11. Cell Differentiation

Differentiation is the process where cells become specialized.
It involves both making more of a specific cell type and increasing the total number of cells.
This process occurs as cells go through different stages of division.

Stem cells can divide without becoming specialized.
Other cells can function and divide (e.g., bone cells).
Some cells can function but not divide (e.g., neurons).
Stem cells divide and then differentiate into various cell types needed for tissues.

Every cell with a nucleus has the same DNA.
Gene expression determines which proteins a cell makes at any given time.
Stem cells require a specific set of proteins to be used while others are not.
Differentiation involves turning certain genes on (using them) and others off (not using them).
It's a stepwise process.

Stem cells are general and can become a number of different cells.
Specialized cells perform unique functions (e.g., making albumin, generating electrical signals).
Highly differentiated cells are specialized cells, like neurons.
Stem cells are undifferentiated or poorly differentiated.

A stem cell undergoes mitosis to make identical daughter cells.
Each cell starts to express specific genes and turn off genes that allowed it to be a stem cell.
Expressing neuron-specific genes leads to becoming a neuron.
The process continues with different genes being turned on and off to get the specific proteins needed for the cell's function, but there is more than one type of neuron and epithelial cell.

Differentiation is generally a stepwise irreversible process.
Once a cell commits to a pathway (e.g., becoming a neuron), it cannot revert to being a stem cell and then become a different cell type.
This one-way process is crucial to prevent making the wrong structures, such as skeletal muscle cells in the liver.

In a laboratory setting, differentiation can be manipulated.
By providing specific signals, cells can be induced to differentiate in a particular way or even revert to a less differentiated state. This is done by applying or removing specific signals.

Cancer cells can become less differentiated as the disease progresses.
Less differentiated cancer cells are more damaging.
Well-differentiated tissue cancers are less severe than poorly differentiated ones.

Embryonic stem cells (at about eight cells) are totipotent: they can become any cell type.
As development progresses to the blastocyst stage (around 32 cells), stem cells become pluripotent: they can become most cell types, but not all.
Adult stem cells are multipotent: they can become multiple, but limited, cell types.
- Examples include stem cells in bone marrow and skin.
Research focuses on reversing this process to allow for different differentiation pathways.

The normal pathway is one-way: totipotent → pluripotent → multipotent → differentiated tissues.
Cancer and research aim to reverse this.

The goal is to take easily accessible stem cells (e.g., skin stem cells) and induce them to become other cell types (e.g., neurons) for tissue regeneration.
This is possible because all the necessary information is present; it just needs to be controlled.