Beyond Binary: Cardiac Patterning as Probabilistic Fate Restriction.

The mammalian heart is classically described as arising from two cardiac lineages, first and second. Yet, recent lineage tracing of Mesp1 mesoderm and live imaging show that cardiac progenitors are already partly biased toward specific heart regions during gastrulation, despite extensive cell mixing. We propose that cardiac fate is probabilistic, analogous to stochastic fate strategies in the retina. Epiblast cells carry probability distributions over fates that resolve during gastrulation into specific regional allocations. Reanalyzing a retrospective clonal dataset, we find that the clones do not single out one fixed lineage tree. Instead, the data permit a family of 361 distinct restriction topologies, none dominant, and only 40 of these are binary. Among the binary trees, the best-supported recover both where progenitors sit along the proximal-distal axis of the streak and the order in which they leave it, as recent prospective lineage tracing shows. Rather than replacing the first/second lineage concept, this framework shifts the focus from fixed lineage identity to fate probabilities. Testing this idea will require prospective live imaging from epiblast through heart tube formation.