Embryology

The left atrium of the heart forms from two sources:

• the initial auricle:
  • forms after division of the common atrium by septum formation during the fourth week of fetal life
  • eventually forms the minority of the left atrium - represented mainly by the left auricle - in the mature heart due to relative growth of the pulmonary veins
• the pulmonary veins:
  • forms the majority of the mature heart and is smooth-walled
  • after the formation of the initial auricle:
    • an embryonic pulmonary vein develops as a projection from the posterior wall passing posteriorly
    • the vein joins those of the developing lung buds
    • all the veins enlarge
    • the main embryonic vein becomes incorporated into the posterior wall of the left atrium
    • due to relative growth, one superior and one inferior pulmonary vein enter the left atrium directly on each side

The fusion of the primitive pulmonary vein with the pulmonary veins derived from the lung buds may stop prematurely or progress excessively. This may lead to, respectively, only one main pulmonary vein or five to six entering the left atrium. Three veins instead of four is the most common deviation from the normal pattern. Usually, two or fewer veins entering the left atrium is associated with gross cardiac abnormalities and fetal abortion.