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Human follicular cells were separated according to their isopycnic densities. Three populations were isolated and identified in terms of their secretion of progesterone and oestradiol. Cells in the least dense population secreted approximately 60% as much progesterone and 20% as much oestradiol per cell as did cells in the two denser bands. It is proposed that cumulus cells compose the least dense band and that another band may be made up of antral cells.

The widespread clinical application of in vitro fertilization (IVF) and other advanced techniques of assisted conception has emphasized the relative inefficiency of human reproduction. Currently, only 10%–15% of in vitro-fertilized oocytes implant following transfer into a “synchronous” endometrium. This figure is still well below 20%, even when adverse factors associated with superovulation are negated by transfer of frozen-thawed embryos during natural or hormone replacement cycles. Making some fundamental assumptions regarding the frequency of unprotected intercourse and in vivo fertilization rates, it has been estimated that 25%–30% of naturally created embryos implant [1]. So it would seem that, at present, assisted conception techniques can only match nature when three or more embryos are transferred. A failure to achieve pregnancy in both natural and IVF cycles can be attributed to embryonic factors, endometrial factors or a combination of both.