Development of the Ventricular Conduction System of the Crocodilian Heart

Abstract

Unlike other cold‐blooded vertebrates, crocodilians have a fully septated heart, similar to birds and mammals. It is not known whether they have a ventricular conduction system (VCS) as wellWe studied a group of 8 embryos of the Siamese and Mugger Crocodile between 3 and 84 days of incubation, and serial early embryo sections with HNK‐1/Hematoxylin staining of the Nile Crocodile. We employed optical mapping, ultrasound biomicroscopy, histo‐ and immunohistochemistry, in situ hybridization and 3D reconstructionAt the pre‐septation stage (12 days of incubation), the ventricular activation pattern progressed in a left‐to‐right sweep, similar to that observed at early stages of avian and mammalian cardiogenesis. At two post‐septation stages investigated (45 and 84 days) , epicardial activation patterns showed dual apical breakthroughs on either side of the interventricular septum. Immunohistochemistry using smooth muscle actin (SMA), sarcomeric actin, periostin and HNK‐1 antibodies showed that myocardial SMA staining was present even after septation, while it disappears in comparable chick or mouse hearts. HNK‐1 staining was prominent in the interventricular septum, similar to the situation in the chick and rat where it is considered a marker of the developing ventricular conduction system. In situ hybridization for Cx40 (VCS marker) showed a uniform expression in the entire trabeculated myocardiumWe conclude that this is an evidence of evolutionary correlation between ventricular septation and presence specialized VCS in vertebratesSupported by PRVOUK P35/LF1/5.