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In the previous lessons, you have learned:
how healthy cells divide through the process of mitosis and,
how uncontrolled cell growth can lead to cancer when the cell cycle is not regulated.
In this lesson, you will learn about another important cellular process: how cells within our body become different. All the cells in your body have the same genetic instructions called your DNA, but they look different and do different things. If all cells divide into two identical cells, why aren’t we all just a blob of identical cells?
Humans and other sexually producing organisms start out as fertilized eggs. This single cell divides over and over again to produce the billions of cells that make up a human. The process of mitosis explains how organisms grow larger, how organisms heal, and how organisms replace old cells. However, mitosis does not explain how an organism can be made up of so many different types of specialized cells. If mitosis was the only process in play after fertilization, humans would be a mass of cells all identical to one another in form and function. Instead, humans are made of hundreds of different cell types ranging from skin, blood, and muscle to brain and cardiac cells. These specialized cell types are organized into tissues, organs, and organ systems.
To generate such a wide diversity of cell types, something besides mitosis occurs. There must be a process that creates cells that are unique from one another in form and function, like a nerve cell vs a stomach cell.
“Like most other bodily processes, [this process] is controlled by genes, the genetic instructions encoded in the DNA of every cell. Genes instruct each cell how and when to build the proteins that allow it to create the structures, and ultimately perform the functions, specific to its type of cell.”
“Surprisingly, every nucleus of every cell has the same set of genes. A heart cell nucleus contains skin cell genes, as well as the genes that instruct stomach cells how to absorb nutrients. This suggests that in order for cells [...] to become different from one another, certain genes must somehow be activated, while others remain inactive.”
From: PBS Learning Media
In other words, chemical signals inside your body tell your blood stem cells when you need more red blood cells or white blood cells. When the blood stem cell receives a signal to specialize, specific genes inside the cell turn on and off changing how and what the cell develops into.
Have you heard of stem cells? Before a skin cell, muscle cell, or nerve cell becomes specialized it is a stem cell. Stem cells have the full potential to develop into any cell type. Early embryos have a lot of stem cells that can turn into nerve cells, skin cells, and heart cells as the fetus develops. A stem cell that can become any other type of cell is called an embryonic stem cell.
As children and adults, our bodies contain other types of stem cells. These stem cells are considered adult stem cells because they have partly specialized. These cells have already started on a path towards becoming one type of cell. One example is a blood stem cell. Blood stem cells can become any of the different blood cell types like a red blood cell or a neutrophil (type of white blood cell). While the stem cells in an embryo can turn into many different types of cells, blood stem cells in a child or adult can either stay as blood stem cells that multiply to make more blood stem cells, or they can become different types of blood cells. A blood stem cell cannot become a skin cell, intestinal cell, or brain cell.
Human blood is made up of many different types of blood cells including red blood cells that carry oxygen throughout your body and white blood cells that are a part of the immune system and help fight infection. These specialized blood cells travel through the body via your circulatory system which consists of the blood vessels that carry blood away and towards your heart. Hoever, all of the cells in your blood come from the spongy tissue inside your bones called the bone marrow.
The bone marrow works as the blood-making factory for your body. In the bone marrow, blood stem cells divide and specialize into whatever blood cell type your body needs.
The scientific name for a blood stem cell is "Hematopoietic Stem Cell" or "HSC", for short.
The life of a blood stem cell has two parts: the cell cycle and differentiation. During the cell cycle, the cell divides into two daughter cells. After division, the blood stem cells can either stay as blood stem cells in the bone marrow and continuing to divide when needed, or they can specialize or mature into a specific blood cell type and be released into your circulatory system.
Figure 1 Divide and Differentiate. This figure shows how these two processes of division and becoming a specialized cell work together. In the bone marrow, blood stem cells divide by going through the cell cycle. The resulting cells have the potential to either stay a blood stem cell or mature into a specific type of blood cell. The blood stem cells (HSCs) that do not specialize stay in the bone marrow where they can divide into more HSCs.
Once a blood stem cell specializes into a specific blood cell type it loses its ability to become another blood cell type. Much like a child matures into an adult but cannot go back to being a baby, a blood stem cell can mature into a red blood cell but the red blood cell then cannot go back to being a blood stem cell or become a white blood cell. Moreover, once a blood stem cell becomes specialized, it becomes a lot more difficult to divide. Once a blood stem cell becomes a red blood cell, that cell can no longer divide. That is the same for most white blood cells, too.
In order to make more blood, your blood stem cells divide through the process of mitosis, and then those new blood stem cells can change and mature into red or white blood cells.
See Figure 2. Red blood cells cannot divide. They live in the bloodstream where they bring oxygen from the lungs to your tissues. The blood stem cells (HSCs) that do not specialize stay in the bone marrow where they can divide into more HSCs.
If each time the HSC divides into two daughter cells, one HSC stays an HSC and one HSC becomes a red blood cell after each cycle, how many HSCs and RBCs would you have after 5 cell divisions?
Figure 2. Imagine if after each cell division one daughter cell becomes a red blood cell and one daughter cell stays a HSC. After 3 cycles, you would have 3 red blood cells and 1 HSC.
Figure 3: Imagine if after each division both daughter cells stayed HSCs. After the 1st cell division, there would be 2 HSCs. After the 2nd cell division, you would have 4 HSCs.
Figure 4: Imagine if after each cell division both daughter cells become red blood cells. Because RBCs lose the ability to divide, no more cell divisions can occur.
Read the questions and record your answers on a separate document to be turned in later.
Based on information provided from previous lessons, explain why cells need to divide? In other words, what is the purpose of mitosis?
Humans start out as a single cell (a fertilized egg). Based on information from the reading, explain why cell division alone (through mitosis) cannot be the only process involved in human development from a single cell to a fully functioning being.
What controls how a stem cell specializes into a more specific cell type. In other words, how can two cells with identical DNA go on to be completely different cell types?
(a) Define stem cells. (b) How are embryonic stem cells different from adult stem cells?
(a) Define and describe hematopoietic stem cells (HSC’s). (b) What types of cells do HSC’s mature into and where does this process occur?
Using the model of blood development, compare the two symmetric cell division situations (1) and (3). In what types of situations would an organism's HSCs produce only more stem cells (1)? And when would an organism’s HSCs produce only differentiated cells such as red blood cells (3)?