The nucleolar channel system reliably marks the midluteal endometrium regardless of fertility status: a fresh look at an old organelle.

OBJECTIVE To determine whether nucleolar channel systems (NCSs) in the midluteal endometrium are associated with overall fertility status and/or with unexplained infertility. DESIGN Retrospective and prospective clinical studies. SETTING Repository of stored specimens from prior multicenter study and private infertility center. PATIENT(S) Retrospective study that included 97 women (49 fertile couples, 48 infertile couples) who had been randomized for endometrial biopsy during the midluteal or late luteal phase. The prospective study included 78 women with a variety of infertility diagnoses. INTERVENTION(S) Endometrial biopsies were obtained and assessed for the presence of NCSs by indirect immunofluorescence. MAIN OUTCOME MEASURE(S) The presence of NCS was graded semiquantitatively and dichotomized as normal versus low or absent. RESULT(S) Normal presence of NCS was significantly associated with the midluteal phase compared with the late luteal phase (80% vs. 29%). However, there was no association between presence of NCS and fertility status or between presence of NCS and unexplained infertility. CONCLUSION(S) Midluteal phase endometrium consistently forms NCSs regardless of fertility status, including unexplained infertility. This indicates a possible role for the NCS in initiating the window of endometrial receptivity. However, the consistent presence of NCSs across several different types of infertility challenges the likelihood that inadequate secretory transformation is a cause of infertility.