Hematopoiesis

Image attribution: Keith Chambers, CC BY-SA 3.0, via Wikimedia Commons

Types of Blood Cells

Blood is made up of different types of cells that have important roles in our bodies. The three main types of blood cells are red blood cells, white blood cells, and platelets.

Red Blood Cells: These are the most abundant cells in the blood. Their primary job is to carry oxygen from the lungs to all parts of the body. They look like tiny, round discs and are red because of a special protein called hemoglobin. Hemoglobin helps them grab onto oxygen in the lungs and release it where it's needed.
White Blood Cells: These cells are like the body's defenders. They help protect us from infections and illnesses. There are different types of white blood cells, and each has its own way of fighting off germs and harmful invaders. They work together to keep us healthy by identifying and attacking harmful substances.
Platelets: These cells are small and play a vital role in the clotting process. When we get a cut or injury, platelets rush to the area to form a clot, which helps stop bleeding. This process is crucial because it prevents excessive blood loss and helps the body heal.

Each type of blood cell has an essential role in maintaining our health and keeping our bodies functioning properly. They work together as a team to keep us feeling good and protect us from diseases.

How are new blood cells made?

New blood cells are continuously produced in the body through a process called hematopoiesis. This process mainly occurs in the bone marrow, which is the spongy tissue found inside certain bones, such as the hip bones and the ends of long bones like the femur and humerus.

Hematopoiesis involves stem cells, which are special cells that have the ability to develop into different types of blood cells. Hematopoietic Stem Cells (HSCs) are blood stem cells. HSCs are the "parent" cells that can give rise to all types of blood cells – red blood cells, various types of white blood cells, and platelets. HSCs are located in the bone marrow and are responsible for continuously replenishing the blood cell population.

The process of hematopoiesis involves several stages of cell differentiation and maturation. As the cells progress through these stages, they become more specialized and take on the characteristics of the different blood cell types.

Once the blood cells reach maturity, they are released into the bloodstream, where they circulate throughout the body and carry out their specific functions. Red blood cells carry oxygen, white blood cells fight infections, and platelets help with blood clotting.

This continuous process of blood cell production and renewal is vital for maintaining a healthy blood supply and ensuring that our bodies can function properly to keep us alive and well.

The process of making new blood cells starts with a signal that more blood cells are needed. This signal is received by the hematopoietic stem cells causing the stem cells to divide by mitosis. This creates more stem cells that can then differentiate into the types of cells that are needed while also retaining some stem cells in the bone marrow so that a supply of HSCs is maintained.

Differentiation is the process in which unspecialized or immature cells undergo changes to become more specialized and take on specific functions and characteristics. During differentiation, cells acquire unique structures, functions, and features that enable them to perform specific roles in the body. Differentiation is tightly regulated by various signaling molecules and genetic factors that control the expression of specific genes in the cell. While all body cells - except red blood cells - contain the same DNA, different types of cells are different because they have different combinations of genes turned on and off.

In order to differentiate into a red blood cell, specific genes have to be turned on or off in a specific order which changes the shape and function of the cell, resulting in a red blood cell. To form a B cell or T cell the same process of turning genes on and off occurs but the specific genes and the order of genes will be different for these different cell types.

The process of differentiation is essential for the proper functioning of our bodies. Without it, our cells would not be able to perform their specialized functions, and the complex structures and systems in our bodies would not be possible. Differentiation plays a central role in shaping our bodies and maintaining the health and functionality of various tissues and organs throughout our lives. Differentiation of blood cells is called hematopoiesis but all the specialized cells in your body go through differentiation in order to become functional mature cells.

Hematopoiesis Clay Modeling

You can model hematopoiesis using some common school supplies. Watch the video to the left to be walked through this activity or click on the button above and work through it on your own.

Identifying Blood Cells

White blood cells can be identified on a blood smear. White blood cells are much less common than red blood cells and need to be stained in order to be seen and identified. You can identify white blood cells based on their size compared to RBCs and the shape of the nucleus. They are very easy to misidentify.

Granulocytes (includes Neutrophils, Eosinophils and Basophils).

Neutrophils - making up 60-70% of the total white blood cells, neutrophils are the most common type of WBC. The neutrophils are bigger than red blood cells. They contain a single nucleus, which is multilobed.

Eosinophils are rarely found in blood smears as they make up between 1-6% of the total white blood cells. When counting cells for a complete blood count (CBC) you only count the first WBCs. Eosinophils are larger than neutrophils, and about 3 times the size of a red blood cell. They have two lobes to their nucleus and granules that stain reddish purple are seen throughout the cell.

Basophils are the rarest type of white blood cell, making up only 1% of the white blood cells found in a blood smear. The nucleus can be hard to see due to the number of darkly stained granules in the cell.

Lymphocytes are the second most common white blood cell (20-50%), and are easy to find in blood smears. There are two main types of lymphocytes (B-celsl and T-cells). These cells are a major component of the adaptive immune system. Most lymphocytes are slightly larger than red blood cells but can be up to twice as large as a RBC. Lymphocytes do not have a kidney-bean shaped nucleus.

Monocytes are the largest type of white blood cell. They have a kidney-bean shaped nucleus surrounded by lots of cytoplasm. The shape of the nucleus can be difficult to identify so don't rely on it to determine if the cell is a monocyte or a lymphocyte. Monocytes are the third most common white blood cell, making up 2-20% of the white blood cells found on a blood smear.

Blasts are immature blood cells and they are not supposed to be seen on a blood smear. These cells are found in the bone marrow but can be forced into the blood stream when too many are made, such as in cases of leukemia or other blood disorders. Blasts are easily confused with monocytes due to their large size but monocytes have more cytoplasm and a more lobe shaped nucleus. Blasts can also be confused with lymphocytes but are larger and can contain granules.