Immunohistochemical evaluation of the developing outflow tract in the rat: achieving aortic to mitral fibrous continuity.

Objectives: To study the development of aortic to mitral fibrous continuity in the normal rat heart.

Methods: The hearts and great vessels of normally developed rat embryos and fetuses aged between 13.25 and 19.75 days of gestation were studied in conjunction with those of newborns aged 2 and 7 days post-partum. Standard histological methods and monoclonal antibodies raised against alpha smooth muscle actin (clone 1A4) and ventricular beta myosin heavy chain were used to demonstrate the ventricular outlets, ventriculo-arterial junction, inner heart curve and aortic infundibulum from the early stages of aortopulmonary septation to attainment of their definitive morphology.

Results: The two antibodies demonstrated temporal specificity (actin specificity increased post-partum; myosin specificity maximal during fetal period) in the labelling of their intended structures which correlated with their known developmental profile. Full-thickness fibrous continuity between aortic and mitral valves was not complete until 1 week after birth. After ventricular septation was complete, and thereafter towards the end of fetal life and beyond, separation was maintained by a muscular structure histologically identical to the vestigial netro-aortic root branch of the conduction tissue, a structure known to be derived from the primitive ventricular myocardium within the environs of the inner heart curve.

Conclusions: Ventricular septation (occurring relatively early) and the attainment of fibrous continuity (occurring relatively late in development) are two independent processes. Muscular tissue separating left-sided arterial and atrioventricular valves is not derived from the aortic infundibulum but from the inner heart curve. Persistence of this structure is a feature of normal rat heart development and needs to be recognised when working with rodent-based animal models of congenital heart disease aimed at studying the disruption of the development of the ventricular outflow tracts.