The anatomy and development of the cardiac valves.

Advances made in the understanding of the molecular biology of the cardiac valves have been truly spectacular. Not all of those investigating these aspects, however, have an appropriate understanding of the underlying anatomy. Partly, this reflects problems in describing the components of the various valves, a difficulty also emphasised by surgeons who repair or replace the valves. In this review, we describe briefly the overall anatomy of the cardiac valves, pointing to their similarities and differences. We then suggest that uniform terms can be developed to account for the components of the valves, treating them as complexes that guard the atrioventricular and ventriculo-arterial junctions. The atrioventricular valvar complex is made up of an annulus, leaflets, tendinous cords, and papillary muscles. The tension apparatus is required to hold the leaflets together against the force of ventricular systole. The ventriculo-arterial complex is also based on the leaflets, but supported within the valvar sinuses, and limited distally by the sinutubular junction. It is the semilunar nature of the leaflets that underscores their snug closure during ventricular diastole. The complexes thus defined can be separated to produce paired valves in the normal arrangement, or to produce common valves in the congenitally malformed hearts. Knowledge of development now permits accurate inferences to be made regarding the origin of the various components, and their relevance to valvar disease. The valvar leaflets are developed from the endocardial cushions formed in the atrioventricular canal and the outflow tract by a process of endothelial-to-mesenchymal transformation. The papillary muscles of the atrioventricular valves are then derived from the trabecular layer of the developing ventricular walls, whereas the sinuses of the ventriculo-arterial valves are formed by additional growth of the non-myocardial tissues, concomitant with excavation of the outflow cushions to form the leaflets.