Where is hematopoiesis occurring at birth?

• A. Liver
• B. Thymus
• C. Bone marrow
• D. Spleen

Answer: (C)

Hematopoiesis is the production of blood cells, and different organs take turns hosting this process during development. In early gestation, the liver is a major site, with the spleen contributing as well, while the thymus handles T-cell maturation rather than broad blood cell production. By the time of birth, the bone marrow becomes the primary site for hematopoiesis, producing red cells, white cells, and platelets. The marrow provides the hematopoietic stem cells that differentiate into all blood cell lineages under growth factors and niche signals. The liver’s role diminishes after birth, and the thymus isn’t the main site for hematopoiesis. Extra-medullary hematopoiesis can occur under certain conditions, but normally at birth the marrow is responsible for most blood cell formation.