Stage-specific uptake of apolipoprotein-B in ovarian follicles and corpora lutea of the menstrual cycle and early pregnancy

A monoclonal antibody (mAb), HCL-2, was raised which reacts with apolipoprotein-B, and it was shown by immunohistology that HCL-2 can be used to analyse the uptake of apolipoprotein-B by steroid-producing cells in vivo. In this study we have investigated the dynamic utilization of low density lipoprotein (LDL) in human ovary by immunohistological localization of apolipoprotein-B and LDL receptors using HCL-2 and anti-LDL receptor mAb. In antral follicles, including those of <1 mm in diameter, both apolipoprotein-B and LDL receptors were localized to theca interna cells, but not granulosa cells. In pre-ovulatory follicles, the LDL receptor was expressed on all granulosa cells. Apolipoprotein-B was also detected in granulosa cells located at the basal layer, suggesting that they utilize LDL through the basal lamina before ovulation. In mid-luteal phase, large luteal cells seemed to stain more intensely for apolipoprotein-B than did small luteal cells, suggesting that large lutal cells are the main sites of LDL utilization. In regressing corpora lutea, the expression of LDL receptor was weak, and apolipoprotein-B was rarely detected. In corpora lutea of early pregnancy, LDL receptor and apolipoprotein-B were localized to both luteal cells. These findings show the precise dynamic changes in LDL uptake by human ovarian cells during their differentiation in vivo.