3.16 THIS IS HOW WE DO IT: Can cells change their composition to adapt to their environment?

Figuring out how to approach a question in biology can be one of the most important steps in trying to understand how things work. In most cases, the best strategy is to take the simplest approach. Researchers wondered, for example, how an animal’s cells might respond to exposure to a much colder environment. Here’s what they did.

They used 15 cats from five litters. From each litter, one cat served as a control, while the others were exposed to extreme cold—a temperature of –30° C—for two periods of 1 hour per day. The rest of the time, all of the cats were kept at 20° C.

After one week of this treatment, the team collected samples of fat tissue from all of the cats, which they examined microscopically. In the fat tissue, they measured the number and size of mitochondria, the number of tiny blood vessels (capillaries) per cell, and the size of the cells.

The results were clear and dramatic, as shown here (note that a micrometer, μm, also called a micron, is one-millionth of a meter).

The experimental approach was elegantly simple. The researchers used just a single manipulation—exposure to cold—and then observed the consequences. This made it straightforward for drawing conclusions. Cold stress caused a change in fat cells. In the animals exposed to cold, the mitochondria became more numerous, with each tripling in size. The number of blood vessels doubled. And the fat cell size was reduced to less than a quarter of the size found in the cells of the control group animals.

Essentially, it appears that cells in the fat tissue of cold-exposed animals upgraded their heat-generating system. And by making use of stored lipids to fuel the intensive heat production, they depleted a significant portion of their energy reserves.