Bicuspid evolution of the arterial and venous atrioventricular valves.

Atrioventricular valves were studied in pisces (shark), amphibians (frog), reptiles (turtle, snake and crocodile) and aves (duck, chicken and quail). Dog shark, Tiger shark and Hammerhead shark have a contractile common atrioventricular orifice with anterior and posterior bridging leaflets. Angel shark resembles the frog in having a contractile oval common atrioventricular orifice guarded by two lateral (right and left) and two central bridging (anterior and posterior) leaflets. In turtles and snakes the interatrial septation is complete. The common oval atrioventricular orifice is divided into two D-shaped orifices by the lower edge of the interatrial septum. Each orifice is guarded by a flap valve hanging down from the interatrial septum with rudimentary chordae tendinae, functionally corresponding to the septal leaflet of the human venous atrioventricular valve (VAVV) and the aortic leaflet of the human arterial atrioventricular valve (AAVV). There is no curved mural leaflet on either side related to curved contractile segments of the annuli. In crocodiles, the interventricular septation is complete. The VAVV is bicuspid with a prominent septal and diminutive mural component with no chordopapillary support. The AAVV is bicuspid with direct attachment of the fused commissural lines between the two leaflets to the ventricular musculature. In the aves, with four chambered hearts the VAVV has a curved mussular mural leaflet with a primitive muscular chorda and no septal leaflet. The AAVV resembles closely the human mitral valve. This study lends support to the view that VAVV and AAVV in man are similar in design and function with a D-shaped annulus and a skirt of leaflet tissue divisible into two functional units. The relatively straight part of the annuli suspend the straight leaflets; septal leafleft of the VAVV and aortic leaflet of the AAV. The curved annular components suspend the curved mural leaflets in both valves. Since this segment is contractile, the related mural leaflets of both valves need to have slits which split them into varying number of scallops to enable them to adapt themselves to the reduced systolic orifice and open widely during diastole. These scallops are best numbered as subsegments of the curved mural leaflet and not considered as separate leaflets. The curved leaflet of the VAVV needs to negotiate the acute border of the heart. Hence a major slit is usually present at this region which artificially divides it into an 'anterior' and 'posterior' portion resulting in assumption of a 'tricuspid' configuration.(ABSTRACT TRUNCATED AT 400 WORDS)