larva. Lineage of the muscle cells is well documented. Twenty-eight of them are derived from a pair of B4.1 cells of a bilaterally symmetrical 8-cell embryo. This B-line presumptive cells are able to differentiate autonomously into muscle cells without cell-cell interaction depending on so-called muscle determinants localized in the egg myoplasm. In this article we reviewed results of recent analyses on the molecular nature of the muscle determinants, the expression of an ascidian homolog of vertebrate MyoD, and the structure and expression control of a muscle actin gene. All of the results suggest that muscle differentiation in the ascidian embryo is an appropriate experimental system to studying molecular mechanisms of cellular differentiation during animal development.
Cleavage-arrested ectodermal blastomeres isolated from the 8-cell stage ascidian embryo differentiate into neural cells after cell-contact with a vegetal blatomere over a critical period. In this two-cell system, neural differentiation can be exemplified by the appearance of Na-dependent action potentials and the transcription of a Na+ channel gene, TuNaI. This system enables us to precisely study gap-junctional communication between early developing blastomeres by microinjection of tracer molecules and by double voltage clamp method. Gap junctions are likely to determine timing of functional expression of Na+ channels and K+ channels.
The cell fate determination in ascidian embryos is mediated by various cell interactions. Two sensory pigment cell precursors constitute an equivalence group, in which the ocellus (eye) pathway is the dominant fate. It is suggested that the unique cell interaction, which is called lateral inhibition, is involved in determination events of these equivalent cells. It is likely that the mode of specification which utilizes the lateral inhibition within equivalent cells plays a fundamental role during early embryogenesis in both invertebrates and vertebrates.
Thermal stability of proteins can be improved by evolutionary molecular engineering, without the knowledge about 3D structures of the proteins nor the principle that governs the protein stability. The method will provide a way of improving the thermal stability of industrially important proteins. It can also be used as a tool to study protein stability. There are a substantial number of examples now. Our recent study suggested that it is easy to obtain thermo-stabilized proteins by single-base substitution. Although, it requires further improvement of the method to obtain mutants with double-base substitution within a codon.