Questions: Define stem cells and their function. Define atrophy, hypertrophy, hyperplasia, and necrosis.

Stem cells are undifferentiated cells that have the unique ability to develop into various specialized cell types in the body. They serve two main functions:

1. Self-renewal: Stem cells can divide and produce more stem cells, maintaining the stem cell pool throughout an organism's life.
2. Differentiation: They can differentiate into specialized cells, such as muscle cells, nerve cells, or blood cells, which is crucial for growth, development, and tissue repair.

Now, let's define the other terms:

1. Atrophy: Atrophy refers to the decrease in size or wasting away of a body part or tissue. It can occur due to a variety of reasons, including lack of use, poor nutrition, or disease. For example, muscle atrophy can happen when a limb is immobilized in a cast for a long period.

2. Hypertrophy: Hypertrophy is the increase in the size of an organ or tissue due to the enlargement of its component cells. This is often seen in muscles as a result of exercise, where muscle fibers increase in size to accommodate increased workload.

3. Hyperplasia: Hyperplasia is the increase in the number of cells in an organ or tissue, leading to its enlargement. Unlike hypertrophy, which involves cell size, hyperplasia involves cell number. It can occur as a normal response to a stimulus, such as the proliferation of cells in the lining of the uterus during the menstrual cycle.

4. Necrosis: Necrosis is the premature death of cells in living tissue, often resulting from factors such as infection, toxins, or trauma. It is characterized by the uncontrolled breakdown of cell structures and can lead to inflammation and damage to surrounding tissues.

In summary, stem cells are crucial for growth and repair due to their ability to self-renew and differentiate. Atrophy, hypertrophy, hyperplasia, and necrosis are terms that describe different cellular and tissue responses to various stimuli or conditions.