Meeting Abstract

54.2  Tuesday, Jan. 5  A Dual Origin of the Pituitary Primordium in the Ascidian KANO, S*; SATOU, Y; DESCHET, K; MARTIN, P; HAEUSSLER, M; JOLY, JS; CNRS, Gif-sur-Yvette, France; Kyoto University, Kyoto, Japan; CNRS, Gif-sur-Yvette, France; INRA. Jouy-en-Josas, France; CNRS, Gif-sur-Yvette, France; CNRS, Gif-sur-Yvette, France kanoic@ciona.info

The pituitary is an essential endocrine organ in the vertebrates. While the urochordate possesses no distinct pituitary organ, it has been long proposed that the ascidian neural complex (NC) composes homologous organs to the brain and the pituitary. Recent molecular data suggests that the ciliated funnel (CF), one of the NC components, is the most probable candidate as several placodal genes including CiPitx are expressed at the ectodermal stomodaeum and the CF, respectively, in embryonic and the adult stages (Boorman and Shimeld, 2002; Christiaen et al., 2002). To reinforce this hypothesis with insights from the developmental process, we conducted lineage analyses mainly using a photo-convertible fluorescent protein Kaede driven with promoters recapitulating endogenous expressions of CiPitx and CiNut (the neural tube marker). We validated that a part of the ectodermal stomodeaum invaginates, separates from the major part and gives rise to the distal tip of the CF. Furthermore, we revealed that the proximal domain of the CF, which is adjacent to the distal tip, is derived from the neuroectodermal “dorsal duct”. Thus, the ascidian CF has a dual origin and its organogenesis resembles the vertebrate pituitary. We then investigated whether the ectodermal CF expresses genes relevant to an endocrine activity like the vertebrate pituitary. However, we could not confirm it. It is likely that a pituitary primordium is present in the ascidian, but that the endocrine activity of the pituitary is a vertebrate innovation presumably with evolutions of adenohypophyseal transcription factors which terminally differentiate hormone-secreting cells.