[The development of the atrioventricular node and bundle of His in the human embryonic period]

To study the development of the atrioventricular specific system, together with the closely related mesenchymal tissue (from which the fibrous skeleton of the heart later develops). Thirty human embryos ranging from 3 mm to 30 mm crown-rump length (Carnegie stages 10-23) were used. The primordium of the atrioventricular specific system was observed for the first time in human embryos of 10 mm Crown-Rump length (Carnegie stage 16) as a cellular aggregate located below the inferior endocardial cushion. The primordium cells originate from the myocardium of the posterior wall of the atrioventricular canal. The primordium later forms both the atrioventricular node and the His bundle. The mesenchymal tissue which surrounds these features originates from the endocardial cushions (particularly from the inferior endocardial cushion). Todaro's tendon and the central fibrous body are later formed from the inferior endocardial cushion. The atrioventricular node and His bundle are both formed from the primordium of the atrioventricular specific system. The node is produced from the cranial-dorsal extension of the primordium while the His bundle is produced from the growth of the primordium in a ventral-caudal direction. The central fibrous body is anatomically defined in the stages post-dating morphogenesis of the atrioventricular specific system.