Morphological basis for the study of the interatrial septum in the human fetus.

OBJECTIVE To describe morphological features of the interatrial septum in normal fetuses, especially foramen ovale (FO) and septum primum (SP), in order to compare septum primum excursion with foramen ovale diameter. METHODS Septum primum excursion (SPE) toward the left atrium (LA) and foramen ovale diameter (FOD) were measured in the hearts of ten formaldehyde-fixed human fetuses ranging from 28 to 36 weeks of gestation. Histological sections were obtained from the foramen ovale (FO), septum primum (SP), septum secundum (SS), left atrium (LA), and right atrium (RA). RESULTS FOD and SPE measurements were the following: FOD 3.1-3.5 mm and SPE 2.8-3.1 mm in three fetuses with presumed gestational age (GA) of 28 weeks; FOD 3.3-3.5 mm, and SP excursion 4.0-5.0 mm in four fetuses with presumed GA of 34 weeks, plus FOD 3.3-4.5 mm and SPE 6.0-9.0 in three fetuses with presumed GA of 36 weeks. Cardiac muscular fibers were identified in both the septum primum and secundum. CONCLUSION Based on its muscular components, it may be suggested that SP is active in character, influencing blood flow through the FO, SP mobility, and its excursion into the LA.