Induction and improved embryonic development by the nucleus of Pander in associated avian blastoderm parts: influence of delta or gamma ooplasm

After placing in vitro, central subgerminal ooplasm (containing a central nucleus of Pander) from a quail germ disc of a prelaid egg (before symmetrization) on the upper layer of an isolated chicken antisickle, we observed the induction of a radially oriented preneural plate (without interference of chordamesoblast). This observation suggests the primary existence during the period of symmetrization in utero of an until now unknown temporospatially linked "vertical" effect, emanating from the nucleus of Pander, on the parallel (pre)neural plate anlage forming part of the area centralis in the overlying blastoderm. For comparison, we "sandwiched" in vitro a quail sickle endoblast fragment between the deep side of the upper layer of an isolated chicken antisickle region and a central subgerminal ooplasmic mass. This resulted in a colonization of the subgerminal ooplasmic mass by quail sickle endoblast cells followed by improved neurulation and/or gastrulation phenomena. The latter never occurs in the absence of central subgerminal ooplasm. In both types of experiments there seems to exist a common link between the observed induction phenomena: the presence of delta ooplasm in the involved deep structures. Indeed, the nucleus of Pander contains delta ooplasm as well as the structures derived from it, i.e., endophyll with primordial germ cells and sickle endoblast-derived cells after colonization of the neighboring central ooplasm (present study). Therefore, we think that the preneural plate-inducing effect observed after placing a nucleus of Pander on the antisickle region is due to the presence of a factor in the delta ooplasm that diffuses in the neighborhood. The appearance of gastrulation phenomena in the second type of experiment seems to be due to colonization of the more peripheral part of the central subgerminal ooplasm containing the more superficial and peripheral gamma ooplasm in which Rauber's sickle material can develop. This suggests that the kind of involved ooplasm (delta or gamma) can predetermine the inductive activity of the deep structures that contain it: the central part of the nucleus of Pander and/or endophyll for preneurulation phenomena and sickle endoblast (in the presence of central subgerminal ooplasm) for gastrulation and/or neurulation phenomena.