Fig. 4

Myofibers in the systemic RV tend to be organized within individual, small trabeculae (*) and papillary muscles a few mm up to ~ a cm or so in diameter. In contrast, larger muscle bundles (enclosed in white ellipses), which are composites of the small trabeculae (*) and measure up to a few cms, appear disorganized with many crossing fibers. a – Parallel fiber tracts in small trabeculae (*) in the MA/AA heart. These small trabeculae cross each other as they converge into a larger bundle (ellipse). b – A small papillary muscle on the wall of the DIRV heart. c – A large muscle bundle (ellipse) in the MA/AA heart showing the many crossing trabeculae (*). d – A similar complex, large bundle (ellipse) in the DIRV heart