Contributor: Gordon K. Klintworth
Hematopoiesis is a migratory phenomenon in the developing fetus and begins within within 3 weeks of fertilization in the mesenchyme of the extraembryonic tissues (chorion, yolk sac, and body stalk). By a gestational age of 4-6 weeks the liver becomes the main site of hematopoeisis and retains this function during bone marrow development. Extramedullary hematopoiesis is uncommon after fetal life and is most often observed in patients with neoplastic (myelofibrosis with myeloid metaplasia, polycythemia vera [spent phase]) and non-neoplastic (thalassemia) hematologic disorders.