Scanning and Transmission Electron Microscopic Studies on Sinusoidal Endothelium in the Embryonic, Neonatal and Adult Mouse Liver
During intrauterine life, the liver serves as a hematopoietic organ. To clarify the relationship between the development of liver hematopoiesis and the surface structures of the sinusoidal endothelium, we examined ICR mouse livers from 11 days of gestation to 90 days after birth by scanning and transmission electron microscopy. The sinusoidal endothelium of neonatal and adult livers was generally fenestrated. On the basis of their size, endothelial pores could be classified into three types : small-sized pores (S pores) were 250 nm or less in diameter, medium-sized pores (M pores) were 0.5-2.5 μm in diameter, and large-sized pores (L pores) had a diameter larger than 4 μm. At 11 days of gestation, the sinusoidal endothelium of the liver was commonly non-fenestrated, although a few M pores could be seen at the endothelial cell periphery. At 13 and 15 days of gestation, large numbers of both M and L pores were observed on the sinusoidal endothelium, and reticulocytes and hematopoietic cells were passing through the L pores. In addition, large-scale defects of the sinusoidal lining were also observed at 15 days of gestation. In the neonatal liver, both L and M pores decreased in number, and, due to a marked increase in S pores, the sinusoidal surfaces of 7-day-old mice began to resemble those of adult mice. Therefore, the surface structures of the sinusoidal endothelium are considered to undergo remarkable changes in relation to the developmental stages of liver hematopoiesis.