Title:
Anterior-posterior axis formation in amniotes

Shinichi Aizawa
Laboratory for vertebrate BodyPlan, Center for Developmental Biology (CDB), RIKEN Kobe, Japan

Abstract:
Vertebrates have dramatically changed in their early development prior to gastrulation as they have evolved. The yolk has been stored in various amounts, and cleavage development has been markedly deformed from the holoblastic cleavage seen in amphibian, bichir and lamprey to the meroblastic one in teleosts, reptile and avian. Extraembryonic structures in amniotes, especially those in mammal, have been acquired later in vertebrate evolution, but they have been formed by altering post-cleavage development. With these alterations it is obscure how the mechanisms of the anterior-posterior (A-P) axis and three germ layer formations are related and diverged among each family of vertebrates; they are the most fundamental events in vertebrate development.
In egg cylinder-shaped mouse embryos the A-P axis is initially formed along the distal-proximal orientation by the emergence of a unique population of cells, which express a series of head organizer genes in the distal visceral endoderm (DVE). The genes include transcriptional factors such as Otx2 and Hex and signaling antagonists such as Dkk1, Cerl and Lefty1. The DVE cells subsequently move to the future anterior generating the anterior visceral endoderm (AVE). The questions are: (1) how DVE is formed, (2) how DVE cells move and (3) whether and how the direction of the movement is determined.
Most of mammalian embryos are disc-shaped, and it is not certain how the mouse pattern of the A-P axis formation is common in mammal. Rabbit and pig embryos lose the polar trophectoderm, which generates extraembryonic ectoderm in rodent, when ICM transforms into epiblast. The mouse extraembryonic ectoderm expresses BMP4 and the pronodal convertases, Furin and PACE4, and plays essential roles in the embryonic development.
The initial landmark of the A-P axis formation in avian is the posterior marginal zone, that expressVg1 and Wnt8c , and Koller’s sickle, that expresses Gata6 and Gsc , in the future posterior. Hypoblast in avian is considered homologous to visceral endoderm in mammal. Transcriptional factors of head organizer genes are expressed, but signaling antagonists are not, in the hypoblast. No DVE/AVE-like structure is formed in avian. Moreover, nodal that is expressed in entire epiblast and visceral endoderm and plays essential roles in the development of epiblast, extraembryonic ectoderm and visceral endoderm in mouse embryos, is not expressed in avian embryos by stage XII. How are the mechanisms of the A-P axis formation is related and diverged between mammal and avian? A comparative analysis of the A-P axis formation in non-rodent mammalian and reptilian embryos will be discussed.