The early development of the AV node and bundle in the ferret heart

The development of the atrioventricular (AV) junctional tissues in the ferret embryonic heart was studied on days 16, 18, and 21 of gestation. This important region of the heart was examined with PAS and toluidine-blue staining at the light microscope level and with transmission electron microscopy at the ultrastructural level. By day 16 of gestation the ferret heart was in the initial stages of convolution. The heart was at the primitive four-chamber stage by 18 days postcoitum. On day 21 of gestation the heart was in the process of being septated. The AV nodal primordia were first observed as two clusters of cells in the dorsal wall of the common atrium of the 16 day ferret embryo heart. These nodal primordial cells were morphologically different from working myocardial cells or cells of the AV canal. The AV canal cells were particularly numerous in the dorsal wall of the canal and eventually gave rise to the AV bundle in this region. On day 18 of gestation the morphological differences between the AV nodal, AV canal, and myocardial cells were readily apparent. By 21 days postcoitum, the AV node with its two regions had reached its definitive anatomic position. The AV bundle was also present in its normal adult location. The AV nodal cells were distinctly different when compared to the ventricular or atrial myocytes at this stage in development. In addition, the AV bundle cells were morphologically different from the AV nodal cells and working myocardial cells. A discussion of these findings relates this information to current descriptions of how the AV node and bundle develop.