Transition from fetal to adult murine hematopoiesis. Histologic changes in the liver, spleen, and BM of newborn mice during the first 2 weeks of life, compared with adults. Fixed tissue sections were stained with H&E and photographed with a ×20 objective lens (Bar: 50 μm). (Left column) The liver of P1 and P3 newborn mice contained numerous hematopoietic islands (arrows), which rapidly decreased after P5. (Middle column) The BM of newborn pups during the first week of life was markedly hypocellular, containing large sinusoids and limited hematopoietic elements. (Right column) Clearly distinct red and white pulp zones were formed in the spleen and increased in size during the first 2 weeks of life.