Septation of bulbus cordis (embryology)

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