In this explainer, we will learn how to describe the processes of test-tube baby formation (IVF) and renucleation and outline the role of gamete banks in artificial reproduction.
In 2006, Dr. Patricia Rashbrook (born in 1943) gave birth to a healthy baby boy. Dr. Rashbrook and the father of her child were in their 60s when they had this child, and biologically, this seems to not make sense. Menopause, the point in a woman’s life when egg production stops, is typically reached around the age of 51. So, how did this couple manage to conceive a healthy new baby in their 60s?
This baby was born through in vitro fertilization (IVF), which is a form of artificial reproduction that allows egg and sperm cells to be joined in a laboratory environment, in order to help with infertility. The word in vitro means “in glass” and is meant to describe how experiments can be done outside the living organism, as in the glass of a test tube or a plastic petri dish.
Artificial, or assisted, reproduction is the creation of new life by artificial means. By combining the egg and sperm in the lab, the resulting embryo can be transferred into the uterus where implantation and pregnancy occur.
The term test-tube baby is sometimes used when referring to IVF, but it can be misleading and inappropriate for some people. The term was first coined because of the idea that an embryo is formed using laboratory equipment instead of being formed in the woman’s fallopian tube. This is only true for the very early stages of an embryo’s development because the embryo is eventually transferred into the uterus for embryo implantation and pregnancy. But as the term was being used casually in public, it created a false impression that the whole pregnancy takes place outside the uterus. For that reason, many countries lean toward using the term in vitro fertilization to refer to the process instead.
Key Term: Artificial Reproduction
Artificial reproduction is the creation of new life in ways that are not natural.
Key Term: In Vitro Fertilization (IVF)
In vitro fertilization (IVF) is when an egg is fertilized by a sperm in a petri dish in a laboratory, usually to allow parents to reproduce, where they would otherwise be unable to.
IVF is a process with multiple stages.
The first stage involves stimulation with hormones, that can be synthetic, in order to promote egg development in the ovaries. Typically, only one egg develops and undergoes ovulation, and in the early stages of IVF, this one egg is what is extracted. However, the use of hormones has allowed multiple eggs to develop simultaneously in a process called “controlled ovarian hyperstimulation.”
Key Term: Ovulation
Ovulation is the part of the menstrual cycle when an egg is released from one of a woman’s ovaries.
Next, the eggs, or oocytes, can be extracted in a clinical setting in what is sometimes called an “egg retrieval” or “oocyte pick up.” While the eggs are being extracted, the sperm can be prepared from the ejaculated semen.
Then, the eggs and sperm are combined for fertilization. Normally, this stage occurs in the fallopian tubes, but in IVF, this occurs in the IVF lab. Typically, a petri dish is used, which is a plastic (or glass) dish where the eggs and sperm can be deposited. They are suspended in a specialized liquid culture media that is meant to mimic the physiological environment of the fallopian tube. You can see this in the photograph below.
By combining the egg and sperm together, the sperm will bind and penetrate the egg to fertilize it over time.
Once the eggs and sperm are combined, they are placed in incubators that maintain the same pH and temperature found in a fallopian tube environment where they normally grow. After fertilization, the next stage involves monitoring the resulting embryos over the course of about 3–5 days before they can be transferred to the uterus. There are different stages of an embryo’s development, day 3 is called the cleavage stage, day 4 is called the morula stage, and day 5 is called the blastocyst stage.
During the final stage of IVF, the embryo is loaded into a specialized catheter that is inserted into the uterus. The embryo is then transferred into the uterus where it will hopefully implant and lead to a pregnancy. Any excess embryos created during an IVF procedure can be frozen and transferred at another time.
In some cases, women may transfer more than one embryo. This can lead to an increased chance of having fraternal, or nonidentical, twins.
The basic steps of IVF are demonstrated in Figure 2.
Example 1: Describing the Stages of IVF
Where are embryos placed in the final stage of the IVF process?
- Fallopian tube (oviduct)
Infertility can impact many couples around the world. In vitro fertilization (IVF) is a technique that allows the eggs and sperm to be combined outside of the body in order to overcome some of the causes of infertility.
IVF has multiple stages.
First, the female is treated with hormones, which can be synthetic, to develop multiple eggs in the ovaries. These eggs can then be extracted and brought to the IVF lab along with the sperm.
