Apolipoprotein B-related gene expression and ultrastructural characteristics of lipoprotein secretion in mouse yolk sac during embryonic development

In mice, the yolk sac appears to play a crucial role in nourishing the deve loping embryo, especially during embryonic days (E) 7-10, Lipoprotein synth esis and secretion may be essential for this function: embryos lacking apol ipoprotein (apo) B or microsomal triglyceride transfer protein (MTP), both of which participate in the assembly of triglyceride-rich lipoproteins, are apparently defective in their ability to export lipoproteins from yolk sac endoderm cells and die during mid-gestation. We therefore analyzed the emb ryonic expression of apoB, MTP, and alpha-tocopherol transfer protein (alph a-TTP), which have been associated with the assembly and secretion of apoB- containing lipoproteins in the adult liver, at different developmental time points, MTP expression or activity was found in the yolk sac and fetal liv er, and low levels of activity were detected in E18.5 placentas. alpha-TTP mRNA and protein were detectable in the fetal liver, but not in the yolk sa c or placenta, Ultrastructural analysis of yolk sac visceral endoderm cells demonstrated nascent VLDL within the luminal spaces of the rough endoplasm ic reticulum and Golgi apparatus at E7.5 and E8.5, The particles were reduc ed in diameter at E13.5 and reduced in number at E18.5-19. The data support the hypothesis that the yolk sac plays a vital role in providing lipids an d lipid-soluble nutrients to embryos during the early phases (E7-10) of mou se development.-Terasawa, Y., S. J. Cases, J. S. Wong, H. Jamil, S. Jothi, M. G. Traber, L. Packer, D. A. Gordon, R. L. Hamilton, and R. V. Farese, Jr . Apolipoprotein B-related gene expression and ultrastructural characterist ics of lipoprotein secretion in mouse yolk sac during embryonic development .