Metaphase II (MII) oocytes obtained at different time points in the same in vitro fertilization cycle

Abstract

Oocyte maturation is required to complete meiosis and to produce a competent oocyte able to sustain embryo development, implantation and pregnancy. In humans, meiosis begins in the developing ovaries at 11–12 weeks of gestation [4]. The oocytes progress to diplotene stage of prophase I and then arrest until ovulation, an event that can occur decades later. Oocytes resume meiotic maturation in response to the midcycle luteinizing hormone (LH) surge. This final oocyte maturation can be induced medically with either human chorionic gonadotropin (hCG) or a gonadotropin-releasing hormone agonist (GnRHa). The former binds the LH receptor, while GnRHa promotes the release of endogenous gonadotropin stores from the hypophysis. The resumption of meiosis (so called nuclear maturation) occurs 14–18 h after the beginning of the LH surge; meiosis I is completed within 35 h and the oocytes reach the metaphase II stage (MII) [16]. Meanwhile, in response to the LH surge, the cumulus cells display almost complete expansion by 20 h; then the mature cumulus-oocyte complex detach from the follicular wall before ovulation [2]. While nuclear maturation and cumulus expansion are closely linked, it is unclear if different LH levels are needed to regulate the two processes and if different follicles have different LH thresholds. In this report, we describe a case of an oocyte donor who received a GnRHa trigger that resulted in low number of retrieved oocytes; all oocytes had minimally expanded cumulus cells but showed evidence of nuclear maturation (MII). Due to low oocyte yield, the patient received an hCG trigger on the evening of this first retrieval. Repeat retrieval 34 h after hCG resulted in recovery of an appropriate number of mature oocytes.