Takeuchi Masaki    Department   Kawasaki University of Medical Welfare  ,    Position   Associate Professor
Language	English
Title	Holoblastic Embryogenesis in a Basal Actinopterygian and Lamprey Represents the State as Evolutionary Origin of Meroblastic it
Conference	14th Japanese Medaka and Zebrafish Meeting
Conference Type	Seminar
Presentation Type	Poster notice
Lecture Type	General
Publisher and common publisher	◎Masaki Takeuchi, Maiko Takahashi, Shinichi Aizawa
Date	2008/11/15
Venue (city and name of the country)	Okazaki, Japan
Summary	Diversification of embryonic form in vertebrates was caused by the acquisition of massive egg yolk as nutrition for development at first of this lineage and alterations of yolk distribution or quantity in each taxon. In turn, the meroblastic cleavage of eggs was often derived in parallel from ancestral holoblastic it. It was believed that all blastomeres of holoblastic development contain yolk granules and contribute to primary germ layers, because in amphibian embryos even the yolk-rich vegetal mass cells (VMC) is identified as a part of endoderm. In contrast, meroblastic developments are accompanied with extra-embryonic tissues such as the yolk cell. Here, we report on pattern of primary germ layers during gastrulation in a basal actinopterygian fish, Polypterus known as bichirs, and lamprey, Lethenteron japanicum. Their embryos represent primitive holoblastic development, and their VMC consists of non-endodermal cells that have no expression of sox17 ortholog as definitive endoderm marker. But, bichir VMC has an activity of meso-endoderm induction like as the nature of yolk syncytial layer (YSL) in teleost fishes. Furthermore, ortholog of VegT, playing central roles of germ layer patterning and dorsal determination in Xenopus, is not expressed maternally in bichirs. These data show that homologous tissue of just yolk cell exists in holoblastic embryos as preliminary states toward meroblastic, and suggest that the whole-patterning of germ layers in amphibian correlates with maternal VegT expression rather than holoblastic cleavage. There is a possibility that the holoblastic development with local-patterning of germ layers as in bichirs and lampreys is the default of embryogenesis in not only teleost fishes but also all living vertebrates.