Early immune cell development precedes gastrulation in annual killifish

Following cleavage, deep blastomeres disperse as individual cells across the yolk surface, remaining independent until undergoing self-organized aggregation and germ layer formation. This dispersed phase lacks the tissue-level organization characteristic of other vertebrate embryos, providing an opporunity to investigate how lineage-specifying GRNs function in an altered morphogenetic environment.

To investigate this question, we performed single cell RNA-sequencing on killifish embryos using PIP-seq technology, thereby producing the first coarse-grained atlas of early killifish embryogenesis. Here we show that primitive myelopoiesis, which in most studied vertebrates invariably develops from ventral mesodermal progenitor domains, is uncoupled from this tissue source in N. furzeri. Instead, neutrophils emerge prior to gastrulation.