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Development of the conduction system in the rat heart as determined by Leu-7 (HNK-1) immunohistochemistry and computer graphics reconstruction.

BACKGROUND
The development of the heart conduction system is controversial. Our previous study demonstrated anti-Leu-7 antibody to cross-react with the cells of the conduction system in the embryonic rat heart. Thus, detailed analysis of the development of the conduction system was performed by immunohistochemical reaction with the use of the antibody.


DESIGN
Horizontal serial sections of hearts obtained from embryonic and neonatal rats were treated for immunohistochemical reaction with anti-Leu-7 antibody, and three-dimensional images were reconstructed by computer graphics. Images were analyzed by superimposing the results on scanning electron micrographs.


RESULTS
The development of the rat heart conduction system starts at 10 days and is completed by 18 days of gestation. The presence of three internodal tracts (INT1, -2, and -3) and two primordia of the atrioventricular node were confirmed in this process. INT1 and -3 pass through the septum spurium. INT2 follows the same route as the posterior internodal tract of James, but the other two INTs showed no correlation to previously described tracts. The sinuatrial node and INTs were found to be derivatives of the so-called S-A ring, and the bundle of His and bundle branches were found to be derived from the B-V ring. In this study, no immunoreactivity was observed in cells of the truncobulbar portion.


CONCLUSIONS
Three internodal tracts and two atrioventricular node primordia were observed in the developing heart. The paths of the three internodal tracts showed intimate relationships with the internal structures of the heart. The developmental significance of the septum spurium, sinus septum, and venous valve was discussed.