The morphology of heart development in Xenopus laevis

We have used serial histological sections to document heart formation in Xenopus laevis, from the formation of a linear heart tube to the appearance of morphologically distinct atrial and ventricular chambers. 3D reconstruction techniques have been used to derive accurate models from digital images, revealing the morphological changes that accompany heart differentiation. To demonstrate the utility of this approach in analysing cardiac gene expression, we have reexamined the distribution of Hand1 transcripts in the linear and looped heart tube. Our results demonstrate that prior to looping, an initial asymmetric, left-sided pattern is replaced by more symmetrical localisation of transcripts to the ventral portion of the myocardium. After the onset of looping, Hand1 expression is restricted to the ventral ventricular myocardium and extends along the entire length of the single ventricle.