Abstract
In mice, the yolk sac appears to play a crucial role in nourishing the developing embryo, especially during embryonic days (E) 7;-10. Lipoprotein synthesis and secretion may be essential for this function: embryos lacking apolipoprotein (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 embryonic expression of apoB, MTP, and alpha-tocopherol transfer protein (alpha-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 liver, 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 sac or placenta. Ultrastructural analysis of yolk sac visceral endoderm cells demonstrated nascent VLDL within the luminal spaces of the rough endoplasmic reticulum and Golgi apparatus at E7.5 and E8.5. The particles were reduced 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 and lipid-soluble nutrients to embryos during the early phases (E7;-10) of mouse development. secretion in mouse yolk sac during embryonic development.
Highlights
In mice, the yolk sac appears to play a crucial role in nourishing the developing embryo, especially during embryonic days (E) 7–10
In our initial gene expression studies, we performed reverse transcriptase (RT)-Polymerase chain reaction (PCR) assays for apoB, microsomal triglyceride transfer protein (MTP), and ␣-␣-tocopherol transfer protein (TTP) transcripts on cDNA generated from embryos at early (E7.5), middle (E13.5), or late (E18.5) stages of development
The highest levels of apoB transcripts were in the yolk sac and liver; the highest levels of ␣-TTP were in fetal liver
Summary
The yolk sac appears to play a crucial role in nourishing the developing embryo, especially during embryonic days (E) 7–10. The data support the hypothesis that the yolk sac plays a vital role in providing lipids and lipid-soluble nutrients to embryos during the early phases (E7–10) of mouse development.—Terasawa, Y., S. Several mammals, including rodents, rabbits, moles, shrews, armadillos, and some bats, have yolk sacs whose absorptive surface is partially or completely “inverted” [1] In such cases, the absorptive surface faces the surrounding uterine decidual tissue, where it is thought to be important in the absorption of maternal nutrients during the early phases of development. The yolk sac develops around embryonic day (E) 7 and probably plays a vital nutritive role until at least E9 –10, when placental function is established. ␣-tocopherol is undetectable in E9.5 mouse embryos lacking apoB [10]
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