septation of bulbus cordis (embryology)

Last reviewed 01/2018

The bulbus cordis, the most distal swelling of the primitive heart tube, has two subsections which are divided by septation from the start of the fifth week:

  • the truncus arteriosus; forms the smooth outflow tract of the heart:
    • the most distal part of the bulbus cordis
    • two mesenchymal swellings appear adjacent to each other on its walls:
      • right superior bulbar ridge; sited on the right superior wall
      • left inferior bulbar ridge; sited on the left inferior wall
    • the swellings elongate distally and spiral around each other by about 180 degrees
  • eventually, the ridges fuse to form a sheet - the aorticopulmonary septum; the septum divides the truncus arteriosus into aortic and pulmonary trunks
    • the spiralling of the ridges around each other precursors the spiralling of the aorta and pulmonary trunk around each other in the mature heart
    • truncus swellings also produce the semilunar valves
  • the conus cordis; the middle part of the bulbus cordis forms the outflow tracts of the adult heart:
    • two equivalent swellings appear which are equivalent to those on the truncus arteriosus:
      • right dorsal conus ridge
      • left ventral conus ridge
    • the ridges elongate distally and fuse with the truncus swellings
  • the ridges fuse with each other to form a septum that is continuous with the aorticopulmonary septum superiorly
    • therefore, the conus ridges fuse to demarcate two outflow tracts for the adult heart:
      • the outflow tract of the right ventricle anterolaterally
      • the outflow tract of the left ventricle posteromedially