Figure 2. The monolayer of myocardium displays apical-basal polarity.
Mouse embryonic heart tube at E9.5-E9.0 has a smooth inner surface and contains a monolayer myocardium and a monolayer endocardium. The monolayer myocardium displays transmural polarity with basement membrane inside the heart lumen. ?: The orientation of apical-basal polarity shows some controversy, as a group says the domain that abuts the heart lumen is the apical domain or basal domain by another group.