Postnatal development of the spleen in Didelphis virginiana.

The postnatal development of the spleen has been examined in 85 opossums ranging in age from newborn to adult. At birth the spleen consists of a well vascularized mass of mesenchymal tissue and lacks lymphatic tissue or any evidence of haemopoietic activity. Haemopoiesis is evident at seven days, increases to a maximum at about two to three weeks and thereafter gradually declines. Although production of granulocytes has disappeared by 60 days postnatum, a small degree of erythropoiesis and megakaryocyte formation continues throughout life. Lymphatic tissue appears by the third week, but germinal centres do not appear until after weaning. A feature of the spleen during the first three to four days is the presence of a population of primitive 'blast' cells.