The changing morphology of the ventricular walls of mouse and human with increasing gestation.
That the highly trabeculated ventricular walls of the developing embryos transforms to the arrangement during the fetal stages, when the mural architecture is dominated by the thickness of the compact myocardium, has been explained by coalescence of trabeculations, often erroneously described as "compaction". Recent data, however, supports differential rates of growth of the trabecular and compact layers as the major driver of change. Here, these processes were assessed quantitatively and morphologically using a larger dataset than has previously been available of mouse hearts from embryonic day 10.5 to postnatal day 3, supported by images from human hearts. The volume of the trabecular layer increased throughout development, in contrast to what would be expected had there been "compaction". During the embryonic-fetal transition, fast growth of the compact layer diminished the proportion of trabeculations. Similarly, great expansion of the central cavity reduced the proportion that intertrabecular recesses make of the total cavity. Using the median value of left ventricular trabeculation, we provided illustrations for each gestational day so as to provide pictorial evidence of the changes. The illustrations confirmed a pronounced growth of the compact wall, and prominence of the central cavity. This corresponds, in morphological terms, to a reduction in the extent of the trabecular layer. Similar observations were made in the human hearts. We conclude that it is a period of comparatively slow growth of the trabecular layer, rather than so-called compaction, that is the major determinant of the changing morphology of the ventricular walls of both mouse and human.