Then, the eggs and sperm are combined in a petri dish and are allowed to fertilize. A petri dish is a plastic or glass dish that contains a specialized culture media fluid to mimic the environment of the fallopian tubes and encourage fertilization. Once the eggs and sperm are combined, they are placed in special incubators to maintain specific pH and temperature levels that are optimal for fertilization and embryo growth.
After fertilization, the resulting embryos grow for around 3–5 days and are transferred back to the uterus. Here, they will hopefully implant in the uterine lining and lead to a pregnancy.
Therefore, the correct answer is A: uterus.
IVF success rates are mostly dependent on maternal age, which may have to do with decreases in egg quality over time. According to the Centers for Disease Control and Prevention in the USA in 2016, women under the age of 35 have the highest IVF pregnancy rates at about . Meaning that out of 100 women (aged less than 35) who undergo IVF, about 30 to 40 of them will get pregnant. The pregnancy rate then begins to decline to less than by the age of 40. Note that these are statistics for infertile women undergoing IVF, while the rest of the population (without diagnosed infertility) may have higher pregnancy rates even with advanced age.
Due to the decline in pregnancy rates with increased age, couples who are planning to delay parenthood are encouraged to freeze their gametes at an earlier age. This is important because IVF can be very expensive, and for some older couples, they may not be able to afford the multiple rounds of IVF needed to get pregnant, so they save their gametes when they are young.
Gametes are also encouraged to be frozen and cryogenically stored for those preparing for chemotherapy, as chemotherapy may reduce the viability or quality of the gametes. These gametes can be stored under liquid nitrogen at very cold temperatures in egg banks or sperm banks (collectively gamete banks) for up to 10 years and more. You can see an example of samples stored in a part of a cryogenic storage tank in the photograph below.
Besides humans, gametes of animals can also be preserved by cryopreservation. This can be helpful for endangered animals that may soon become extinct.
Key Term: Cryopreservation
Cryopreservation is a process that enables the long-term conservation of biological material by cooling at very low temperature (typically when using liquid nitrogen). At this very low temperature, enzymatic and chemical reactions that could damage the biological material are stopped.
Key Term: Gamete Banks
Gamete banks are storage facilities that specialize in storing cryopreserved gametes like eggs or sperm.
Example 2: Describing Gamete Banks
What is stored at very cold temperatures in gamete banks?
- Fertilized eggs
- Sperm only
- Unfertilized eggs and sperm
- Unfertilized eggs only
Artificial reproduction is the creation of new life by artificial means. One prominent example is in vitro fertilization (IVF), where eggs and sperm are removed from the body and combined in a laboratory to form embryos that can then be transferred into the uterus to create a pregnancy. Excess embryos can be frozen in liquid nitrogen for future use.
Gametes, such as unfertilized eggs and sperm, can also be frozen in egg banks or sperm banks (collectively called gamete banks), and this has some advantages. For example, somebody who needs to undergo chemotherapy may want to freeze their gametes since the treatment may damage their gametes.
Let’s look at the different answers and see which one best defines what is stored in gamete banks.
In A, “fertilized eggs” are not stored in gamete banks because fertilized eggs are not gametes. A fertilized egg is a zygote.
In B, “embryos” are not stored in gamete banks because, again, they are not gametes.
In C, “sperm only” is half true because sperm can be stored in gamete banks but so can eggs.
In D, “unfertilized eggs and sperm” seem like the best choice because both of these are gametes and can both be stored in gamete banks.
In E, “unfertilized eggs only” again is only half true because sperm can also be stored in gamete banks.
Therefore, the correct answer is D: unfertilized eggs and sperm.
Besides IVF, another popular technique is artificial insemination, where sperm is transferred to the reproductive tract in the hopes of starting a pregnancy. In one type of artificial insemination, called intrauterine insemination (IUI), sperm is transferred directly to the uterus. These techniques are used in humans and are a very popular method to inseminate farm animals.
Definition: Artificial Insemination
Artificial insemination is the process of transferring sperm directly into the female reproductive tract to achieve pregnancy.
