DEVELOPMENTAL ANATOMY OF HNK-1 IMMUNOREACTIVITY IN THE EMBRYONIC RAT-HEART - CODISTRIBUTION WITH EARLY CONDUCTION TISSUE

To investigate the origin and development of the cardiac conduction sy stem, the distribution of HNK-1 immunoreactivity in embryonic rat hear ts was studied in histological sections and in three-dimensional compu ter reconstructions. Earliest HNK-1 reactivity was found along the end ocardial surface of the fusing tubular heart at 9.5 embryonic days (ED ) and subsequently within individual myocytes scattered widely along t he looped tubular heart. Immunopositive myocytes appeared along the ea rliest ventricular trabeculae as they coalesced to form the developing interventricular septum during day 11, spreading to either side to gi ve 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, coal escing during ED 13 along the anterior wall of the right sinus horn or developing coronary sinus, respectively. In the atria, several distin ct tracts of immunoreactive myocytes were defined by 14.5 ED, ramifyin g from the sinoatrial junction to the atrial appendages or to the atri o-ventricular (AV) junction near the AV node. The timing and distribut ion of these immunostaining patterns suggest that ventricular conducti on tissue develops within the earliest trabecular and septal myocardiu m, and is distinct from later immunopositive atrial tracts and extraca rdiac cell populations, such as neural crest, that appear to contribut e to formation of the sinus node and autonomic innervation of the hear t.