What Are Bone Marrow Macrophages?

J. Finnegan

Bone marrow macrophages are a type of white blood cell, or leukocyte, that attack and digest invading bacteria, tumor cells, protozoa, which are single-celled organisms, and other hostile germs that can invade and infect a living host organism. They are part of the immune system and have the ability to stimulate the activity of other types of immune cells. As their name suggests, bone marrow macrophages are created in the marrow, or the inner tissue, of the bones of animals and humans. The term bone marrow macrophages can refer to either naturally produced cells, or cells created in an in vitro environment, meaning that they're produced in a laboratory setting rather than made naturally in the body of a living animal. The term bone marrow-derived macrophages refers specifically to macrophage cells that are created in vitro.

Three pieces of bone with the marrow in the middle.
Three pieces of bone with the marrow in the middle.

Macrophages can be created in a laboratory by artificially growing mammalian bone marrow. This is achieved by exposing undifferentiated cells, or immature cells without a defined structure, to the macrophage colony-stimulating factor (M-CSF), which is a type of growth factor, or more simply, a hormone that controls the growth and maturation of macrophage cells. These new bone marrow cultures are grown in Petri plates, which are also known as culture dishes.

A diagram of the anatomy of a bone, showing the bone marrow.
A diagram of the anatomy of a bone, showing the bone marrow.

Hematopoiesis, which can also be spelled haematopoiesis, is the process by which the body naturally creates all the different types of blood cells, or blood components, including bone marrow macrophages. Hematopoietic stem cells are responsible for the process of hematopoiesis. All multicellular organisms have stem cells. They can divide, reproduce, and turn into a variety of specialized cell types, meaning they can turn into different types of cells that perform different biological functions. They also have the ability to repair damages cells.

All blood cells are divided into three categories, referred to as lineages. Bone marrow macrophages belong to the myelocyte lineage. The process called myelopoiesis refers to both the production of cells that belong to the myelocyte lineage, and also to the process that creates bone marrow tissue.

When marcophages are first created in bone marrow, they are referred to as monocytes. After they migrate to other parts of the body via the bloodstream, they mature into macrophages. These new macrophage cells stay permanently in bodily tissues where they protect against invading germs and tumor cells by both engulfing and dissolving them, and by alerting other immune cells to the presence of invasive cells or organisms.

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