Introduction. Blood cells undergo progressive differentiation from stem cells originating in the bone marrow

Blood cells undergo progressive differentiation from stem cells originating in the bone marrow. In the presence of differentiation inducing growth factors B lymphocytes mature in the bone marrow and T lymphocytes mature in the thymus; both of these lymphoid organs are classified as primary lymphoid organs. Peripheral blood, lymph nodes, the spleen, and the tonsils are classified as secondary lymphoid organs since they contain mature, fully functional lymphocytes. By comparing cells from the bone marrow with cells from peripheral blood and the lymph nodes some of the cytological differences between immature stem cells and mature lymphocytes may be observed.

The bone marrow contains lymphoid and myeloid cells at various stages of differentiation. The larger more primitive cell types show less dense nuclei and are more closely related to immature stem cells. These undifferentiated cells are non-functional and differences between lymphoid and various myeloid cell types are more difficult to distinguish.

Peripheral blood smears show well defined different cell types. Functional myeloid include erythrocytes, granulocytes, monocytes and platelets. Lymphocytes show a circular dark staining nucleus under normal conditions. It is not possible to distinguish T lymphocytes from B lymphocytes in peripheral blood using standard Wright’s-Giemsa stain.

Distinct circular region of cells in the lymph nodes called germinal centers are present near the outer, cortical edge of the organ. These centers represent sites where antigen processing cells like macrophages have stimulated B lymphocytes with antigen and induced clonal expansion of those B lymphocytes that were able to respond to antigen. T lymphocytes, including key T helper cells, are located in the lymph node in the paracortical region that lies closer to the central portion of the lymph node.

There may be a need to use standard definitions for the histology and cellular terms. A proposed set of definitions follows:

B-cell. Lymphocyte that is the key cell in the production of antibodies following stimulation.

Bone marrow. Source of stem cells for all cellular components of blood.

Clonal expansion. The stimulation of a single cell to undergo cell division and produce clonal descendants.

Cortical. Outer layer or region of an organ or tissue.

Differentiation. The process of selective gene activation as a cell progresses in development from a stem cell.

Erythrocyte. Differentiated red blood cells.

Germinal centers. Areas of lymphoid tissue where stimulated lymphocytes are dividing.

Granulocytes. Phagocytic white cells that show prominent cytoplasmic granules and are abundant in peripheral blood.

Lymph nodes. Secondary lymphoid organs along length of lymphoid vessels that act as sites for antigen trapping and B cell stimulation.

Macrophage. A white cell derived from a monocyte that helps process and present antigen.

Monocyte. A white cell type found in peripheral blood that, like more abundant granulocytes, has phagocytic activity.

Myeloid cells. A family of cells which develop from a bone marrow stem cell to become erythrocytes, monocytes, granulocytes, and platelets.

Paracortical region. A region outside of the cortex of an organ or tissue, often from the external edge.

Spleen. A secondary lymphoid organ which stores white cells and red blood cells.

Thymus. A primary lymphoid organ which is essential for the differentiation of T lymphocytes.

Tonsils. Secondary lymphoid organ that stores differentiated lymphocytes.

T lymphocytes. Lymphocyte that is a key cell in stimulating immune cells and forming T killer cells for rejecting grafts and tumors.

T helper cell. Type of T lymphocyte that helps stimulate both functional T and B cells through production of soluble effectors called lymphokines.

Дата добавления: 2015-10-29