IVF including ICSI needs CASA sperm functionality more than ever before!

Fig. 1: Acrosome intact sperm with green acrosomes. Acrosome reacted are only blue.

The question then is what is the role of CASA in fertility assessment and in IVF procedures? SCA Microptic in Barcelona, Spain quickly realized that a big paradigm shift is required and the emphasis needs to be on sperm functionality. A semen sample may have a high sperm concentration and even a good percentage motile sperm. However, most of these sperm may be deficient in crucial sperm functional aspects. Which sperm functionalities should one focus on?

Sperm needs to cross three very important barriers. They need to penetrate the cervical mucus, they need to become capacitated and accordingly the ability to become hyperactived (essential for fertilization) and finally need to undergo the acrosome reaction and penetrate the oocyte. Three of these crucial aspects that relate to fertility has been converted in quantitative sperm functional CASA tests in SCA.

Sperm cervical mucus penetration ability is best measured in the seminal plasma which has about the same viscosity as cervical mucus. A special combination of sperm kinematics is used to express the number of sperm in an ejaculate that will cross sperm cervical mucus and will relate to potential fertilization success.

Capacitation and hyperactivation of sperm have been discussed in a previous blog.  Suffice to state that the SCA system incorporates this as a sperm functional test. In this instance different kinematic parameters are combined to define hyperactivation. It appears that at least 20% of sperm needs to become hyperactivated to relate to fertilization success. In both humans and animals this test is one of the best predictors of live birth outcome.

Fig. 2: Red boxes showing pink – dead sperm. Blue boxes live sperm (x20 objective)

It is furthermore important to assess the percentage sperm with intact acrosomes. However, in addition these sperm also need to undergo the acrosome reaction. SCA has now developed a fully automated sperm functional test which measures both the percentage intact acrosomes as well as the percentage sperm that has undergone the acrosome reaction (Fig. 1). The acrosome reaction is induced with a Ca based medium that simulates what happens in the vicinity of the oocyte.

Which other tests can be used to measure the functional integrity of sperm? The percentage vitality measurement is extremely important in IVF and particularly ICSI if the percentage motility is very low as in the case of asthenozoospermia. Microptic now offers two options to measure this important aspect. Firstly, in fluorescence mode but now also in brigtfield mode using a newly defined nigrosin-eosin stain combination. Unique about this development is that the percentage vitality can be accurately measured at a low magnification with brightfield microscopy (Fig.2) but in addition a further sperm functional aspect, namely hypo-osmotic swelling (HOS), can be evaluated. In Fig. 3 swollen sperm will have thick folded tails (good quality cell membranes) while those that cannot osmotically adapt and burst and have thin straight tails (Fig. 3).

Fig. 3: In this example all white sperm show osmotic swelling of tails –usually folded back in a loop as it contains a swelling droplet at that point. In contrast the pink sperm have thin straight tails – burst. So here HOS = Percent vitality

The rationale of this blog is to emphasize that we require in the “work up” of a patient to IVF procedures a combination of sperm functional tests that relate to fertilization success. In this context IVF including ICSI procedures should greatly benefit from the focus of CASA to sperm functionality and in the future would rather be more dependent on CASA than before. The idea of embryologists/gynaecologists that only one sperm is needed is really redundant! The better we understand sperm functionality of a sample the better we can decide on the strategy to follow to the benefit of the patient. We now have the tools within SCA CASA analysis to evaluate this.

Gerhard van der Horst (PhD, PhD)
Senior Consultant
MICROPTIC S.L.