For a few months in utero the normal human spleen is hematopoietic. By the close of the fifth fetal month, the function has moved permanently into the marrow. Except for some lymphocytes and plasma cells, which continue to be contributed from the white pulp of the spleen and other lymph tissues, the production of blood cells—monocytes, granulocytes, erythrocytes, and megakaryocytes—remains entirely in the marrow. The basis of this obligatory behavior probably resides in the quality of the marrow's unique sinusoidal microcirculation, which provides an essential environment.1
In adult small mammals, the spleen remains a normal hematopoietic organ. In the mouse, for example, all available space in the marrow cavities contains what we would call, in the human condition, hypercellular marrow; there is no fat. In these tiny animals, the proportion of marrow cavity to solid bone is small and the life span of blood cells is relatively short (requiring
Crosby WH. Hematopoiesis in the Human Spleen. Arch Intern Med. 1983;143(7):1321–1322. doi:10.1001/archinte.1983.00350070037004
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