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Choosing the Best — Selecting the Best Gametes and Embryos for Use in Assisted Reproduction

Do perfect-looking embryos always yield the best result? How do we select which embryos to use? How can we provide the best chances for pregnancy?

by Dr Sam Prien and Dr Lindsay Penrose

On July 25, 1978, the world changed with the announcement that Louise Brown had been born in England. She was the first human “test-tube” baby to be born from in vitro fertilisation (IVF). IVF is now just a part of what globally is referred to as Assisted Reproductive Technologies (ARTs), and while many of the techniques used during IVF have changed and improved over time, one technique that has changed very little is how we choose the sperm, eggs, and embryos we use to help couples have babies.

So, how do we choose? Is there a highly scientific reason why some sperm, eggs, and embryos are selected? Simply put, we use morphology, or appearance, and pick the ones that look the best. This is based on the idea that what looks the best must be the best, but in 35 plus years of assisting couples to have children, our lab has seen many couples with “perfect embryos” go home without children, while couples where we transfer the embryo because “it is all that we have” take home beautiful babies.

Let’s practice deciding which embryo goes back in our patient. These embryos (Figure 1) are both at the blastocyst stage, which is currently the best stage to transfer embryos grown outside the body into our patients. So, which one would you select and put back in the potential mother?

Why doesn’t appearance always work? Cells and embryos can look perfect and yet contain a number of flaws. About 10 years ago, techniques were developed to assess the genetic materials (i.e. DNA) inside the cells of an embryo. Similar techniques were also developed for sperms. It was found that sperm and embryos, normal-looking or not, often contain flaws in their DNA. Flaws at this level often lead to cells that function abnormally, i.e. have abnormal physiological or biochemical activity. Flaws in biological activity lead to less than healthy sperm and egg cells, less functional embryos, and fewer pregnancies.

So, if simple appearance is not always the best measure of which is the best sperm, egg, or embryo, are there alternate means we can use to assess which one to pick? Is there a way to assess reproductive materials that allow for rapid assessment of quality? This has been the focus of our research for the last 20 plus years – focusing first on the male partner, and more recently on the female.


Helping the Male

While some might argue otherwise, the male is the forgotten partner in ART treatment. The attitude of the industry has been that since most men make at least a million sperms, and with the techniques of intracytoplasmic sperm injection where one sperm is injected into each egg, there is always sperm available to achieve fertilisation, or as is commonly believed in the field, “It only takes one”. This assumes that just because a sperm is swimming and looks normal, it is physiologically normal. However, mounting research continues to demonstrate that a large portion of most sperm pools have issues that limit their ability to fertilise the egg cell. Unfortunately, there is currently no way to assess the “health” of each individual cell without destroying it. Therefore, we assess the pool, or population, of cells produced at each ejaculate and hope what we see accurately reflects the sperm we use for procedures.

But some of the issues on the male side begin with the collection process itself. Traditionally, semen samples have been collected in a standard lab specimen cup – the same cup used for collecting urine, defecates, and other clinical samples. These samples can easily fill the 90 to 150-millilitre containers, but human semen samples are small. A normal human semen sample is only 2 to 5 millilitres, meaning it becomes a thin layer spreading across the bottom of the container. This environment allows for damage to the sample from multiple factors, leading to changes in the sperm cells that make them less capable of fertilising an egg. It is well documented that rapid temperature changes lead to sample degradation. The thin pool of fluid has a large surface that can also lead to rapid evaporation of the seminal plasma and dehydration of the sperm cells, again, causing detrimental changes in sperm quality. There is also evidence that the plastics in the container are at least mildly toxic to the sperm cells.

Recognising the potential of each of these conditions to affect sperm quality, our lab has spent a great deal of time over the last 25 years exploring means of improving the collection environment. We started our exploration by trying to determine the reasoning behind using the current collection methods. While one might have expected to find an important scientific reason behind the process, we determined that the method, which dates back at least 100 years, was based solely on tradition. We then set out to answer a simple question, what does sperm need to stay optimally functional? What makes a “happy sperm”?

Working across many species, we designed species-specific collection vessels, including one for humans, that minimised all the bad properties of a standard specimen cup and maximised those that promoted sperm health. The device that we call the Device for Improved Semen Collection (or DISC), led to a patent and is now being marketed by Reproductive Solutions Inc (rsifertility.com).

Human and animal studies have shown that our simple and easy-to-use device maintains better semen parameters, including biochemistry and physiology. The use of the device results in more pregnancies that make it to heartbeat and beyond.

We are now working on both second- and third-generation devices, which we hope will improve the collection environment even more.


