A study of the ultrastructure of the developing spleen in the albino rat

Spleens from developing rats of ages 16 days' gestation to 4 days after birth were prepared for electron microscopic study. At 16 days the spleen consists of a meshwork of intercommunicating spaces surrounded by mesenchymal or early reticular cells which show indications of collagen-forming activity. By 17 days these spaces are connected with the blood vessels and an open circulation is established. Some spaces are dilated and by 20 days begin to assume the characteristics of sinuses while the reticular cells are crowded together to form the sinus walls. Similar spaces, when collapsed against the contained cells, resemble the pulp cords. The endothelium of the blood vessels is differentiated by the 21st day and by 4 days after birth, basement membranes have appeared between cells lining adjacent sinuses. Erythropoiesis, established by 17 days, persists at least through the fourth postnatal day. White pulp formation is initiated on the second postnatal day.