Temporal and anteriorly positioned mitotic zones drive asymmetric microtubule patterns needed for Left-Right Organizer development.

Cellular proliferation plays a crucial role in tissue development, including the development of the Left-Right Organizer (LRO), the transient organ essential for dictating the vertebrate LR body plan. Here we investigate cell redistribution mechanisms and the dominance of specific progenitor cells in LRO formation, addressing cell lineage and cell behavior questions. Using zebrafish as a model, we mapped all LRO (Kupffer's Vesicle, KV) mitotic events, revealing an FGF-dependent, anteriorly enriched mitotic pattern. Using a KV-specific fluorescent microtubule (MT) line, we found that mitotic events align their spindle along the KV's longest axis until the rosette developmental stage, where "spinning" spindles followed by exclusion from KV occur. Daughter cells that remain are linked by cytokinetic bridges, shaping anteriorly focused MT patterns that precede apical actin recruitment. Our findings underscore the importance of spatially regulated mitotic events in establishing MT and actin pattern formation essential for LRO development.