72. monocytes - development, monocytopoiesis

Monocytes are a type of agranulocyte, which is a subgroup of leukocytes (white blood cells). They serve as precursor cells for various members of the mononuclear phagocyte system, including macrophages and osteoclasts.

structure

• Size: 13-20 µm in diameter.

• Nucleus:

  • Large, eccentric (off-center) in position.

  • Oval or kidney-shaped.

  • Contains 1-2 nucleoli.

• Cytoplasm:

  • Basophilic (light blue to gray in standard staining).

  • Contains fine lysosomal azurophilic granules, contributing to the bluish-gray color.

  • Small amounts of rough endoplasmic reticulum (rER).

  • Prominent Golgi apparatus and mitochondria.

  • Presence of pinocytic vesicles and numerous microvilli on the cell surface.

functions

• Phagocytosis: Monocytes are actively involved in phagocytosis, a process where they engulf and digest pathogens, dead cells, and debris.

• Antigen Presentation: Monocytes can differentiate into antigen-presenting cells (such as dendritic cells and macrophages), which play a crucial role in the immune response by presenting antigens to T lymphocytes (T cells) and initiating adaptive immune responses.

lifespan

• In Blood: Monocytes circulate in the bloodstream for approximately 1-3 days. During this period, they are not fully differentiated and do not perform their primary functions.

• In Connective Tissue: Once they migrate from the bloodstream into tissues, they differentiate into macrophages or dendritic cells and can survive for several months to years, performing various immunological roles.

Monocytopoiesis: Development of Monocytes

• Monoblasts:

  • Monoblasts are the progenitor cells for monocytes. They are large, immature cells found in the bone marrow.

  • These cells differentiate into promonocytes.

• Promonocytes:

  • Large cells with basophilic cytoplasm.

  • Undergo two rounds of cell division (mitosis).

  • During this stage, the cells decrease in size and begin to develop azurophilic granules (lysosomes).

• Monocytes:

  • The final stage of maturation results in mature monocytes, which then enter the bloodstream.

  • Monocytes remain in the blood briefly before migrating to tissues.

• Maturation into Macrophages:

  • Once monocytes leave the bloodstream and enter tissue, they further differentiate into macrophages or dendritic cells.

  • These mature cells are capable of phagocytosis and antigen presentation and can live for several months, playing essential roles in tissue homeostasis, immune defense, and repair processes.