Anatomical development of the left atrioventricular valvar complex

Much less is known regarding the changes taking place during development to produce the different components of the left atrioventricular valvar complex than is now known regarding the lineage and origins of the tissues forming the definitive valve. In this chapter, we review our own findings of serially sectioned human embryos and foetuses, along with mouse embryos prepared using the technique of episcopic microscopy. We also take advantage of our access to a graded series of human embryos prepared to permit their interrogation using interactive pdfs. The latter series is helpful in showing the early stages of development, revealing the basis arrangement of the atrioventricular cushions that develop within the atrioventricular canal. The reconstructions show the details of the expansion of the right atrioventricular junction and fusion of the cushions, producing the primordiums of the developing mitral and tricuspid valves. When first formed, the developing mitral valve has an obvious trifoliate arrangement, which mirrors the situation found in hearts in which the atrioventricular junction retains its commonality in the setting of an atrioventricular septal defect. In normal development, subsequent to separation of the left atrioventricular junctions, it is the commitment of the aortic root to the left ventricle that underscores the changes in which the mural leaflet of the developing mitral valve expands so as to guard two-thirds of the circumference of the left atrioventricular junction. It is during the same process that the aortic root loses its myocardial support, with the leaflets of the aortic valve coming into continuity with those of the developing mitral valve so as to produce its aortic leaflet. At the end of the period of embryonic development, however, the leaflets of the developing valve are of similar size, and at this stage, there has been no development of the tendinous cords anchoring the leaflets to the ventricular myocardium. Nor at this stage has the myocardium of the atrioventricular canal been separated from the crest of the developing ventricular wall. All of these changes take place during the early part of fetal development. By 11 to 12 weeks of development, it is possible to recognise the papillary muscles and tendinous cords, with the fibrous tissues of the atrioventricular groove now insulating the atrial wall from the ventricular myocardium. As the insulating tissues are formed, however, within the same heart it is possible to recognise features that, in the postnatal heart, are often described as "disjunction." These changes are no more than part of the normal sequence of development.