In the zebrafish, amphibian and avian gastrulae, the establishment of the body plan is influenced by the activity of a group of specialized cells known as the organizer (Gilbert

In the zebrafish, amphibian and avian gastrulae, the establishment of the body plan is influenced by the activity of a group of specialized cells known as the organizer (Gilbert and Saxen, 1993; Harland and Gerhart, 1997; Shih and Fraser, 1996; Schier and Talbot, 1998; Smith and Schoenwolf, 1998). Two unique functional attributes of the organizer are its ability to exert an inductive influence on the differentiation of the tissues with which it interacts and the provision of morphogenetic cues that regulate the dimensions, the orientation and the placement of the induced tissues in the body plan. Experimentally, the organizing activity can be demonstrated by the induction of a secondary body axis in a host embryo following the transplantation of tissue fragments containing the organizer. Cells in the organizer of different vertebrates express a common set of transcription factors and other molecules that are involved with intercellular signalling, and these cells contribute to the formation of the axial mesendoderm and the midline neuroectoderm (Harland and Gerhart, 1997; Smith and Schoenwolf, 1998; Camus and Tam, 1999). In the mouse, the node of the late-gastrula (late-streak to early-bud stage) embryo has been shown to possess organizing activity by virtue of its ability to induce axis formation following heterotopic transplantation (Beddington, 1994) and the expression of genes that are associated with the organizer of other vertebrate gastrulae (Tam and Quinlan, 1996, Tam and Behringer, 1997). In other vertebrates, cell population that displays organizer activity has been identified at the onset of gastrulation (Shih and Fraser, 1996; Zoltewicz and Gerhart, 1997; Ipzisúa Belmonte et al., 1993). However, the existence of an organizer that precedes the node in the mouse gastrula has not been demonstrated until recently by the discovery of a group of cells in the posterior epiblast of the early-streak embryo that displays cell fates, gene expression and tissue patterning activity that are characteristic of other vertebrate organizer (Tam et al., 1997; Camus and Tam, 1999). Our previous study has shown that a fragment of the posterior epiblast that is localised outside the early primitive streak can induce the formation of ectopic neural tissues, which in some cases are organized into a rudimentary axis. Ectopic neural tissue can be found in about 26% of the recipient embryos, which is comparable to the frequency of neural tissue induction by the node of the late-gastrula (Beddington, 1994; Tam et al., 1997). This group of cells may therefore be regarded as the equivalent of the organizer in the early gastrulae of other vertebrates and has been termed as the early gastrula organizer (EGO). When the node of the mouse is tested for its organizing activity by transplantation, the induced axis is typically made 5171 Development 126, 5171-5179 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV4211