Underpinning cell behavior during development are fundamental dynamic changes that drive cell fate processes away from a pre-existing equilibrium. Within a short window of developmental time hematopoietic stem cells (HSC) originate from a natural transdifferentiation of endothelial cells through a dynamic process called endothelial to hematopoietic transition (EHT). The Gata2 transcription factor is an important dosage dependent molecular regulator of this process as revealed by genetic studies in mice. Using a Gata2-Venus reporter mouse with unperturbed Gata2 expression, we followed Gata2 expression in single cells of the midgestation aorta (the site of EHT and HSC generation) by vital confocal time-lapse imaging. Rapid pulsatile Gata2 expression level changes were observed in the cells undergoing morphological/fate transition. Moreover, Gata2 pulsatile expression behavior was found to be dramatically altered in Gata2 haploinsufficient embryonic aorta cells, which undergo fewer endothelial-to-hematopoietic transitions and are reduced in hematopoietic cell potential. This highly unstable genetic state during EHT suggests that Gata2 is the rate limiting intrinsic regulator influencing the transition to HSC fate. Dynamic changes also occur at the cell-cell interaction level, as observed by vital confocal time-lapse imaging. Macrophages briefly interact with cells undergoing EHT. Since macrophage production in the early yolk sac occurs prior to HSC generation, we tested whether macrophages are part of the HSC generative niche in the embryonic aorta. Imaging of CSF1rGFP (macrophage reporter) aortic sections revealed highly motile macrophages that undergo mitosis, intravasate into the aorta and interact with endothelial and emerging hematopoietic cells. The CX3CL1/CX3CR1 axis is involved in macrophage migration to the aorta and HPSC generation is reduced in the absence of macrophages. CX3CR1 deficient embryos show a decrease in the frequency of macrophages (microglia) and HPSC in the hind brain, suggesting that in both the embryonic aorta and brain, macrophages provide an extrinsic inductive signal involved in EHT and HPSC generation.