Developmental anatomy of HNK-1 immunoreactivity in the embryonic rat heart: co-distribution with early conduction tissue

To investigate the origin and development of the cardiac conduction system, the distribution of HNK-1 immunoreactivity in embryonic rat hearts was studied in histological sections and in three-dimensional computer reconstructions. Earliest HNK-1 reactivity was found along the endocardial surface of the fusing tubular heart at 9.5 embryonic days (ED) and subsequently within individual myocytes scattered widely along the looped tubular heart. Immunopositive myocytes appeared along the earliest ventricular trabeculae as they coalesced to form the developing interventricular septum during day 11, spreading to either side to give rise to the right and left bundle branches in the 12.5 ED heart. In the venous pole of the heart, primordia of the sinus node, and of the transient left sinus node, appeared immunopositive from 12.5 ED, coalescing during ED 13 along the anterior wall of the right sinus horn or developing coronary sinus, respectively. In the atria, several distinct tracts of immunoreactive myocytes were defined by 14.5 ED, ramifying from the sinoatrial junction to the atrial appendages or to the atrio-ventricular (AV) junction near the AV node. The timing and distribution of these immunostaining patterns suggest that ventricular conduction tissue develops within the earliest trabecular and septal myocardium, and is distinct from later immunopositive atrial tracts and extracardiac cell populations, such as neural crest, that appear to contribute to formation of the sinus node and autonomic innervation of the heart.