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 andlor 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 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 0 1994 WILEY-LISS, INC. results of a recent study in which only the ventral layer of the node was marked (Beddington [19941 Development 1206134320) 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 [19911 Development 112615-626). 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 a...

Fluorescent dyes were used in the present study to map three rostrocaudal levels of the primitive streak of mouse embryos at 7.5 days of gestation. Injections at the most rostral level, the 114 streak level, qrincipally labeled cells of the paraxial mesoderm, including mesenchyme in the head, and somites and segmental plate mesoderm in the trunk. Injections at the intermediate level, the 1/2 streak level, principally labeled cells of the lateral plate mesoderm of the trunk. Injections at the most caudal level, the 3/4 streak level, principally labeled cells of the extraembryonic mesoderm at the periphery (i.e., caudolateral portion) of the embryo adjacent to the developing amnion, yolk sac, and allantois. However, injections placed at all three primitive-streak levels provided some labeled cells to the areas of the ingressed mesoderm derived from immediately adjacent levels, suggesting that prospective mesodermal areas are not fully spatially segregated from one another at this stage. These data combined with those from a companion study in which the structure of the node was defined and its derivatives were mapped (Sulik et al. [1994] Dev. Dyn. 201:26&278) allowed us to construct a prospective fate map of the mouse primitive streak at 7.5 days of gestation. This map revealed that mediolateral subdivisions of the ingressed mesoderm were arrayed in rostrocaudal sequence within the primitive streak, such that more rostral levels of the streak contained prospective medial cells (e.g., notochord and paraxial mesoderm), intermediate levels contained prospective intermediate cells (e.g., intermediate mesoderm or nephrotome and lateral plate mesoderm), and more caudal levels contained prospective lateromost cells (e.g., extraembryonic mesoderm). Prospective mesodermal cells, with the exception of those of the notochord, ingressed bilaterally from the primitive streak, contributing to the mesoderm on both the right and left sides of the axis. Our prospective fate map of the mouse primitive streak at 7.5 days of gestation, when compared to a map of the avian primitive streak at a comparable stage of development (Schoenwolf et al. [ 19921 Dev. Dyn. 193:235-248), demonstrates that considerable congruity exists between the locations of mesoderm precursor cells in birds and mammals. Thus, sim-0 1994 WILEY-LISS. INC ilar morphogenetic movements underlie mesoderm formation during avian and mammalian gastrulation. Moreover, prospective mesodermal locations within the mouse primitive streak correspond to recently described patterns of early gene expression (Sasaki and Hogan 119931 Development

OBJECTIVE To examine mechanisms that mediate increased intramembranous solute and water absorption. STUDY DESIGN Intramembranous solute and water fluxes were measured in fetal sheep under basal conditions and after intraamniotic infusion of lactated Ringer’s solution of 4 L/d for 3 days with and without lung liquid diversion. RESULTS Intramembranous sodium, potassium, chloride, calcium, glucose, and lactate fluxes increased 2.5- to 7.9-fold, were linearly related to volume fluxes (r = 0.83–0.99), and were unaffected by lung liquid. All clearance rates, except that of lactate, increased to equal the intramembranous volume absorption rate during infusion. CONCLUSION Under basal conditions, passive diffusion makes a minor and bulk flow a major contribution to intramembranous solute absorption. During high absorption rates, the increase in solute absorption above basal levels appears to be due entirely to bulk flow and is unaffected by lung liquid. The increased bulk flow is consistent with vesicular transcytosis.

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Introduction:Isomerism of the atrium is the result of a defect in lateralization associated with bilateral morphologically identical atrial appendages. It is commonly associated with gross cardiac and vascular anomalies. Many of the affected individuals die at an early age. Objective: To present our experience in the prenatal diagnosis of anomalies of cardiac isomerism using high-resolution color Doppler ultrasound. Methodology: There were 116,859 low-risk patients scanned in the Antenatal Diagnostic Center as part of their routine screening over a 8-year period from 1998 to 2005. A retrospective review of congenital heart disease (CHD) associated with isomerism was conducted. Results: A total of 36 cases of suspected isomerism were observed among the cases of CHD diagnosed. Isomerism was confirmed by postmortem or postnatal 2D echo in 22 cases. In eighteen out of twenty-two (82%) of these cases, a right-sided isomerism was noted. They typically presented with AVSD (11/18), dextrocardia (8/18), situs inversus (9/18) in antenatal scan. In two of them a large azygous vein was presented. Postmortem showed 3-lobed or 4-lobed lung bilaterally. In four left-sided isomerism cases, only one had azygous drainage of IVC. None of them associated with heart block. Karyotypes were done in 13 of 22 cases and were all normal. Conclusion: There is geographical heterogeneity in the presentation of isomerism. Right isomerism is exceptionally common in our local population. We postulate that there may be a racial predisposition to the presentation of the type of isomerism. There was also a low association with karyotype abnormality.