Buzz
Cats and mice split apart roughly 95 million years ago. In the wild, it's nearly impossible for a cat and mouse to unite—except when a cat does what cats do best: crushes a mouse with its teeth, which is a fairly common occurrence.
In laboratory settings, however, the rules can be bent. There are times when pushing the cat-mouse boundary makes sense from a scientific perspective, even if it's ethically contentious.
10. Hybrid Embryos
A cat-mouse hybrid might be a deeply conflicted creature. It could end up attacking its own body, driven by the urge to destroy it. Or, it could live in a constant state of fear, terrified by its own scent.
We don't know for sure because cat-mouse hybrids don't survive long enough to be born. Not even close. However, scientists have made some progress in creating such creatures by merging a cat egg with mouse sperm and a mouse egg with cat sperm. After fertilization, many of these hybrids begin dividing into two cells, and a few even reach a more developed stage known as the blastocyst.
In the wild, hybrids would never get this far. The very idea of cat-mouse mating seems impossible, partly due to the size differences. Even if they somehow managed to mate, the egg and sperm wouldn’t be able to merge. The egg is protected by a layer called the zona pellucida, which prevents sperm from unrelated species from fusing with it.
In the lab, scientists can overcome these issues by directly injecting sperm into the egg. This method even works if they remove the sperm tails and use only the sperm heads. With this technique, researchers have tried other animal pairings beyond just cats and mice. For instance, they’ve used hamster sperm with cat eggs and even injected two sperm into a single mouse egg: one from a cat and one from a mouse.
These experiments, however, haven’t progressed much further either.
9. Mice Inherit A Cat’s Cancer
In the wild, cats typically kill mice with their sharp teeth and claws. However, in a 1985 experiment, one cat got to cause harm in a far stranger way. Researchers began by extracting tumors from 30 cats, all of which had been diagnosed with breast cancer. Some of these cats, for whom there was still hope, underwent a straightforward surgery. Others were put down.
Next, the scientists injected a special breed of mice with cells from one of the tumors. These mice were missing two key characteristics. First, they had no fur, earning them the name “nude mice.” Second, they lacked a thymus, an immune organ in the chest. Without the thymus, these mice couldn’t produce T cells.
Without T cells, the nude mice couldn't reject the transplanted cancer. The cat's cells established themselves and began dividing to form new tumors. Given enough time, these tumors would likely have led to the mice's deaths. However, in this case, the scientists killed the mice themselves before that could happen.
Cancer cells, when taken from bodies, can sometimes survive in lab conditions for extended periods. This was certainly the case for this cat's cancer, which resurfaced in another study published 28 years later.
According to the 2013 study, scientists injected the cat’s cells into two different environments. The first was SCID mice, a new breed also lacking T cells. The second was chicken eggs, specifically targeting the membranes outside the embryos. In both instances, the cells grew into tumors.
What happened to the original cat is unclear. It likely succumbed to its cancer in the 1980s. However, there’s a slim possibility it survived and lived for several more decades. By the time the 2013 study was published, though, the cat was almost certainly long gone.
But that's the fascinating part about cell culture. Even after the cat's days of hunting were over, a part of it lived on. Incredibly, that part continued to act like a cat, causing chaos for mice and baby birds.
8. Assisting Mouse Cells in Catching a Cat Virus
HIV infects humans, while FIV, a related virus, infects cats. In both species, infected cells can merge with healthy cells, forming multicellular clusters known as syncytia. However, this fusion is dependent on the presence of specific proteins.
According to a 1998 study, one key protein is CXCR4. Researchers managed to get FIV-infected cat cells to merge with cells from a mouse, hamster, and mink by first introducing CXCR4 into these cells.
The scientists experimented with different types of CXCR4, including versions from both humans and cats. Surprisingly, the human version of CXCR4 proved to be the most effective. Through these experiments, they created several multispecies hybrids. In one case, mouse cells carrying a human gene fused with cat cells to form a large, FIV-infected mass.
7. Cat DNA Illuminates a Mouse Heart
In a 2002 study, researchers combined DNA from two very different animals, a firefly and a cat. The firefly’s DNA carried the luciferase gene, which allowed it to glow at night. The cat’s DNA, originally linked to a gene called NCX1, contained instructions for producing NCX1 in the heart.
To test the DNA, scientists injected it into the eggs of lab mice. In some cases, the DNA successfully integrated into the mouse genome and became a permanent part of it. Five of the modified eggs developed into mice, and two of those mice had offspring that also carried the firefly-cat DNA.
The cat DNA began giving instructions in the hearts of these mice. Normally, this DNA would tell cells to produce NCX1, but since that gene was replaced, the cat DNA directed the cells to produce luciferase instead.
When the heart cells followed the instructions, the hearts of these mice became filled with luciferase, just like in fireflies. To make the heart cells actually glow, scientists removed them from the animals, broke them open, and added luciferin, a special molecule. After that, they lit up.
6. A Cat Gene Stops a Mouse Virus
Just as mouse viruses thrive inside mouse cells, cat viruses multiply in cat cells. In a 1976 study, researchers combined infected cells from both species. In these new cat-mouse hybrid cells, both viruses were much less active. It appeared that something from each animal’s cells—likely a gene—was preventing the other’s virus from spreading.
However, both cat and mouse genomes are large, making it difficult to identify the specific gene or narrow down the source of the blockage. Fortunately, the hybrid cells were unstable. While most of them retained their mouse chromosomes, they often lost the cat chromosomes, which helped to focus the search.
