A bioelectrical phase transition patterns the first vertebrate heartbeats.

Area of Science:

• Developmental biology
• Cardiovascular physiology
• Biophysics

Background:

• A regular heartbeat is crucial for vertebrate survival, typically driven by a localized pacemaker in mature hearts.
• In early embryonic hearts, pacemaking is broadly distributed, posing questions about the establishment and maintenance of tissue-scale activity.
• The initial transition from a silent to a beating heart and the spatiotemporal dynamics of early heartbeats are poorly understood.

Purpose of the Study:

• To characterize the very first heartbeat in a zebrafish embryo at the timescale of individual electrical events.
• To analyze the development of cardiac excitability and conduction surrounding the initial heartbeat.
• To understand how asynchronous single-cell development leads to coordinated tissue-scale cardiac activity.

Main Methods:

• Utilized all-optical electrophysiology to capture the earliest cardiac electrical activity in zebrafish embryos.
• Analyzed the spatial and temporal dynamics of the initial heartbeats and subsequent development.
• Modeled the bioelectrical dynamics using a noisy saddle-node on invariant circle bifurcation framework.

Main Results:

• The first few heartbeats appeared abruptly with irregular intervals.
• Coherent propagation of electrical activity was observed across the primordial heart.
• Pacemaking initiation sites varied between individual embryos and over time.
• Action potential upstroke was identified as being driven by Ca_V1.2 channels.

Conclusions:

• The transition from quiescence to coordinated beating is a robust tissue-scale phenomenon.
• This transition arises from the gradual and asynchronous development of single-cell bioelectrical properties.
• The study provides a detailed characterization of early cardiac electrophysiology and conduction dynamics.