Determination of embryonic polarity in a regulative system: evidence for endogenous inhibitors acting sequentially during primitive streak formation in the chick embryo

Abstract

Avian embryos have a remarkable capacity to regulate: when a pre-primitive streak stage embryo is cut into fragments, each fragment can spontaneously initiate formation of a complete embryonic axis. We investigate the signalling pathways that initiate primitive streak formation and the mechanisms that ensure that only a single axis normally forms. As reported previously, an ectopic primitive streak can be induced by misexpression of Vg1 in the marginal zone. We now show that Vg1 induces an inhibitor that travels across the embryo (3 mm distance) in less than 6 hours. We provide evidence that this inhibitor acts early in the cascade of events downstream of Vg1. We also show that FGF signalling is required for primitive streak formation, in cooperation with Nodal and Chordin. We suggest that three sequential inhibitory steps ensure that a single axis develops in the normal embryo: an early inhibitor that spreads throughout the embryo (which can be induced by Vg1), a second inhibition by Cerberus from the underlying hypoblast, and finally a late inhibition from Lefty emitted by the primitive streak itself.

The importance of the posterior midline region for axis initiation at early stages of the avian embryo

“…This suggested that there must be a control mechanism which contributes to the formation of a polarized axis and also inhibits the tendencies of other regions to initiate an axis. These reports support the notion that a region responsible for axis initiation exists at the posterior side of the area pellucida (Spratt and Haas, 1960;Spratt, 1963;Khaner and Eyal-Giladi, 1986;Khaner and Eyal-Giladi, 1989;Khaner, 1993;Khaner, 1998, Bertocchini et al, 2004. Moreover, it was found that the midline region of the avian blastoderm has a crucial role in the process of axis formation (Khaner, 1996).…”

“…A recent publication indicated that Vg1 induces an inhibitor that travels across the embryo (3mm distance) in less than 6 hours (Bertocchini et al, 2004). Although such inhibitors exist, they could not prevent the initiation of more than one embryonic axis (series Aa, Ba and Db).…”

“…In addition, it is known that several other manipulations (such as marginal zone grafts, or certain types of hypoblast rotation), as well as in some series in this report, also generate ectopic axes. In these cases, this is not considered as conflicting with the proposed inhibitors, but rather, they help to explain where and when the various inhibitors might act (Bertocchini et al, 2004).…”

“…This inhibitor acts early in the cascade of events downstream of Vg1. It was suggested that three sequential inhibitory steps ensure that a single axis develops in normal development of the embryo (Bertocchini et al, 2004).…”

Germline development in amniotes: A paradigm shift in primordial germ cell specification

“…When the chicken egg is laid, the embryo, which already contains about 50,000 cells and is organised in a flat round single‐layered blastodisc (epiblast) can be cut in pie‐shaped slices, and each slice can generate an embryonic axis 62, 63, 64, 65, 66. This property lasts until the initiation of gastrulation, 12 hours after the egg has been laid, when a lower extra‐embryonic layer has grown below the epiblast and the embryonic axis, or primitive streak, is about to appear.…”