Developmental niches for embryonic erythroid cells.

Primitive erythroid cells (EryP) are the first differentiated cell type to be specified during mammalian embryogenesis. EryP arise from a pool of lineage-restricted progenitors in the yolk sac (YS) and then enter the newly formed embryonic circulation to mature in a stepwise, synchronous fashion. Numbering in the millions in the mid-gestation mouse embryo, EryP are the dominant circulating blood cell prior to the rapid generation of adult-type definitive erythroid (EryD) cells in the fetal liver. The identification of maturational events in this lineage presented a significant challenge, as EryD begin to outnumber EryP in the bloodstream from approximately E14.5 onwards. We used human epsilon-globin gene regulatory elements to drive lineage-specific expression of a histone-H2B::EGFP fusion protein, allowing us to label the chromatin of EryP during their development and to track and quantify EryP nuclei following their expulsion from the cell. Using this transgenic fluorescent reporter mouse line, we have monitored primitive erythropoiesis in three distinct niches: the YS, where EryP progenitors arise; the circulation, where EryP continue to divide and mature; and the fetal liver, where EryP complete the terminal stages of their differentiation.