Has the concept of polyspermy prevention been invented in the laboratory?

There is no evidence, nor need, for a fast block to polyspermy in animal oocytes. The idea that oocytes have evolved a mechanism to allow the entry of one spermatozoon and repel all others has, however, gained consensus over the last century. The main culprit is the sea urchin, which has been used for over a century in studies of the fertilization process.

DNA Methylation Patterns in the Early Human Embryo and the Epigenetic/Imprinting Problems: A Plea for a More Careful Approach to Human Assisted Reproductive Technology (ART).

An increasing number of publications indicate that babies born after IVF (in vitro fertilization) procedures have higher rates of anomalies related to imprinting/epigenetic changes, which may be attributed to suboptimal culture conditions. Appropriate maintenance of DNA methylation during the first few days of an in vitro culture requires a supply of methyl donors, which are lacking in current in vitro culture systems. The absence of protection against oxidative stress in the culture increases the risks for errors in methylation.

Metabolic enhancers supporting 1-carbon cycle affect sperm functionality: an in vitro comparative study.

The sperm plasma membrane is a sensitive target to oxidative stress. The most representative reactive oxygen species (ROS) scavengers in the genital tract, hypotaurine and glutathione, require, for their synthesis, cysteine whose availability is associated with the 1-carbon cycle (1-CC). Human, bovine and ascidian spermatozoa were incubated with compounds supporting the 1-CC (Vitamin B6, Methylcobalamin, 5 Methyl Tetrahydrofolate, Zinc Bisglycinate and N-acetyl-cysteine) (TRT) and compared to the effects induced solely by N-acetyl-cysteine (NAC).

Time to re-evaluate ART protocols in the light of advances in knowledge about methylation and epigenetics: an opinion paper.

DNA methylation is a biochemical process that modifies gene expression without changing the underlying DNA sequence, and this represents the molecular basis for imprinting and epigenetics. Recent reports have revealed alterations in DNA methylation profiles in the placenta of babies born from assisted reproductive technologies (ART). This supports several previous observations that suggested an increase in the prevalence of imprinting diseases following ART treatment, and also fits our observations regarding the metabolism and requirements of early human embryos.

Oxidative stress and alterations in DNA methylation: two sides of the same coin in reproduction.

The negative effect of oxidative stress on the human reproductive process is no longer a matter for debate. Oxidative stress affects female and male gametes and the developmental capacity of embryos. Its effect can continue through late stages of pregnancy.

Assisted yes, but where do we draw the line?

In a recent report in Reproductive Biomedicine Online by Ebner et al., a comprehensive multi-centre study was presented on the use of a calcium ionophore, A23187, to artificially activate oocytes from patients who had poor fertilization rates in previous cycles. Under physiological conditions, the calcium increase in oocytes at activation is caused by influx and release from specific stores and ion channels, and has precise temporal, quantitative and spatial patterns.

Trends, fads and ART!

Morphological selection techniques of gametes and embryos are of current interest to clinical practice in ART. Although intracytoplasmic morphologically selected sperm injection (IMSI), time lapse imaging morphometry (TLIM) or quantification of chromosome numbers (PGS) are potentially useful in research, they have not been shown to be of statistically predictive value and, thus, have only limited clinical usefulness. We make the point that morphological markers alone cannot predict the success of the early embryo, which depends on the correct orchestration of a myriad of physiological and biochemical activation events that progress independently of the maternal or zygotic genome.

Is the idea of a fast block to polyspermy based on artifact?

This purpose of this review is to look at the experimental evidence, both kinetic and electrophysiological, that led to the hypothesis of a fast electrical block to polyspermy in sea urchin eggs. The idea of a fast partial block, forwarded in the 1950's, that would reduce the receptivity of the egg surface by 1/20th following its interaction with the fertilizing spermatozoon, was based on experiments that treated fertilization as a first order chemical reaction. Here, I outline the criticisms of the Rothschild theory and demonstrate that the hypothesis of a fast partial block to polyspermy is unfounded.

Polyspermy prevention: facts and artifacts?

The purpose of this review is to open a debate as to whether or not oocytes actively repel supernumerary sperm or in nature final sperm: oocyte ratios are so low that polyspermy preventing mechanisms are not necessary. Before encountering the oocyte, spermatozoa need to be primed, either by environmental factors as in animals exhibiting external fertilization, or by factors from the female reproductive tract, as in mammals. The spermatozoon must then recognize and interact with the outer layers of the oocyte and progression of the fertilizing spermatozoon through these layers is further controlled and modulated by a precise sequence of signals in situ.