As is well known, a woman’s age is a key factor determining pregnancy outcomes. With increasing age, the rates of miscarriage and congenital fetal deformities rise, while pregnancy and live birth rates decline; the risk of complications for advanced maternal age significantly increases.
In contrast, the role of sperm is often downplayed, and statements like “a 90-year-old man can still make a woman pregnant” abound. In the process of in vitro fertilization (IVF), poorly developing embryos have various underlying causes. Historically, issues with embryos were attributed solely to the eggs, but modern research has shown that sperm quality also directly or indirectly affects embryonic development.
What indicators of sperm quality influence embryonic development?
Currently, the main indicators used to assess sperm quality include sperm concentration, motility, morphology, and sperm DNA fragmentation index (DFI).
1. Conventional indicators of sperm
Most studies believe that sperm concentration, motility, and the total number of motile sperm (TMC) have no significant relationship with embryo quality in ICSI cycles. A very low normal morphology rate can negatively impact embryo development.
In a foreign study involving 666 patients, researchers observed 1456 ICSI cycles and found that the total number of motile sperm did not significantly affect embryo development during the cleavage stage. However, domestic scholars retrospectively analyzed 4756 IVF/ICSI cycles, classifying sperm by normal morphology rates into >15%, 4%-15%, and <4% groups. They found that regardless of IVF or ICSI, the group with <4% had a significantly lower rate of high-quality embryos (P<0.05).
2. Sperm DFI fragmentation rate
Some studies suggest that sperm DFI affects the quality of embryos in IVF/ICSI cycles, while others indicate no significant correlation between early embryo development in IVF/ICSI and sperm DFI. Currently, there is no recognized correlation between embryo quality in IVF/ICSI and sperm DNA damage indicators, although around half of the published studies suggest that sperm DNA damage impacts embryo quality, most of which are retrospective in nature.
3. Sperm head morphology
Vacuoles in sperm heads are one potential indicator. The IMSI system can magnify sperm by 6500 times to select sperm without head vacuoles for ICSI injection, which may improve blastocyst formation rates. However, there is currently insufficient evidence from evidence-based medicine to demonstrate that IMSI can improve live birth rates or decrease miscarriage rates; further research is needed.
4. Genetic and epigenetic factors of sperm affecting embryonic development
(1) Since gene expression in embryos begins during the second cleavage (4-cell to 8-cell stage), early studies believed that sperm nuclei lacked DNA transcription products (RNA); thus, it was thought only egg transcription products mattered, leaving sperm out of consideration (which must be frustrating for the sperm). Research over the past decade has challenged or overturned this traditional view, revealing that sperm nuclei contain transcription products such as mRNA, tRNA, rRNA, and a large number of small RNAs, including miRNA, siRNA, and piRNA, which are carried into the egg upon fertilization.
Domestic researchers found that levels of sperm miRNA-34C significantly correlate with embryo quality after ICSI, with patients having better embryo quality exhibiting higher levels of sperm miRNA-34C than those with poorer quality. This suggests that sperm miRNA-34C may become a future factor in predicting embryonic development. Other researchers have reported that sperm DNA methylation patterns also affect embryo development. These molecular biology studies are progressing rapidly, with more sperm regulatory genes being discovered, ultimately piecing together a complete mechanism to explain the mysteries of embryonic development.
(2) Small segments of balanced translocations at the ends of male chromosomes can quietly cause small deletions and duplications in embryonic chromosomes, leading to infertility, poor embryonic development, and recurrent miscarriages. Routine pre-pregnancy G-banded karyotype analysis often fails to detect these hidden “killers,” which can leave doctors perplexed if there are no embryonic materials as clues, such as infertility, poor embryonic development, or repeated implantation failure, necessitating the use of more advanced detection and analysis methods to uncover the “culprit.”
[Summary 1] Egg quality is a key factor in embryonic development, and sperm equally influences embryo quality. Current research confirms that a very high sperm morphology rate may have some impact on embryonic development, along with sperm miR
