Electron microscopic study of virus particles in yolk sac and placenta and in hematopoietic organs of newborn C3Hf mice.

Examination of yolk sac from a C3Hf and a C3H mouse with the electron microscope revealed the presence of C-type virus particles in the blood islands. Particles were observed budding from the plasma membrane of hemocytoblasts, from erythroblasts, and occasionally from reticulocytes. C-type particles were also found in similar cells in hematopoietic foci in the liver, spleen, and bone marrow of embryos, and they continued to be present in newborn C3Hf mice up to 11 days of age. Particles consistently appeared in the thymus, even in older suckling mice. A comparison is made between the presence of C-type particles in organs of embryonic, newborn, and adult C3Hf mice. C-type particles were not observed in the chorioallantoic placentas from mice that were given injections of mouse leukemia virus (Gross) or from normal noninjected mice; however, intracisternal A-type particles were present in cytotrophoblast cells from these placentas.