Through a series of chemical manipulations, scientists were able to select hybrids that had lost the cat’s X chromosome. In these hybrids, the mouse virus became active again. From there, they were able to demonstrate that the cat gene responsible for blocking the mouse virus was located somewhere on the X chromosome.
5. Snow Leopard Cells Transformed Into Mouse Cancer
In an early-stage embryo, cells are considered ‘pluripotent,’ meaning they have the potential to become any cell type in the body. As the embryo develops, this ability diminishes. Liver cells specialize in the liver, and brain cells specialize in the brain.
Reversing this process to restore pluripotency in an adult cell is difficult, but it can be achieved by introducing new copies of several genes. After this procedure, the cell becomes an ‘induced pluripotent stem cell’ (iPSC). These cells hold great potential for various uses, some in medicine and others in conservation efforts for endangered species.
Apart from the domestic cat, many feline species are facing challenges. If we could transform easily accessible cells, like skin cells, into iPSCs, we might also be able to convert them into rarer cells, like eggs. With these eggs, we could potentially create embryos to be implanted into surrogate mothers.
As a first step, scientists began by working with ear cells from a snow leopard. To turn these cells into iPSCs, they introduced a special virus containing five human genes, which played vital roles in encouraging the cells to behave like embryonic cells.
When the virus infected the snow leopard cells, it successfully transferred the human genes. To verify that the transfer worked, the scientists injected the cells into a live mouse, where the snow leopard iPSCs formed a unique tumor known as a teratoma. This tumor contained three types of tissue found in early embryos: ectoderm, mesoderm, and endoderm.
This was textbook iPSC behavior, confirming that the scientists’ genetic transfer had been successful. The scientists maintained this cat-mouse hybrid for 10 weeks before euthanizing the mouse and extracting the snow leopard tumor.
4. Cat Cells Merged With Mouse Cancer
To combat infections, many animals produce various types of specialized proteins known as antibodies. These antibodies attach to invaders and signal them for destruction. Each antibody type is made by a distinct kind of B cell, and each binds to the invader slightly differently. This process is called polyclonal.
However, in the lab, scientists often prefer working with monoclonal antibodies, which come in only one form. To create these monoclonal antibodies, scientists fuse B cells with cells from a blood cancer called myeloma. This fusion results in new cells known as hybridomas.
Each hybridoma produces a single type of antibody (like its B cell parent) and replicates many times (like its myeloma parent). The outcome is an endless supply of monoclonal antibodies, ready for use in the lab.
When a specific species lacks a myeloma line, scientists borrow one from another species, like the laboratory mouse. Fusions like cow-mouse, mink-mouse, and goat-mouse have all been successfully used to produce antibodies.
In a 1993 study, the researchers attempted to create cat antibodies by extracting cells from the spleens of seven cats and fusing them with two distinct mouse myelomas. These fusions resulted in many cat-mouse hybrid cells. Each cell contained a mix of chromosomes, and many were larger than their parent cells. However, none of the hybrid cells produced any antibodies.
To view this failure in a positive light, the scientists suggested that it might be possible to fuse these hybrids with another cat cell, creating a mouse-cat-cat cell. These new cells might produce antibodies, similar to how mouse-human-human hybridomas were made in previous experiments.
3. A Mouse With A Cat’s Immune System
The immune system is made up of various cells and organs scattered throughout the body, making transplantation a complex process that involves multiple steps.
In a 1994 study, scientists transplanted a cat’s immune system into mice. They started by dissecting one-day-old kittens from a mother who was free of infections. From each kitten, they carefully extracted various body parts that are known to play vital roles in the immune system.
The researchers then carried out multiple transplant surgeries, using a special strain of mice as the recipients. These mice had already had their immune systems disabled, making them suitable for the experiment.
The kittens’ thymus sections were transplanted into the right half of each mouse, while the lymph node sections were placed in the left half. In addition, spleen and bone marrow cells from the kittens were injected into the mice.
Once the surgeries and injections were completed, the mice exhibited several cat-like traits. Their blood contained cat DNA, they started producing a cat-specific immune protein called IgG, and, as noted in a 1995 study, they were successfully infected with FIV, the feline virus.
2. Lion Ovaries Transferred To A Mouse
Ovaries can be transplanted into laboratory mice from various species, such as cows, elephants, wallabies, and domestic cats.
In these cat-to-mouse ovary transfers, a variety of mice have been utilized, including females who have had their ovaries removed and males that have been castrated. The renal capsule, located just beneath the mouse's kidney, is a common site for the transplant.
According to a 2014 study, scientists collected ovaries from zoo lions, sliced them using a punch tool, and implanted the pieces under the skin of laboratory mice. These lion ovary fragments were maintained inside the mice for four weeks. During this period, some development occurred, but not enough to produce mature lion eggs.
1. Lynx Testicles Transferred To Mice
When conducting testicle transplants between species, mice are commonly used as recipients. Testicles from sheep, dogs, and buffalo, among others, have been transplanted under the skin of mice. In some instances, functional sperm can be harvested from these transplants to produce baby animals.
In a 2004 study, scientists extracted testicles from young domestic kittens, cut them into small pieces, and grafted them under mice's skin. Some of these grafts survived for over a year and even produced mature sperm.
In a 2014 study, scientists expanded this technique to the endangered Iberian lynx, harvesting testicles from six young animals, ranging from a six-week-old fetus to a two-year-old subadult.
The testicle fragments from a six-month-old lynx cub proved to be the most effective. After being transplanted into mice, some of these pieces began the process of sperm production but were unable to complete it within the experiment’s duration. They only produced early-stage male germ cells, called spermatogonia, and did not generate mature lynx sperm.
