Monocytes and tissue macrophages
Monopoiesis, the production of monocytes, gives white cells that circulate in blood and, once they enter tissues, become macrophages, which engulf and digest microbes and debris.
Remember
A monocyte is the circulating form and a macrophage is the tissue form: the macrophage is formed only after the cell leaves the blood, so the order is marrow, then blood, then tissue.
All blood cells begin as stem cells in the bone marrow. These split into two main families: the lymphoid family, which makes lymphocytes, and the myeloid family, which makes everything else, including monocytes, red cells, platelets and the granulocytes (white cells with granules in the cytoplasm). A common myeloid progenitor is the parent cell of all the myeloid blood cells, and monocytes come from it.
Monopoiesis is the production of monocytes. Monocytes are large white cells with an indented, horseshoe-shaped nucleus. The young monocyte is released into the blood, which acts as a transport route and not as the final home. Monocytes circulate for roughly 20 hours and then leave the vessels.
In a tissue, signals from the surroundings change the monocyte into a macrophage, a large scavenger cell that engulfs and digests microbes, dead cells and debris. This engulfing is called phagocytosis. Macrophages also release cytokines (small signalling proteins that call other immune cells to the area) and show fragments of microbes to other immune cells, a process called antigen presentation.
The same cell can be called a monocyte or a macrophage, depending on whether it is in the blood or in a tissue. Some tissues give their macrophages names of their own, for example Kupffer cells, the macrophages of the liver. The body cannot predict where an infection will start, so it keeps a mobile supply of cells in the blood, ready to become macrophages wherever they are needed.
Each blood cell line stays within its own pathway. Granulopoiesis is the separate branch of the myeloid line that makes neutrophils and the other granulocytes, and a monocyte does not change into a granulocyte or a red cell.
Key facts
- Starting cell
- monocytes arise from the common myeloid progenitor, a parent cell in the bone marrow.
- Process
- monopoiesis is the production of monocytes.
- Time in the blood
- monocytes circulate for roughly 20 hours before they leave the blood.
- Tissue form
- a monocyte that settles in a tissue is then known as a macrophage.
- Main job
- macrophages engulf and digest microbes, debris and dead cells, which is called phagocytosis.
- Named example
- Kupffer cells are the macrophages of the liver.
Exam traps
- Monocytes do not arise from a lymphoid progenitor, because lymphoid progenitors give rise only to lymphocytes and monocytes are a myeloid cell.
- Monocytes do not turn into neutrophils, because neutrophils are short-lived granulocytes made by granulopoiesis, a separate branch from monocytes.
- Monocytes do not finish becoming macrophages in the bone marrow, because they leave the marrow as monocytes and complete the change only after they have entered tissues.
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