INTRODUCTION During mouse gastrulation, cells recruited from the epiblast ingress through the primitive streak (PS) and are organized into layers of mesodermal cells that constitute the embryonic and extraembryonic mesoderm. Although the formation of the mesoderm has been extensively studied (Hashimoto et al., 1987, Lawson et al., 1991; Lawson and Pedersen, 1992a,b; Parameswaran and Tam, 1995; Tam and Beddington, 1987; Tam et al., 1993, 1997), little is known about the morphogenetic processes that influence the placement of mesodermal cells in different parts of the embryonic axis and the extraembryonic membrane. Fate-mapping studies of the mesoderm of the gastrulating embryo reveal that cells that are destined for the extraembryonic mesoderm constitute the major tissue type in the nascent mesodermal layer. The precursors for cranial and heart mesoderm are present later in the mesodermal layer of the mid-streak stage embryo, but are ahead of those that contribute to the paraxial and lateral mesoderm of the trunk, which form the bulk of the embryonic mesoderm in the late-streak embryo (Parameswaran and Tam, 1995; Tam et al., 1997). These studies also show that the various mesoderm precursors are distinctly regionalized in the mesodermal germ layer and their relative position is concordant with the final location in the fetal body. An examination of the regional distribution of PS-derived cells in the paraxial mesoderm has revealed that PS cells are allocated to the somites in a craniocaudal manner (Tam and Beddington, 1987). Epiblast cells that are recruited to the PS at successive stages of gastrulation are also found to be distributed similarly in a craniocaudal order to the gut endoderm (Tam and Beddington, 1992; Lawson et al., 1987; Lawson and Pedersen, 1986). During gastrulation, cells in the epiblast and their descendants are progressively displaced towards the PS where cellular ingression takes place (Lawson et al., 1991; Lawson and Pedersen, 1992a). It is possible that cells in the epiblast are recruited for ingression in a sequential order that is determined by their geographic proximity to the PS. This may imply that the position of the cells in the epiblast predisposes the order and timing of their recruitment and thus their destination in the anteroposterior axis of the body plan. However, clonal analysis of epiblast cells has revealed significant overlap in the distribution in both the ectoderm and the mesoderm along the anteroposterior body axis of clones arising from cells localized in different regions of the epiblast (Lawson et al., 1991; Lawson and Pedersen, 1992b). Indeed, any spatial restriction of clonal 4691 Development 126, 4691-4701 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV4212 The prospective fate of cells in the primitive streak was examined at early, mid and late stages of mouse gastrula development to determine the order of allocation of primitive streak cells to the mesoderm of the extraembryonic membranes and to the fetal tissues. At the early-streak stage, primitive streak cells contribute predominantly to tissues of the extraembryonic mesoderm as previously found. However, a surprising observation is that the erythropoietic precursors of the yolk sac emerge earlier than the bulk of the vitelline endothelium, which is formed continuously throughout gastrula development. This may suggest that the erythropoietic and the endothelial cell lineages may arise independently of one another. Furthermore, the extraembryonic mesoderm that is localized to the anterior and chorionic side of the yolk sac is recruited ahead of that destined for the posterior and amnionic side. For the mesodermal derivatives in the embryo, those destined for the rostral structures such as heart and forebrain mesoderm ingress through the primitive streak early during a narrow window of development. They are then followed by those for the rest of the cranial mesoderm and lastly the paraxial and lateral mesoderm of the trunk. Results of this study, which represent snapshots of the types of precursor cells in the primitive streak, have provided a better delineation of the timing of allocation of the various mesodermal lineages to specific compartments in the extraembryonic membranes and different locations in the embryonic anteroposterior axis. Key words: Primitive streak, Cell fate, Mesoderm, Gastrulation, Mouse SUMMARY The orderly allocation of mesodermal cells to the extraembryonic structures and the anteroposterior axis during gastrulation of the mouse embryo Simon J. Kinder 1 , Tania E. Tsang 1 , Gabriel A. Quinlan 1 , Anna-Katerina Hadjantonakis 2 , Andras Nagy 2 and Patrick P. L. Tam 1, * 1 Embryology Unit, Children’s Medical Research Institute, Locked Bag 23, Wentworthville, NSW 2145, Australia 2 Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada *Author for correspondence (e-mail: ptam@cmri.usyd.edu.au) Accepted 11 August; published on WWW 6 October 1999