Why should we be cautious about generalizing the results of animal and cell studies?

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Erin McKay

As part of our new series, “Science Bits,” we asked Erin McKay, a graduate student in the Neuroscience department at Michigan State University, to discuss scientific evidence.

We first presented the “Hierarchy of Science Evidence” infographic (provided by the European Food Information Council and shown below). We then asked Erin to provide further insights on the limitations of animal and cell research.

Hierarchy of science evidence

CRIS Bits Discussion with Erin McKay

CRIS Bits: Erin, could you begin by explaining what in vivo and in vitro mean?

Erin: Sure.

In vivo research means occurring “in the living body of a plant or animal.”

In vitro research, on the other hand, occurs “outside the body and in an artificial environment.”

So, for example, an ingredient being administered to a living rat and the observation of the behavior and health of the rat afterwards is in vivo research. If cells are collected from the rat and grown in a dish apart from the animal (a primary cell culture) and then the ingredient is added to these cells for a quick assessment of consequences at a small scale, this is in vitro research.

CRIS Bits: The infographic cautions us to “keep in mind some of the limitations of cell and animal research”. What are some of those limitations we should keep in mind?

Erin: Let’s start with animal research. First and foremost, most animal research is conducted on rodents, and there are important differences between rodents and human beings. Sometimes certain substances have a different effect in humans than in rodents.

As one example, the livers of inbred lab rodent strains are far more susceptible to the development of tumors. This is due to a process called peroxisome proliferation – a process thought to be present in rats, but not active in humans. So, just because a substance causes tumors in rats, doesn’t automatically mean it will do the same in a human. In this case, another species, like a rabbit, could be used to determine whether the result is isolated to rats or a more general response.

The route of administration is another important factor. Sometimes, animals are not always exposed to a substance in the same way that humans would be exposed to it. For example, a rat might be fed a particular substance in the study, but humans would normally apply it to their skin. The differences in route of administration can result in a wide variation of the amount of a substance that is actually available to cause an effect (also known as its bioavailability). For example, when a substance is eaten, it is partially metabolized by the liver before it enters the bloodstream. But when an ingredient is applied to the skin, how soon it reaches the bloodstream depends on the thickness of the skin and how easily the substance can cross it. Therefore, when a study reports negative health events, but the route of administration differs between animal research and projected use in humans, we should cautiously interpret the findings. The research is still valuable and warrants further investigation, but by itself is not irrefutable evidence of a risk to humans.

CRIS Bits: What about cell culture research? What are the limitations here?

Erin: Except for rare exceptions, most cell cultures consist of only one cell type, such as neurons from the brain or epithelial cells from the skin. While using cell cultures can answer some simple questions in initial screening studies, the ability to draw conclusions about effects in an animal or human made up of many types of cells could be difficult. That’s what is meant when cell culture is described as lacking the organizational complexity of a whole organism.

There’s also the environment in which the cells are grown, and the fact that it’s not a perfect replication of what the cells would be experiencing in the body. Briefly, the cells are treated with a mixture containing nutrients and other ingredients to encourage them to grow (cell food basically). They are kept in a clean incubator with a typical body temperature and typical oxygen levels found in the air we breathe.

An issue that has been raised recently concerns this oxygen level. Consider that the oxygen you get from the air you breathe has to go through your lungs and blood to reach your cells. This means that cells in a living body actually experience a lower percentage of oxygen than is in the air. It’s possible that this excess oxygen could be damaging the cells in ways separate from any experimental treatment effects. However, this is still the standard used in most labs.

CRIS Bits: What about the new technology called “organs on a chip”? Will this help alleviate any of these limitations?

These Organs on a Chip, under development by Emulate, Inc. out of Harvard University, are clear containers with human cells (right now primarily liver) that experience fluid and air flow that can better mimic the environments inside a creature that breathes and has flowing blood. There is also the ability to link several chips so that the effect of a substance on several cell types as it moves through, for example, the digestive tract, can be assessed. However, the way the cells are exposed to the ingredient does not cover all the ways humans could be exposed to an ingredient, and the current cost of analyzing the results with the necessary machines and software could potentially limit how accessible this technology is to researchers.

CRIS Bits: The infographic tells us that isolated cells in culture act differently than cells in the body. Could you tell us about that?

Erin: That depends on the type of culture you’re using, specifically, whether it’s a primary culture or a continuous cell line. I’ll expand on that a little and what makes each different from cells in a living body.

A primary cell culture is made of cells that have been recently harvested from tissue, such as an organ or skin, and grown outside of the body. However, damage can be done to these cells when they are removed from their original source. Some cells adhere to surfaces in our bodies or physically reach out to other cells to tell them how to respond to shifting conditions. These can be severed as they are removed, which causes them stress before they make it into their new home.

A continuous cell line has undergone a procedure called transformation that allows them to divide rapidly and unchecked for a very long time, much like tumor cells. However, since these start from a single cell, if that cell had something wrong with it, such as a mutation, that defect will be passed on to all the cells in that line. That defect might change how the cell reacts to encountering an ingredient. Since all cells in the culture will share this rare defect they will all show this altered response. This could make it look like a common response by these cells to the ingredient, when actually the response could be very rare in a living creature. Put another way, in a whole organ if one cell has a defect that causes it to react poorly to an ingredient, it probably won’t have a negative effect on the entire organ because the normal ones will be fine. Here there is the potential for what might be an extremely rare defect to become rampant and make something appear far more or far less dangerous than it really is.

CRIS Bits: Thanks very much, Erin. Any final thoughts?

Erin: I think it’s important to acknowledge that cell and animal research does play an important role in investigations, whether in studies of ingredient safety to the development of treatments in the medical field. These limitations should not be taken as a reason to do away with such valuable research. Instead, we should consider evidence from these types of studies as preliminary, or a starting point for further investigation.

Erin McKay is a doctoral candidate in neuroscience at Michigan State University in the department of Translation Science and Molecular Medicine. She received a Bachelor of Arts degree from Kent State University graduating with a double major in Biology and Psychology in 2013. Her present work is focused on identifying novel mechanisms of vascular dementia (dementia that follows a stroke) in the hope of finding a modifiable target for treatment. She ultimately hopes to pursue a career in science education and outreach.

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