Haemopoietic cells of yolk sac and liver in the mouse embryo: a light and electron microscopical study.

Haemopoietic cells in yolk sac and liver of mouse embryo were examined by light and electron microscopy with particular reference to nuclear and nucleolar structure. Mitotic index of yolk sac haemopoietic cells increased from 10 days of gestation, showing a peak at 11 days and, at 12 days, the lumen of the yolk sac vessels were full of erythroblasts with pachychromatic nuclei. In 10 and 11 days yolk sacs, the majority of haemopoietic cells were ultrastructurally large erythroblasts, and, in addition, there were a few extra large erythroblasts and free immature haemopoietic cells. These had reticulated nucleoli and nuclear membrane invagination and were classified as transitional forms between late angioblasts and extra large erythroblasts. After 12 days of gestation, not only these immature cells but extra large erythroblasts disappeared from yolk sac vessels. At 11 days of gestation, hepatic tissues contained haemopoietic cells which were classified as extra large erythroblasts as well as immature cells which resembled late angioblasts on the basis of their nuclear and nucleolar appearance. After 12 days of gestation, erythroblasts showing various stages of maturation appeared and the immature cells remained in the liver until 15 days of gestation. Free haemopoietic cells in the yolk sac and liver are discussed in relation to cells which have the properties of haemopoietic stem cells.