What Limits Cell Division?

Instructor: Amanda Robb
In this lesson we'll go over what cell division is in different types of cells and look at the factors that stop cells from dividing. We'll also learn what happens when cell division doesn't stop.

What Is Cell Division?

All life starts with a single cell. That cell multiplies as we grow and develop, leading to multicellular organisms with trillions of cells. How does this happen?

The answer is cell division, when one cell divides into two. But why do we stop growing? What tells our cells to stop dividing and remain in equilibrium?

Human cells have complex signals that limit cell division, but even singled-celled organisms, like bacteria, also have factors that prevent them from overpopulating. Let's talk about that bacterial cell division in more detail.

Cell Division Limits in Bacteria

Bacteria are single celled organisms that do not have a nucleus, or a container holding their DNA. They are the simplest organisms and have the quickest process of cell division, called binary fission.

Binary fission in bacteria
binary fission

Since bacteria can divide rapidly, they grow exponentially, doubling over and over until they reach the carrying capacity of their environment, or the maximum amount of organisms an environment can support. Once they reach carrying capacity, they can no longer divide.

Some things that control the carrying capacity include food, space, and waste. Since cell division takes energy, bacteria can only divide if there is enough food to support new growth. As the bacteria metabolize the food for energy, they create waste products, such as carbon dioxide. Eventually, in a contained environment, the waste products build up and can kill off the bacteria, controlling the population. Space is also an issue. Only so many bacteria can fit in one place, capping cell division.

Exponential cell growth in bacteria
bacterial growth

Cell Division Limits in Humans

Cell division in multicellular organisms, like humans, is a little more complicated. Humans have a multi-step process for cell division, called mitosis, and it occurs slower than bacteria and in a much more controlled way.

Internal Limits

Instead of cells dividing as fast as they can, our cells have an internal limit of divisions and then stop. All cells in the body have this internal control and most cells only divide between 50-70 times before they stop or die.

However, adult stem cells, which are special cells that can make many other types of cells, can divide much longer, and embryonic stem cells can divide nearly indefinitely.

External Limits

But extracellular factors determine division too. Cells only divide when they receive specific signals from proteins called mitogens, thus the presence of mitogens can limit cell division.

Mitogens are needed for cell division in healthy cells and are released depending on what cells the body needs. For example, during injury, like a cut, cells receive signals that tell them more cells are needed. The cells divide and move in to fill the wound, only dividing until the wound is healed.

Mitogens are also released differently depending on what part of the body the cells are in. Skin cells and blood cells divide quickly to replace dead cells. Other cells, like neurons, rarely divide at all. Most neurons don't do mitosis at all and won't usually respond to mitogens.

Mitogens signal cell division in wound healing
mitogens

Spacial Limits

Cells also depend on space. Many types of cells in the human body are anchorage dependent, meaning they need to attach to something to grow. In the body, they attach to other cells or proteins outside of the cells.

For some cells, like fibroblasts, which make up tissue under our skin, the growth factors can only stimulate cell division if the cell is attached to a surface. It's almost as if the cells can't hear the mitogen signal unless they are attached to a surface. Once they are attached, the signal comes in loud and clear to divide. However, once the cells have reached a single layer on the substrate, they will stop dividing.

Anchorage dependence in body cells
anchorage dependence

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