Presence of alkaline-phosphatase-positive reticulum cells in fetal liver and bone marrow of mice and their absence in yolk sac.

One of the conspicuous stromal cells in hematopoietic parenchyma of the bone marrow in adult mice is the alkaline-phosphatase-positive reticulum cell (A1-RC). To determine whether it is present in the fetal yolk sac, liver, and bone marrow of mice, these tissues were examined at the gestational ages when they are sites of hematopoiesis. Fetuses of nine, 16 and 18 days of gestation were fixed and embedded in plastic, and 2-micron sections were stained for alkaline phosphatase and examined by light microscopy. No cells with the characteristics of the A1-RC were seen in the blood islands of the yolk sac, the site of erythropoiesis. Because of the high resolution of this method, however, alkaline phosphatase activity was observed on the border of the adjacent endodermal cells for the first time. By 16 days, the liver contained A1-RC interspersed with hematopoietic cells, as well as alkaline phosphatase activity in bile canaliculi. We found that the A1-RC is the predominant stromal cell of bone marrow at 18 days of gestation. We conclude that, as maturation proceeds, the hematopoietic stromal environment becomes more similar to that of the adult.