Development of the sickle canal, an unrecognized formation in the avian blastoderm, and its spatial relationship with the first appearing blood islands, induced by Rauber's sickle

In the present study, we demonstrate the existence in avian blastoderms of a voluminous (approximately 2-4 mm long), previously unrecognized sickle-sh aped canal (termed sickle canal). It usually bulges into the subgerminal sp ace and is localized near the caudo-lateral border of the area pellucida af ter approximately one day incubation. The sickle canal, which is always vis ible on sections, is found both in the chicken and in the quail blastoderm. It seems to function as an expansion space for lateral migration of mesobl ast cells, between epiblast and endoblast. The origin and evolution of the sickle canal have been followed (using quail-chick chimeras). by apposing q uail Rauber's sickle fragments on fragments of unincubated chicken blastode rms. It was seen that part of the wall of the sickle canal is formed by end oblast derived from Rauber's sickle, i.e, transitional and junctional endob last. Very obvious, on sections through the chimeras, is the intimate conta ct between the V or U-shaped quail junctional endoblast and the first forme d blood islands, developing from mesoblast that migrates peripherally over the sickle canal. Our study demonstrates that even in the absence of the ar ea opaca, a sickle canal forms and blood islands start to develop from meso blast of the area pellucida under the influence of junctional endoblast (de rived from Rauber's sickle). Rauber's sickle and its derivatives seem thus to be the major organizers of the avian blastoderm. During early incubation they induce the formation of endomesoblast ingressing via the primitive st reak (CALLEBAUT & VAN NUETEN, 1994), and somewhat later junctional endoblas t induces the development of blood islands from the most laterally ingresse d mesoderm.