Macrophages are the first differentiated blood cells formed in human embryonic liver.

Up to 70% of free, intravascular hemopoietic cells belonged to the macrophage series in half-thin sections prepared from the liver of a 4.5 week old human embryo (fertilization age) and 50 to 10% were found in the liver sinusoids up to the end of the 7th week. The number of apparent macrophage precursors could reach 10% of cells of the series. Intravascular macrophage clusters were discovered; the majority of contributing cells were phagocytic. The percentage of circulating macrophages in smears was only 0.5% or less during the investigated time period. Peak values for yolk sac macrophages did not exceed 10% of intravascular cells, and apparently followed peak values found in the liver. In half-thin sections large, pale, endodermally located hemocytoblasts of the definitive series also appeared during the 5th gestational week, but the amount of these cells was less than 5%. Their relation to the intrasinusoidal macrophage has not been proved till now. So, the first differentiated definitive blood cells apparently formed in, or at least preferentially attracted to the liver are not definitive erythroblasts, but macrophages.