The mesodermal cell layer is created by ingression and migration of the cells from the primitive streak region in mouse embryos on day 7 of pregnancy. In order to study the mechanisms of mesodermal cell migration during development, the mesodermal cells isolated from the primitive streak were cultured on various substrata, and cell behaviour and motility were analysed with a time-lapse video system. The mesodermal cells on the surface of extracellular matrix (ECM)-coated dishes (ECM produced by bovine corneal endothelial cells) showed extensive migration at a mean rate of approx. 50 micron h-1. They also showed frequent cell division and exhibited contact paralysis of lamellipodia and contact inhibition of movement. On plastic or glass surfaces, however, the mesodermal cells became more flattened and less motile (approx. 20–30 micron h-1). Cell shape and mean rate of movement on the ECM were very similar to those in situ, as investigated in a previous study (Nakatsuji, Snow & Wylie, 1986). Therefore, this culture condition could provide a useful experimental system for analysing the cellular basis of normal and abnormal morphogenetic movements in mouse embryos. Employing such a culture system, we studied motility of the mesodermal cells from embryos homozygous for Brachyury (T) mutation, which are lethal at the midgestation stage in utero. Histological observations have suggested that anomalous morphogenesis of the T/T embryos may be brought about by defects in migration of the mesodermal cells derived from the primitive streak. When mesodermal cells from the primitive streak of the T/T mutant embryos on days 8–9 were cultured on the ECM substratum, mean rate of cell migration was significantly reduced compared to cells from normal embryos. Results support the idea of retarded migration by the mutant mesodermal cells as an important factor causing abnormalities in morphogenesis.

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