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-shaped canal (termed sickle canal). It usually bulges into the subgerminal space and is localized near the caudo-lateral border of the area pellucida after approximately one day incubation. The sickle canal, which is always visible 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 mesoblast cells, between epiblast and endoblast. The origin and evolution of the sickle canal have been followed (using quail-chick chimeras). by apposing quail Rauber's sickle fragments on fragments of unincubated chicken blastoderms. It was seen that part of the wall of the sickle canal is formed by endoblast derived from Rauber's sickle, i.e, transitional and junctional endoblast. Very obvious, on sections through the chimeras, is the intimate contact between the V or U-shaped quail junctional endoblast and the first formed blood islands, developing from mesoblast that migrates peripherally over the sickle canal. Our study demonstrates that even in the absence of the area opaca, a sickle canal forms and blood islands start to develop from mesoblast of the area pellucida under the influence of junctional endoblast (derived 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 streak (CALLEBAUT & VAN NUETEN, 1994), and somewhat later junctional endoblast induces the development of blood islands from the most laterally ingressed mesoderm.