Nuclear heterogeneity is prevalent in high-quality fractionated human sperm cells typically used for assisted conception

Abstract

Study question: What is the nuclear heterogeneity of high-density purified human spermatozoa typically used for IVF purposes.

Summary answer: The data show that while density gradient separation has improved the overall sperm population, there is still a large degree of nuclear heterogeneity within these cells.

What is known already: Chromomycin A3 (CMA3) is an important DNA binding fluorochrome for the assessment of male-factor fertility. It is typically used to predict IVF outcomes on entire sperm ejaculates with very high receiver operating characteristic. Here we used CMA3 to characterise typical populations of human spermatozoa that would be used for IVF purposes after density gradient separation.

Study design, size, duration: We compared the intensity of CMA3 binding within high-dense sperm populations obtained from men. Binding heterogeneity was confirmed through fluorescence microscopy and FACS analysis independently. We also looked at CMA3 staining directly with head morphology in this sperm population. Finally, we looked at electron micrographs of nuclear heterogeneity (vacuoles, chromatin compaction) of spermatozoa following density gradient sorting of CMA3-stained cells.

Participants/materials, setting, methods: We used sperm donors who had fathered one or more children. Semen was collected after 2 days abstinence and purified over Percoll gradients. Only the high-quality spermatozoa, the same used for assisted conception, were then used. Cells were stained with CMA3 and sorted using FACS. Following this, electron micrographs were used to assess nuclear heterogeneity of CMA3-dependent sorted spermatozoa.

Main results and the role of chance: CMA3 staining occurs within morphologically normal as well as abnormal spermatozoa. High-intensity CMA3-stained sperm possessed large vacuoles that were not seen in the low-CMA3 population. In addition, the high-CMA3 stained cells possess higher amounts of nuclear granulation.

Limitations, reasons for caution: The present study only describes the issues within the chromatin of these cells and does not suggest an alternate selection technique.

Wider implications of the findings: CMA3 is one of the better reported prognostic assays in predicting pregnancy outcomes, especially in cases where the male is at fault. However, it is clear that even in fractionated populations of human spermatozoa, there are sperm cells that are morphologically normal yet possess high levels of CMA3 staining and chromatin granulation. The implication of this is that the embryologist, whom selects on the basis of sperm morphology, may choose a cell with poor chromatin, which may lead to poor embryo outcomes.

Study funding/competing interest(s): The project was funded by the National Health and Medical Research council, APP1118943. The authors have no conflict of interest to declare.

Trial registration number: N/A.

Keywords: chromatin; chromomycin A3; epididymal; infertility; male; morphology; sperm; testis.