by: Gkalia Tsangkalova – Embryology and Sperm Laboratory Director

Sperm maturation is a complex process which is accomplished at several stages and concerns structural and biochemical alterations inside the male gamete. Several researches all over the world manifest a strong correlation between sperm maturation and infertility issues. There is a range of techniques that can be used to assess sperm maturation. The presence of hyaluronic acid (HA) receptors on spematozoa’s surface and the DNA structure of the nucleus, are two different parameters that can be used as indicators for sperm maturity.

Hyaluronic acid receptors and HBA (Hyluronan Binding Assay)

The presence of HA (Hyaluronic Acid / Hyaluronan) receptors on sperm membrane allows the initial binding on HA, the main component of cumulus-oocyte-complex (COC). COC is the structure that consists of the oocyte, the surrounding “cumulus cells” and their product, HA. Both at natural conception and IVF, once the spermatozoa bind on the HA, special proteins with enzyme activity start to deconstruct the COC complex with final purpose one spermatozoon to reach the oocyte and fertilize it. HBA test determines the proportion of spermatozoa that are able to bind HA due to the presence of adequate hualuronan acid receptors on their surface, therefore HBA score is an indicator of a successful, non invasive fertilization (natural conception/IVF). PICSI (Physiological intracytoplasmic sperm injection) allows the selection of a spermatozoon that binds HA and is considered as the appropriate fertilization method when a semen sample appears to have decreased HBA score. Sperm with high percentage of spermatozoa that bind HA, seem to have low chances of transmitting aneuploidies in offsprings. A positive correlation has been found between HA binding capacity, sperm count, motility, head morphology, DNA integrity and correct nuclear DNA structure.

Nuclear DNA structure and AB (Aniline Blue) STAINING

At the stage of spermiogenesis, sperm DNA is subjected to major restructuring to facilitate a tighter, less vulnerable packaging. This is achieved by an 85% protein replacement of the loose “histone” network by the “protamines”. These protamines pack nuclear DNA into a highly condensed structure so the sperm nucleus demonstrates an important mechanical and chemical stability. When packaged in this manner the paternal genome appears to be protected from several factors as enzymes and mutagens that may damage sperm DNA. In addition, highly condensed nuclear DNA helps the sperm head to obtain its hydrodynamic shape, which allows the spermatozoon to swim fast and progressively. AB staining allows the identification of spermatozoa in which the protein substitution has occurred. Compared to fertile sperm donors, normozoospermic samples of couples undergoing IVF cycle, showed declined chromatin (DNA) maturation whereas ICSI patients, appeared to have even lower rates of mature spermatozoa. Diminished chromatin maturity is connected with head and acrosomal abnormality and lower levels of progressive motility. Moreover, teratozoospermic samples when stained with AB appeared inadequate replacement of histones and therefore diminished maturation. All these findings indicate that semen analysis by itself doesn’t ensure sperm quality and fertilization ability. Lastly, it has been reported that low nuclear maturation status affected in a negative way the zygote development following ICSI and was related to low cleavage rates of zygotes, when Day 2 embryos were observed. Therefore implantation success was low in these cases as well.

It is well established that sperm maturation is an indicator for fertilization capacity and embryonic development. A really important finding is that unexplained recurrent pregnancy losses seem to be significant higher for couples with semen samples presenting really low HA binding ability and low nuclear chromatin maturation. Studies suggest that chromatin condensation constitutes a valuable parameter in the assessment of male fertility, completely independent of conventional semen parameters, and should be recommended to routine laboratory investigations of semen prior to assisted reproduction. Therefore, sperm maturation can be assessed not only to help embryologists choose the right fertilization technique but it can be considered as a reliable predictor of a couple’s ability to conceive as well.