Cellular adaptations

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Alright so today I’ll be talking about how organs and tissues respond to environmental changes. These changes occur at the cellular level, so we call them cellular adaptations. As you know, many of our organ systems are regulated to maintain homeostasis, and in this video, I’m going to discuss changes that result from excess stress or prolonged disuse.

In the image we have in the middle here, we can see an organ, in this case, the bicep muscle in the arm. If the bicep muscle is constantly stressed, such as during strength training, the muscle can experience hypertrophy in which it increases in size to the larger bicep on the left. If muscles aren’t used for extended periods of time, which might be the case for the handicapped or bedridden patients, the muscles might undergo atrophy, which is a decrease in size to the muscle on the right. I’ll be discussing the mechanisms that can causes changes like the ones shown here.

Let’s start with mechanisms in which tissues and thus organs are enlarged or become bigger. There are two ways this can happen.

The first is hypertrophy, literally meaning over-nourishment, which is an increase in size of the cells making up a tissue or organ. You can see this left side of the diagram where the same four cells are increased in size during hypertrophy

The second way an organ can become enlarged is through hyperplasia, which literally means over-formation of cells; so the cells are becoming more numerous. On the right side of this diagram you can see the cells multiplying as they go from four to twelve cells. There is an increase in the NUMBER of cells. Again, this is in contrast to the cells increasing in SIZE during hypertrophy on this side.

One last thing to note is that oftentimes hypertrophy and hyperplasia occur together. So when an organ increases in size in the body, we are really seeing an increase in the size and number of cells. An example of hypertrophy and hyperplasia occurring together is in the growth of the uterus during pregnancy.

Now in order to have hypertrophy, the cell volume needs to physically increase. In a normal animal cell, a protein structure known as a cytoskeleton is what holds up the cell membrane to maintain cell shape and rigidity. This is a microscope image of cells where the cytoskeleton components are stained red and green, and you can see how they kind of stretch out that plasma membrane and define the cell size and volume.

So, to increase the cell size, this cytoskeleton needs to become bigger. Because the cytoskeleton is made of proteins, the process of hypertrophy requires increased protein production, which of course means the activation of more genes to synthesize those proteins. A larger cell also requires more organelles spread about the cell to produce ATP, remove wastes, synthesize molecules, and do everything that organelles do. This means that organelle production increases during hypertrophy as well.

An example of pathologic hypertrophy is in the ventricles of the heart, as shown in this picture here. We see a substantial increase in the walls of the right ventricle in this heart right here. This occurs when the heart struggles to pump blood to the body during heart failure, or when a person has chronic hypertension. Hypertrophy in the walls of a hollow organ or chamber is called eccentric hypertrophy, and the word “eccentric” here means “away from the center.” The walls surrounding the chamber -- in this case, the right ventricle -- are enlarged because of the increased stress on the right ventricle. The right ventricle pumps blood to the pulmonary circulation, so a patient with this condition might have pulmonary hypertension or COPD.

Hyperplasia, or the increase in cell number, is a bit simpler to explain. Hyperplasia is essentially increased cell proliferation or mitosis, and the cells are usually dividing and differentiating themselves from a set of stem cells.

Normal (or physiologic) hyperplasia occurs in the body and is often triggered by hormones, cytokines, or other signaling molecules.
One example of normal hyperplasia is the proliferation of the milk ducts in response to increased estrogen during pregnancy. Estrogen and progesterone also trigger the milk-producing alveolar epithelium for breastfeeding.

Pathological hyperplasia includes benign prostate hyperplasia or BPH, more commonly known as prostate enlargement, which cases urinary issues in men.
And of course, cancer metastasis are a form of hyperplasia, and this moving image here is a proliferation of skin cancer cells from mice.
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