The liver becomes a hematopoietic organ at around 6 weeks of gestation and serves as the major source of hematopoietic cells throughout much of fetal development.
Hematopoietic stem cells and some hematopoietic progenitors also appear by around 6 weeks of gestation in the placenta and umbilical cord blood and persist in these sites until birth. 01. Hematopoietic Cells in Placenta and Umbilical Cord
By 7 to 8 weeks of gestation, lymphoid progenitors derived from the liver begin to seed the newly developed thymus, which is the major site of T-lymphocyte development. Hematopoiesis occurs in the spleen and lymphatic organs between 2 and 4 months.
Around 5 months of age, HSCs derived from the liver and the spleen, go to the bone marrow, which becomes the dominant source of hematopoietic elements by birth, around the time that hepatic hematopoiesis ceases.