Recent advances in developmental hematology have revealed unappreciated hematopoietic waves and origins during development, and unexpected hematopoietic stem cell (HSC)-independent progenitors that provide lifelong hematopoiesis, using in vivo barcoding technologies and various lineage-tracing mouse models. Also, these tools estimate HSC numbers and display HSC behaviors that have not been anticipated. This review introduces such new discoveries in blood development and discusses the data and controversies.
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