Extramedullary, Trilineage & Fetal Hematopoiesis

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

Hematopoiesis is the production of blood cells. In this lesson, we'll learn how blood is made and how that process has changed throughout the course of your development.

Your Blood and You

Have you ever given blood? If you have, you probably gave somewhere around one pint of blood, depending on where you are located and your local standards. That's somewhere around two and a half trillion red blood cells alone! If you're a man, you have about 10-12 pints of blood on average; a woman has 8-9 pints. How does your body manage to replace the blood you lose when you donate?

Hematopoiesis is the production of blood components. Your method of making blood has changed a good deal over the course of your life.

Fetal Hematopoiesis

Embryonic stem cells are totiopotent, meaning that they are capable of differentiating into any type of cell in the body. Embryonic stem cells are therefore interesting to researchers for a lot of reasons. Embryonic stem cell research has led us to many of our current vaccines and to some kinds of breast cancer therapy and cardiac treatments. Researchers are now using embryonic stem cells to investigate treatments for diabetes, amyotrophic lateral sclerosis, and Parkinson's disease, among others.

However, they are interesting for our purposes today for their ability to differentiate into blood cells in the developing fetus. When you were a very young embryo, you had blood islands, clumps of blood cells in your yolk sac that produced more blood. The blood cells you produced were considered 'primitive' because they were structurally different from adult blood cells. For instance, early stage fetal red blood cells have a nucleus, while adult human red blood cells do not.

As you got a little older and started developing organs, you started making blood in your liver, spleen, and lymph nodes. Your spleen took over when you were about a 3-month-old fetus. Eventually, you developed bone marrow, and that's when your bone marrow took over as the primary site of blood cell production. Your spleen and liver helped out but tapered off. By the time you were about a seven-month-old fetus, your spleen and liver would only help when necessary. By the time you were born, your bone marrow was handling the blood production by itself.

Adult Hematopoiesis

However, the sites in your bone marrow where you could produce blood cells continued to decrease. You actually have two types of bone marrow: red bone marrow and yellow bone marrow. Red bone marrow is where most of your blood cells are made. Yellow bone marrow is mostly just fat, although it is capable of making some white blood cells. When you were a baby, almost all your bone marrow was red bone marrow. You needed all this red bone marrow to produce new cells very fast, so you could grow. By the time you became an adult, only about half your bone marrow (in your femur, pelvis, vertebral column, sternum, ribs, and skull) is red. However, if you undergo serious blood loss, your body is capable of converting yellow bone marrow back into red bone marrow so that you can replenish your blood supply more quickly.

Human bone marrow
Human bone marrow

Your red bone marrow consists of two kinds of stem cells. Stromal cells can differentiate into fat, cartilage, and bone. Hematopoietic stem cells are stem cells that are capable of differentiating into any type of blood cell. So they are a bit more constrained than the totiopotent stem cells you had as a fetus, but they're still pretty flexible. All your blood cells are divided into three lineages: erythrocytes, or red blood cells; megakaryocytes, the large cells that produce platelets; and myelocytes, which differentiate into various white blood cells. If we're feeling really ambitious we can refer to the production of all of these cells as trilineage hematopoiesis.

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