Fig. 3. Primary heart tube expansion in Hand1-overexpressing embryos at E8.0-8.5. Bright-field whole-mount left lateral views of hearts from somite-matched Hand1-overexpressing (oe) embryos and control (co) littermates dissected pre-embryonic turning (nine somites; A,B) and during turning (12 somites; C,D). Haematoxylin and Eosin stained frontal sections through the heart tube of a control (E-H) and overexpression embryo (I-L) at the 12-somite stage. Whole-mount in situ hybridisation for Hand1 on control (M,O) and overexpression embryonic hearts (N,P). (A,B) The primary heart tube pre-turning is expanded (white arrowheads) and early stages of looping are accentuated in overexpressing embryos compared with control. Extension and exaggerated ventral and left lateral displacement of the heart tube can be seen as looping progresses during turning (depicted by white arrows; C,D; compare E-H with I-L). (M-P) Confirmation of appropriate spatial overexpression of Hand1 in the mutant embryos at the 12-somite stage. co, control; oe, overexpression; s, somites. Anterior (A) - posterior (P) and left (L) - right (R) axes are indicated for E-L. Scale bar: in D,P, 50 µm for A-P.