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Morphogenesis of the murine node and notochordal plate

Development of the node and formation of the notochordal plate in gestational day 7—9 mice (Theiler stages 10—14) has been documented principally with scanning electron microscopy (SEM) and cell fate analyses utilizing DiI and/or CFSE as a cell label. With SEM, cells composing these two populations are initially identifiable at stage 10 at the ventral midline of the rostral half of the embryo. They can be recognized by their relatively small ventral surface area, as compared to that of the peripherally adjacent prospective gut endodermal cells, and by the presence on the ventral side of each cell of a prominent single, central cilium, which is lacking on endodermal cells. At stage 10, the node is located at the apex of the cup‐shaped embryo. It represents the rostral end of the primitive streak (although its structure differs from that of the rest of the streak), and it consists of a localized two‐layered area (i.e., epiblast and the most caudal aspect of the notochordal plate). By stage 11, the notochordal plate constitutes a relatively broad, circular area (at the level of the node) that tapers rostrally into a narrower midline strip (beneath the future floor plate of the neural tube). The tip of the notochordal plate terminates rostrally at the much broader prechordal plate, which underlies the future forebrain level of the neuraxis. The prechordal plate cells, like the ventral node and notochordal plate cells, each have a relatively small ventral node ventral surface area and displays a single central cilium on their ventral surface. The most caudal aspect of the notochordal plate remains morphologically distinct on the dorsal, midline surface of the open gut through stage 13; the more rostral levels progressively fold off from the roof of the gut to form the definitive notochord. Videomicroscopy reveals that the cilia extending from the ventral surfaces of the cells of node and of the prechordal and notochordal plates are motile. The potential significance of this motile behavior remains unknown. Labeling studies, which marked cells in both the dorsal and ventral layers of the node, reveal that the stage‐10 node contributes cells to the notochordal plate and overlying midline ectodermal cells of the neural plate, the future floor plate of the neural tube. The results of our labeling studies, in which cells in both layers of the node were marked, when compared with the results of a recent study in which only the ventral layer of the node was marked (Beddington [1994] Development 120:613—620) provide strong evidence that the ventral layer of the node forms notochord, whereas the dorsal layer forms floor plate of the neural tube. A similar origin for these two populations of cells has been suggested for the chick embryo (Selleck and Stern [1991] Development 112:615—626). The morphology of the murine notochordal plate and labeling studies support the concept of origin and rostrocaudal elongation of this structure in large part by accretion of cells from the node. In addition, cell division and cell rearrangement within the notochordal plate play important roles in murine notochord extension (Sausedo and Schoenwolf [1994] Anat. Rec. 239:103—112). © 1994 Wiley‐Liss, Inc.