This is a common question for those considering fertility treatments like intrauterine insemination (IUI) or in vitro fertilization (IVF) that use frozen donor sperm or for men freezing their own sperm prior to medical treatments that could affect fertility. Freezing sperm through a process called cryopreservation allows sperm to be stored for future use, but does the freezing process kill the sperm cells? Here’s what to know about whether frozen sperm are alive.
How does sperm freezing work?
Sperm freezing aims to preserve the integrity and function of the sperm cells. Here’s a quick overview of the main steps:
- A semen sample is collected and processed to separate out the healthiest sperm.
- The sperm are mixed with a protective freezing medium containing ingredients like glycerol that help limit damage from freezing.
- The sperm mixture is put into small vials or straws and slowly cooled to very low temperatures using computer-controlled freezers.
- The vials are transferred into liquid nitrogen at -196°C for long-term storage.
This freezing process aims to suspend all metabolic processes and biological activity of the sperm so they can be preserved viability and function when thawed for future use.
Are the sperm cells still alive when frozen?
Even though the freezing process suspends all biological processes, the sperm cells remain living despite being frozen at extremely low temperatures. The cells are still intact but are essentially in a state of suspended animation.
Research shows that the freezing process does not kill or rupture the sperm cells, provided the freeze-thaw process is properly controlled. One study examining DNA integrity found no difference between fresh and frozen-thawed sperm.1 Another study found similar ATP energy production between fresh and frozen-thawed sperm.2
So while frozen sperm are not actively moving or metabolizing like fresh sperm, the cells remain alive and are simply dormant while in the frozen state.
Why does freezing preserve sperm viability?
There are a few key reasons why freezing preserves sperm cell viability:
- The freezing medium contains cryoprotectants that protect the cells from damage.
- The controlled, gradual freezing process minimizes formation of intracellular ice crystals that could rupture cells.
- Storage in liquid nitrogen essentially stops all cellular metabolism and activities.
However, the freeze-thaw process does still have some detrimental effects. Some sperm do not survive the process and lose their ability to swim properly or fertilize an egg after thawing. There is typically some decrease in sperm quality and numbers after freezing and thawing compared to a fresh sample.3
What happens when frozen sperm are thawed?
When frozen sperm cells are thawed, the freezing medium dilutes away, metabolic processes restart, and the cells “come back to life.” However, the thawed sperm only have a limited lifespan and need to be used for conception relatively quickly after thawing.
Sperm thawing protocols are designed to gradually warm the cells in a controlled way to help preserve viability. After thawing, sperm motility and concentration are evaluated to determine how they should be used. Sperm with good post-thaw motility can be used for IUI or IVF, while poorer quality thawed sperm may only be usable for IVF with a technique called intracytoplasmic sperm injection (ICSI).
How is sperm quality affected by freezing?
While sperm remain alive in frozen storage, some aspects of sperm quality are impacted by the freeze-thaw process. Some studies comparing fresh vs frozen-thawed sperm have found:
- Loss of up to 50% of total motile sperm after thawing compared to the original fresh sample.4
- Decline in sperm motility by 10-20% after freezing and thawing.5
- Increased DNA fragmentation in 30-40% of thawed sperm.6
The overall impact depends on the initial semen sample quality and the freezing protocol used. Poorer quality samples are more susceptible to damage from freezing. Newer freezing techniques like vitrification may better preserve post-thaw motility and viability than traditional slow freezing methods.7
How long can frozen sperm remain viable?
Frozen sperm stored in liquid nitrogen at -196°C can remain viable for decades. For example, there are documented cases of sperm remaining viable for up to 28 years in frozen storage.8 However, sperm viability and quality does tend to slowly decline over very prolonged storage times.
One study found sperm motility decreased by about 5% per year of cryostorage, and DNA integrity also worsened slightly over time.9 But overall sperm from most men remain viable for conception for at least 10-15 years when properly frozen and stored long-term in liquid nitrogen tanks.
Does freezing sperm affect success rates?
Pregnancy rates are generally a little lower when using frozen-thawed donor sperm compared to fresh sperm. However, success rates are still good overall:
- IUI success rates per cycle may be reduced by around 5-10% with frozen sperm compared to fresh samples.10
- IVF success rates tend to be around 5-15% lower with frozen donor sperm.11
The reductions in success are due to some sperm dying during freeze-thaw and decreased motility and function. But for the majority of men, there is enough sperm survival after thawing to still allow normal conception rates.
Freezing sperm from men with poor sperm quality
For men with very poor sperm quality like low counts or motility, freezing small numbers of sperm for future IVF use is still possible. However, conception rates are lower than average when using poor quality frozen sperm. Special IVF techniques like ICSI may be required after thawing to achieve fertilization and pregnancy.
Does freezing damage sperm DNA?
There are some conflicting research findings about the effects of freezing on sperm DNA integrity:
- Some studies show increased DNA damage after thawing compared to fresh samples.12
- Other research found no significant difference in sperm DNA fragmentation between fresh and frozen-thawed sperm.13
It’s possible freeze-thaw may negatively affect sperm DNA in some men more than others. Overall, any DNA damage after cryopreservation does not seem to be severe enough to prevent normal conception rates in most cases.
Conclusion
In conclusion, while frozen sperm cells enter a dormant state, they remain living cells and are able to “regain life” after proper thawing procedures. Cryopreservation maintains sperm viability although some aspects of sperm quality like motility decline after freezing and thawing. Pregnancy rates are slightly lower but generally still quite acceptable using frozen sperm compared to fresh samples for most men.