Massive cytoplasmic transport and microtubule organization in fertilized chordate eggs

Eggs have developed their own strategies for early development. Amphibian, teleost fish, and ascidian eggs show cortical rotation and an accompanying structure, a cortical parallel microtubule (MT) array, during the one-cell embryonic stage. Cortical rotation is thought to relocate maternal deposits to a certain compartment of the egg and to polarize the embryo. The common features and differences among chordate eggs as well as localized maternal proteins and mRNAs that are related to the organization of MT structures are described in this review. Furthermore, recent studies report progress in elucidating the molecular nature and functions of the noncentrosomal MT organizing center (ncMTOC). The parallel array of MT bundles is presumably organized by ncMTOCs; therefore, the mechanism of ncMTOC control is likely inevitable for these species. Thus, the molecules related to the ncMTOC provide clues for understanding the mechanisms of early developmental systems, which ultimately determine the embryonic axis. •Cortical rotations of amphibian, fish, and ascidian transport unique maternal factors.•Cortical microtubule (MT) arrays and their functions of axis formation are conserved.•Components of noncentrosomal MT-organizing center (ncMTOC) are various.•Most of the MT structures in fertilized egg are thought to be controlled by ncMTOC.•Massive transport directed by ncMTOC is inevitable for early developmental system.