CHANGING ACTIVATION SEQUENCE IN THE EMBRYONIC CHICK HEART - IMPLICATIONS FOR THE DEVELOPMENT OF THE HIS-PURKINJE SYSTEM

In the mature heart, impulse propagation through the His-Purkinje syst em (HPS) is required for efficient ventricular contraction in an apex- to-base direction. However, the embryonic heart begins to contract as a myocardial tube without a specialized conduction system. To identify the developmental stage when the PIPS begins to function, we mapped t he ventricular depolarization sequence from microvolt-level electrogra ms recorded from embryonic myocardium using 50-mu m extracellular elec trodes, high-gain amplification, and signal-processing techniques. Ana lysis of left ventricular activation in 99 embryonic hearts revealed a transition in the activation sequence that was dependent on developme ntal stage. As the heart develops, a transition in the activation sequ ence occurred from the primitive base-to-apex pattern (in 20 of 33 hea rts) at early stages (Hamburger-Hamilton stages 25 to 28) to the HPS-l ike apex-to-base pattern (12 of 17 hearts) late in development (stages 33 to 36). Immunohistological experiments (n=10) also confirm that th e expression pattern of two biochemical PIPS markers changes in parall el with the change to the mature ventricular activation pattern. These data indicate that the ventricular activation sequence in the chick h eart develops to a mature pattern at stages 29 to 31, suggesting that preferential conduction through the PIPS begins shortly after ventricu lar septation is complete.