TEMPORAL EXPRESSION OF FUNCTIONAL LOW DENSITY LIPOPROTEIN RECEPTORS AND CATALYTIC ENZYMES IN STEROID HORMONE SYNTHESIS OF MOUSE PREIMPLANTATION EMBRYOS

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Mammalian preimplantation embryos, which lack a storage organ for cholesterol which plays an essential role in the composition of cell membranes may exhaust the cholesterol without uptake from lipoprotein during development. LDL is the major lipoprotein carrier in mammals. The aim of this study was to investigate the possibility of de novo steroidsynthesis in mouse preimplantation embryos. Using collected murine oocytes and embryos, the temporal expressions of either LDL receptor (LDLR) or enzymes to synthesize steroid hormones were analyzed at the mRNA level using RT-PCR techniques. The temporal expressions of LDLR and P450scc protein were analyzed by immunostaining. To monitor the uptake of LDL in embryos, an LDL labeled with a probe was used. LDLR mRNA was detected at the oocyte, 8-cell, morula, blastocyst, and hatched blastocyst stages. P450scc mRNA was detected at the oocyte, 1-cell, 2-cell, 4-cell, blastocyst and hatched blastocyst stages. β-HSD VI mRNA was detected at the oocyte, blastocyst and hatched blastocyst stages. P450arom mRNA was detected only at oocyte. The expression of LDLR protein was detected in trophectoderm cells of blastocysts, whereas P450scc was detected in cytoplasm of oocytes and all embryos. Labeled LDL was only taken in by blastocysts, but not oocytes or 4-cell stage embryos. These results suggest that murine blastocysts could utilize LDL as a source of cholesterol for de novo progesterone synthesis.