Hematopoietic stem cells (HSC) are one of the most important sources of blood stem cells in humans. They have been found to be present in all tissues of the body including bone marrow, spleen, thymus gland, liver and heart.[1] HSCs are responsible for repairing damaged or destroyed tissue. When they divide, they produce new blood cells called platelets which are needed for clotting purposes.

The number of HSCs in the human body varies from person to person and depends on several factors such as age, gender, genetic makeup, health status and many others. For example, there are usually between 10 000 – 30 000 HSCs per million in adults,[2] whereas the average adult has about 200 000–400 000.[3][4] The total amount of HSCs in the body is approximately 1 billion.

HSCs are divided into two types: hematopoietic stem cells and myeloid progenitor cells. Myeloid progenitor cells are the ones that give rise to all other blood cell types. These include red blood cells, white blood cells, platelets, granulocytes and monocytes.

Although HSCs are one of the most important types of stem cells, there is still a fair amount of mystery surrounding their exact nature. Described below are some known facts about these important cells.

What are hematopoietic Stem Cells?

Hematopoietic stem cells are found in the bone marrow. They give rise to all blood cell types. This process is known as hematopoiesis. Each HSC can only turn into one type of blood cell, for example a single HSC can turn into both a red blood cell and a white blood cell but it cannot differentiate between the two. This means that the body needs a pool of at least 200 000 HSCs in order to produce 200 000 hematopoietic cells per minute.

What is the Nature of HSCs?

Hematopoietic stem cells are a type of pluripotent stem cell, which means they have the potential to differentiate into any type of cell. Each HSC has the capacity to differentiate into at least one specific type of blood cell and possibly more. For example, a single HSC can turn into either a red blood cell or a white blood cell but it cannot differentiate between the two. The ways that HSCs differentiate into blood cells have been extensively researched.

Hematopoiesis is the process in which HSCs turn into mature blood cells and is broken down into several stages. These stages are:

Hematopoietic Stem Cells – HSCs are immature cells which are found in the bone marrow. These cells have the potential to differentiate into blood cells.

Commitment – Once a HSC differentiates into a specific type of blood cell, it is now known as a progenitor cell. For example, a HSC can differentiate into a myeloid progenitor cell or another type of cell. The blood cell type that a progenitor cell can produce is now set and cannot be changed.

Progenitor Cell or Colony Forming Unit – Some blood cells, such as the red blood cell or white blood cell, are made up of many types of cells. For example, an early forming red blood cell might produce a later forming cell. At this point, both the earlier and later forming cells are known as colony forming units (CFU). The name is justified by the fact that all of the cells in a colony are of the same type.

Mature Blood Cell – When a cell undergoes the final stages of hematopoiesis, it is known as a mature blood cell. Each mature blood cell has a specific function. For example, red blood cells deliver oxygen to the body, whereas white blood cells fight off infection.

These stages are ongoing and never truly end. For example, red blood cells have a lifespan of approximately 120 days but this does not mean the cells die after this time. Rather, they become worn out and are removed from the body. Other cells, such as platelets have a lifespan of only a few hours.

What is the Lifespan of a HSC?

There is a lot of mystery surrounding the lifespan of HSCs in particular. Some believe that these cells can only turn into blood cells and have a lifespan of 40 to 60 years. Others believe that they are essentially immortal as long as the person is still alive. The latter is the more accepted theory as there is no solid evidence either way.

What is Clonality?

The concept of clonality is important in relation to HSCs. This theory suggests that every single person on earth is genetically identical to each other. This means that every single person has the exact same set of genes and they are all passed down in the exact same order.

The concept was proposed by Pierre-Paul Grassé in 1955. In his theory, he stated that a population or species consists of one large clone. For example, all human beings are part of a large clone and therefore have the exact same genes. Grassé believed that this was possible due to asexual reproduction.

This is where one parent contributes half of the genes and the other parent contributes the other half.

Grasse was heavily criticized for this theory and it was generally disregarded for many years. However, some recent studies have suggested that clonality may be possible in very rare circumstances. For example, some sea urchins can regenerate their entire genome from just a single cell.

What Symptoms Cause HSCs to Differentiate into Different Cells?

Hematopoietic stem cells are affected by a large number of factors that cause them to differentiate into different blood cells. The main ones are:

Stem Cell Niche – As stated earlier, HSCs need to be anchored into place in order for them to differentiate into specific types of blood cells. This “anchoring” is known as the stem cell niche. It is believed that there are different niches for different types of blood cell. For example, the niche for a myeloid stem cell may be in the bone marrow, whereas the niche for a lymphoid stem cell may be in the lymph nodes.

Hormones – Hormones also play an important role. They can either encourage or discourage differentiation into specific blood cells. An example of this is thyroid hormone, which encourages differentiation into lymphoid cells.

Signals from Other Cells – Signals from other cells can also cause HSCs to differentiate into specific blood cells. For example, if a cell in the skin is damaged and requires new skin cells, a signal will be sent out to encourage a nearby HSC to turn into a new skin cell.

What is Cancer?

Cancer is by far one of the most common killers in the Western world. It’s believed that around 8% of people will have some form of cancer in their lifetime, with more being diagnosed every year.

Cancer occurs when cells begin to divide and grow at a rate that is out of control. They also cease to follow the normal rules of differentiation. This means that instead of becoming a specific type of cell such as a skin cell, they remain “pre-programmed” to become any type of cell. For example, a cancerous cell could potentially turn into bone, blood, or skin without anything damaging those cells.

Cancerous cells also invade nearby organs and tissues and can also travel through the body via the blood and lymphatic systems. This is why it’s vital to catch cancer early on. If it’s caught early enough, it’s possible to destroy the cancerous cells with therapy (chemotherapy, radiation therapy, targeted therapy, etc.

There are many types of cancer. The most common types are:

Carcinoma – This is a cancer that forms in the skin or internal organs. Carcinomas can form in any type of cell.

Melanoma – This is a cancer that forms in the skin cells that produce pigment (melanin).

Leukemia – Also known as blood cancer, this is a type of cancer that affects the blood and immune system.

Lymphoma – This is a cancer that forms in the cells of the lymph nodes and immune system.

Germ Cell Tumor – This is a cancer that forms in reproductive organs’ embryonic stem cells. It’s very rare and affects children and young adults.

Multiple Myeloma – This is a cancer that forms in bone marrow cells that produce blood cells. Multiple myeloma is incurable at present, although chemotherapy can slow its growth.

In this section, we’re going to look at a specific type of cancer that forms in connective tissue. This type of cancer is called Fibrosarcoma and it’s a very common type of cancer in animals.

Fibrosarcomas (and sarcomas in general) are cancers that form in connective tissue. Connective tissue is a special type of tissue that holds the body together. It also provides support and shapes organs. Connective tissue can either be soft (loosely woven) or rigid (a lattice-like framework).

The types of connective tissue include:

Fascia – This is a layer of connective tissue that covers and connects muscles.

Adipose Tissue – Also known as fat, this is a layer of connective tissue that stores energy in animals.