A popular form of beef comes from the Japanese black cattle breed “Wagyu.” These cattle have a high concentration of fat inside their muscle tissue that gives the meat a lower melting point and a very tender quality unmatched by other cattle. Because of the high demand for Wagyu, breeders can use IVF or artificial insemination to make more of these cattle at a lower cost.
Breeders are able to make many Wagyu embryos from a single cow by first stimulating the cow with hormones to produce multiple eggs in the ovaries. Then, the eggs can either be extracted and mixed with sperm for IVF or the sperm can be transferred directly into the uterus by artificial insemination. The resulting Wagyu embryos can be harvested and then transferred into non-Wagyu cows, which are much cheaper than Wagyu cows. This way breeders are able to make hundreds of Wagyu offspring quickly and for less money.
Other advantages of using artificial insemination or IVF in cattle include the ability to improve the pregnancy rates in breeds with low fertility, to have better control over crossbreeding to create new desirable breeds (for example, through crossing cattle with disease resistance and milk production), or to choose the gender of the offspring.
You will recall that female somatic cells typically have two X chromosomes and males have one X and one Y chromosome. The egg cell always contains a single X chromosome, while the sperm may contain either an X or Y chromosome. Therefore, when the haploid sperm and egg combine to form the diploid embryo, it is the sperm that determines the gender. The same is true in cattle (and most mammals).
The X chromosome contains more DNA than the Y chromosome, and because of this difference, it can be separated using centrifugation and electric fields as well as a technique called flow cytometry. Once separated, the sperm can be frozen and used by farmers to manage the gender of their cattle. So, if they need more female cows for milk production for one particular season, they could use the sperm with the X chromosome to inseminate the females by artificial insemination. This will produce female embryos. This process produces what is known as sexed semen, because it can be used to produce offspring of a desired gender.
Example 3: Using Artificial Reproduction for Gender Selection
Sperm samples from farm animals can be separated into X- and Y-chromosome-containing sperm cells and stored in liquid nitrogen until needed. In which situation might this procedure be beneficial to farmers?
- To prioritize meat production in sheep by using sperm cells containing X chromosomes
- To prioritize milk production in sheep by using sperm cells containing Y chromosomes
- To prioritize meat production in cattle by using sperm cells containing X chromosomes
- To prioritize milk production in cattle by using sperm cells containing Y chromosomes
- To prioritize milk production in cattle by using sperm cells containing X chromosomes
Artificial reproduction is the creation of new life by artificial means. Artificial reproduction in farm animals can be done using in vitro fertilization (IVF) or artificial insemination.
In vitro fertilization (IVF) is a form of artificial reproduction where eggs and sperm are removed from the body and combined in a laboratory to form embryos that can then be transferred into the uterus to lead to a pregnancy. Artificial insemination is the transfer of sperm directly into the female reproductive tract that will lead to a pregnancy.
In mammals, female somatic cells have two copies of the X chromosome, while male cells have one X chromosome and one Y chromosome. Therefore, the female gamete (the egg cell) can only contain a single X chromosome, and the male sperm contains either the X chromosome or the Y chromosome. So, when the haploid sperm and egg combine to form the diploid embryo, it is the sperm that determines the biological sex.
The X chromosome is larger than the Y chromosome and can be separated based on its size. Once separated, the appropriate sperm can be used in IVF or artificial insemination to breed animals of a specific biological sex.
Let’s look at these different answers to see which is beneficial for farmers.
In A, the answer is “to prioritize meat production in sheep by using sperm cells containing X chromosomes.” Sheep, when farmed for meat, can be either female or male, so this answer is not a benefit.
In B, the answer is “to prioritize milk production in sheep by using sperm cells containing Y chromosomes.” The Y chromosome in sperm would make male sheep, which do not produce milk (only the females do), so this would not be a benefit.
In C, the answer is “to prioritize meat production in cattle by using sperm cells containing X chromosomes.” Cattle, when farmed for meat, can be either female or male. Male cattle are more frequently used however, so there may be a benefit in choosing sperm with a Y chromosome and not an X chromosome.
In D, the answer is “to prioritize milk production in cattle by using sperm cells containing Y chromosomes.” Female cattle produce milk, and not males, so using a Y chromosome here would not help because only males would be produced.
In E, the answer is “to prioritize milk production in cattle by using sperm cells containing X chromosomes.” This would produce female cattle that would make milk, so this would be a benefit for the farmer.
