Abstract. Steroid production depends on the cholesterol (CH) substrate supplied by circulating lipoproteins which are internalised in the cells by receptormediated mechanisms. Low density lipoproteins (LDL) stimulate progesterone production in vitro. However, studies on follicles indicate low levels of LDL in follicular fluid (FF1).
In the present study FF1 was obtained by ultrasoundguided punctures just before ovulation from 17 women participating in an in vitro fertilization programme. Serum was obtained simultaneously. Follicular development was stimulated with hMG-HCG or clomiphenehMG-hCG combinations. In another 8 women FF1 was collected in connection with surgery for sterilization. FF1 levels of CH, triglycerides (TG), phospholipids (PL), apolipoprotein Al and B (apoAl; apoB), oestradiol and progesterone were assayed in both groups as were the corresponding serum levels in the stimulated patient group.
The FF1 levels of apoAl, TG and PL were approximately half of the levels in HDL in normal serum in both groups. However, CH was slightly lower in the stimulated group. ApoB was not detectable in FF1. Oestradiol was similar in both groups while progesterone was much higher in the stimulated than in the non-stimulated cycles. FF1 levels of apoAl correlated positively to CH and PL in both groups and to progesterone in the stimulated follicles, while the correlation was negative in the other group. The absence of apoB and the levels of CH, TG, PL and apoA1 1 indicate that high density lipoprotein (HDL), but not LDL is present in FF1. The supply of CH needed for the increased production of progesterone in the granulosa cells in the late follicular phase has to be explained by mechanisms other than transportation with LDL particles. It is possible that gonadotrophins facilitate the delivery of CH from HDL, which has been shown to penetrate the 'blood-follicle' barrier.