Hematology/oncology

In the prenatal period of development, hematopoiesis takes place in the yolk sac, liver, and preterm bone marrow. These can be broken down into three stages of development: mesoblastic, hepatic, and myeloid. Mesoblastic development mainly occurs in the yolk sac between the 10th and 14th gestational days and continues until the 10th-12th week of gestation. Hepatic blood cell production starts at week 6-8 and continues throughout gestation. The hepatic stage remains the dominant source until gestational weeks 20-24. After this point the third stage of hematopoiesis-the myeloid proliferation-within preterm bone marrow takes over. All hematopoietic cells originate from pluripotent stem cells and these progenitor cells differentiate based on the effects of various fetal growth factors. Lymphopoiesis begins at the ninth week of gestation within the spleen and thymus. Cells originating from the thymus are named T-lymphocytes and those originating from the bone marrow are called B-lymphocytes.