Asymmetric cell divisions are concentrated in the dermomyotome dorsomedial lip during epaxial primary myotome morphogenesis
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To determine if somitic stem cell pools could be identified by an intrinsic difference in mitotic behaviour, the orientation of mitoses in the dermomyotome epithelium was analysed. We describe a concentration of apico-basal mitoses within the dermomyotome dorsomedial lip (DML). The occurrence of apico-basal divisions is closely associated with asymmetric localisation of the notch pathway factor numb, allowing description of such divisions as asymmetric. In contrast, planar divisions, occurring in the plane of the epithelium, are symmetric. Further, we show that the DML environmental niche is sufficient to promote numb expression in epaxial dermomyotome tissue that does not normally express this factor. These data provide, for the first time, a non-retrospective tracing analysis of the mechanism by which the DML fulfils the stem-cell pool role it plays during epaxial primary myotome morphogenesis.