Therefore, the correct answer is E.
The manipulation of gametes in a laboratory setting has led to other technologies besides IVF. Suppose you wanted to clone, or make a genetic copy of, a particular breed of cattle; could you use the cells in a steak to do this?
Key Term: Clone
A clone is a genetically identical copy of an organism or DNA sequence. DNA cloning is the process of making more copies of a particular segment of DNA.
Yes, you can! All the genetic information to create a particular breed of cattle is contained in the nuclei of the body cells—or somatic cells—of an individual cow or bull. As meat products, like steak, are mostly the muscle tissue of an animal, we can take the nuclei from these muscle cells! To make a clone of this DNA, a technique called somatic cell nuclear transfer (or renucleation) can be done. This involves several steps.
First, a cell is taken from the steak and its nucleus is isolated.
Then, the nucleus from a cow’s egg cell is removed or enucleated.
The steak cell nucleus and enucleated cow egg are then combined by transferring the nucleus into the enucleated egg.
This can then be transferred to a cow where it can implant to form an offspring that is genetically identical to the cow that the steak was produced from. This is the process they used to clone a particular breed of cattle called Delta in 2014 and can be seen in Figure 4.
Key Term: Enucleation
Enucleation is the process of removing a nucleus from a cell.
Key Term: Somatic Cell Nuclear Transfer (Renucleation)
Somatic cell nuclear transfer is a specialized technique used to clone an animal. It involves replacing the egg nucleus with the nucleus of a body (somatic) cell of a particular animal to create a clone of that animal.
This process was also done with frogs. Embryonic cells at varying stages of development were transferred into enucleated frog eggs. These frogs then developed as clones of the original embryos from which they were derived.
This process of cloning using somatic cell nuclear transfer differs from sexual reproduction to produce an offspring. In sexual reproduction, a haploid egg cell () is fertilized with a haploid sperm cell () to produce a genetically unique diploid embryo () that is a combination of the genetics from the egg and sperm. In somatic cell nuclear transfer, the nucleus of a somatic diploid body cell () of the individual or embryo is transferred into an enucleated egg cell to produce an embryo that is clone of the organism from which the nucleus is derived. This process can be used in other organisms besides cattle, including frogs and sheep for example.
Example 4: Understanding the Technique of Somatic Cell Nuclear Transfer to Create Clones
The two sheep in the figure shown were used in a somatic cell nuclear transfer procedure. Eggs were obtained from sheep 1, and skin cells were obtained from sheep 2.
What would the genetic makeup of the offspring produced by this process be?
- The offspring would be genetically identical to sheep 2.
- The offspring would be genetically identical to sheep 1.
- The offspring would be genetically different to sheep 1 and sheep 2.
- The offspring would be completely black in color.
- The offspring would be completely white in color.
Artificial reproduction is the creation of new life by artificial means. One prominent example is in vitro fertilization (IVF), where eggs and sperm are removed from the body and combined in a laboratory to form embryos that can then be transferred into the uterus to create a pregnancy.
Another form of artificial reproduction is called somatic cell nuclear transfer or renucleation. This is a technique that can be used to clone, or make genetic copies, of animals.
There are multiple steps to clone, so let’s explain this in the context of this question.
First, the skin cell nucleus of sheep 2 is isolated.
Then, the nucleus from the egg cell of sheep 1 is removed or enucleated.
Then, the skin cell nucleus of sheep 2 and the enucleated sheep 1 egg are combined by transferring the nucleus into the enucleated egg.
The result is an embryo that can then be transferred to a sheep where it can implant to form an offspring that is genetically identical to sheep 2. You can see an overview of this process below.
Therefore, the correct answer is A: the offspring would be genetically identical to sheep 2.
Let’s recap some of the key points we have covered in this explainer.
- Artificial reproduction is the creation of life by artificial means.
- IVF is the process of combining eggs and sperm in a laboratory setting to create embryos that can be transferred to the uterus in hopes of achieving a pregnancy.
- Gametes such as eggs and sperm can be stored in gamete banks.
- Artificial reproduction can also be used in farm animals such as cattle.
- Somatic cell nuclear transfer is a technique used to clone animals.