Predicting the Best

Back to the pictures at the top of the article (Figure 1), have you figured out which embryo made a baby?

An embryologist is a technologist who helps create and care for the embryo in the first five to six days of its existence. While embryologists in the lab can move the embryo around, when looking at a photograph, you are also looking at the same information they have when they choose which embryos to put back in the patient, which to freeze, and which they think have no chance of survival.

In the last 40 years, one major addition to the process has been the addition of preimplantation genetic testing or PGT. PGT is the biopsy of a few of the embryo’s cells to allow genetic testing to look for missing or added chromosomes and, more recently, genetic mutations in the embryos. The addition of this test has made us question our use of appearance more and more. PGT has demonstrated that normal-appearing embryos can have major genetic flaws, flaws incompatible with life. However, we can not select on PGT alone as it tells us little else other than if the chromosomes are normal or abnormal.

In 2007, we started looking for alternative methods of embryo assessment. Again, like the concept with the DISC, we wanted to develop a method that would be a rapid and simple means of assessing the embryo’s quality and potential for future growth. The work began in cattle, trying to determine why Jersey dairy cattle had poor rates of embryo survival post-freezing when compared to most cattle breeds, where the conception rate from frozen embryos can be as high as 60 per cent. We theorised that the difference observed in Jersey cattle might be the result of the feature that makes these cattle so valuable to the dairy industry – higher fat content. Jersey cattle are prized for their high-fat content milk; it is known that fat content in the milk is a reflection of the body’s composition. We set out to assess if the body’s fat content was reflected in the fat content of the embryos.

This might sound like an easy task and it is when dealing with a 90 kg human or 800 kg cow. It becomes more problematic when dealing with an embryo that is the size of a grain of sand and weighs picograms. After some initial trials with conventional equipment ended in failure, we went back to the drawing board. We knew our device needed two qualities: 1) to allow us to at least estimate the chemical composition of the embryo and 2) the technique had to be non-invasive because we want the embryo to continue to develop after our assessment.

Our idea is based on the basic physics concept that objects of similar size and shape will descend through a liquid media at different rates based on their density. Density is based on chemical composition. Early in development, embryos have a similar shape and size, so any differences in their density, which reflects their weight, is a difference in their chemical composition. Knowing this, we developed a system to measure the buoyance of embryos. This allowed us to estimate differences in the chemical nature of the Jersey embryo.

In our research, we began to recognise that individual embryos in cohorts of embryos from the same female, such as the multiple embryos from an IVF patient, showed different buoyancies. We observed that differences in buoyance are related to the future growth potential of the embryo. In repeated studies, we have demonstrated that the system can identify approximately 80 to 90 per cent of the embryos that will develop from fertilisation to blastocyst stage. It also can predict which blastocysts will continue to develop, and in one small trial in sheep, it accurately predicted the majority of animals that conceived after embryo transfer.

This discovery recently received a U.S. patent1. We hope to develop this system into a simple device that the embryologist can use to assess embryos and improve pregnancy. Our hope is to create a large database of information that will allow for a more precise selection of embryos of the highest quality to provide the best chances for pregnancy.


What Has This 20 Plus-Year Journey Taught Us

Our goal across this journey has been to change processes in ARTs, to make them more effective. Improving ART techniques will result in better chances to build families. We believe that straightforward solutions are usually better as they are more likely to be incorporated into practice. We also have learned it takes lots of people to turn even a simple idea into a working concept, and we thank the many undergraduate and graduate students who assisted with the experiments to prove these simple ideas that have the potential to change the reproductive world.

Oh, and if you are still trying to decide, the embryo on the left made the baby. [APBN]


  1. Prien, S. D., & Penrose, L. L. (2021). U.S. Patent No. 11,169,064. Washington, DC: U.S. Patent and Trademark Office. Retrieved from https://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO% 2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=prien &s2=penrose&OS=prien+AND+penrose&RS=prien+AND+penrose 

About the Authors

Samuel Prien, Ph.D., director of Clinical and Research Laboratories and professor at the Texas Tech University Health Sciences Center Department of Obstetrics, conducts diverse physiology-based research (animal and human); published almost 100 journal articles; and made approximately 400 presentations. He holds five U.S. Patents, 20 international patents, and three copyrights.



Lindsay L. Penrose, Ph.D., associate professor at the Texas Tech University Health Sciences Center Department of Obstetrics and Gynecology, is a translational researcher improving assisted reproductive technologies (ART) for humans and livestock animals. She holds two U.S. Patents on technologies to improve ART, one international patent,13 publications and 95 presentations.