Ultrastructural studies of hepatoblast junctions and liver hematopoiesis of the mouse embryo

: To clarify the morphological changes in hepatoblast connections during the development of fetal liver hematopoiesis, ICR mouse livers of 11 to 19 days of gestation were studied by means of three-dimensional reconstruction, immunohistochemistry, electron microscopy and freeze fracture replica method. Embryonic liver weight showed rapid increase until 19 days of gestation, and an initial steep increase, due to hematopoietic development, was observed at 13 to 15 days of gestation. Hepatoblast volume appeared to be constant until 13 days of gestation, and, thereafter, showed a gradual increase. An 11-day primitive hepatic cord contained a few immature hematopoietic cells among hepatoblasts, and the hepatoblasts made contact with one another by short cytoplasmic projections. The area of the contact surface had a diameter of 4-5 microns, where E-cadherin-mediated adherens junctions were found. At 12-13 days of gestation, hepatoblasts surrounded large ellipsoidal hematopoietic foci, with long cytoplasmic projections. In addition to the adherens junctions, small desmosomes appeared to bind hepatoblasts together, and biliary canaliculi could be recognized between hepatoblasts. At peak stage of liver hematopoiesis at 14 days of gestation, both tight junctions and gap junctions appeared around the biliary canaliculi, and four types of specialized junctions, i.e., adherens junctions, desmosomes, tight junctions and gap junctions, appeared to be fully developed. After 15 days of gestation, hepatocyte volume showed rapid increase, and the surface areas between adjacent hepatocytes were markedly enlarged. As a result, the involuted hematopoietic foci were forced to move from interhepatocytic spaces to perisinusoidal space at the end of the intrauterine life.