Overview: Primary lymphoid organs

Red bone marrow is the site of blood cell formation (hemopoiesis), producing erythrocytes, granular leukocytes (neutrophils, eosinophils and basophils), B lymphocytes, T cell precursors, monocytes, and platelets. B cells exit the bone marrow and migrate into secondary lymphoid tissues and organs where they differentiate into antibody-producing plasma cells that provide humoral immunity. T cell precursors migrate to the thymus where selection and maturation occur before final migration into secondary structures to provide cell-mediated immunity. Monocytes enter connective tissues and differentiate into macrophages. 400x

Red bone marrow consists of multipotential stem cells and their progeny that form mature blood cells and platelets. Newly formed blood cells enter the circulatory system by traversing thin-walled sinusoids (discontinuous capillaries) in the marrow. Red marrow also contains adipocytes that, with age, increase in number eventually constituting the major cell type forming inactive, yellow marrow.

Red bone marrow consists of multipotential stem cells and their progeny that form mature blood cells and platelets. Newly formed blood cells enter the circulatory system by traversing thin-walled sinusoids (discontinuous capillaries) in the marrow. Red marrow also contains adipocytes that, with age, increase in number eventually constituting the major cell type forming inactive, yellow marrow.

Red bone marrow consists of multipotential stem cells and their progeny that form mature blood cells and platelets. Newly formed blood cells enter the circulatory system by traversing thin-walled sinusoids (discontinuous capillaries) in the marrow. Red marrow also contains adipocytes that, with age, increase in number eventually constituting the major cell type forming inactive, yellow marrow.

Thymus is the second primary lymphoid organ. Immature T cells produced in the bone marrow, first migrate to the thymus where they undergo maturation and selection. Thereafter, they migrate from the thymus to peripheral tissues as well as lymphoid organs, including lymph node and spleen. T lymphocytes are involved with cell-mediated immunity as well as initiation of the humoral immune response. 10x

The thymus is a highly lobulated organ that consists of a cortex and medulla. Cortex, intensely basophilic due to the large number of T cells present, is the site of T cell maturation and selection. Migration of mature T cells from the thymus occurs in the medulla, which is less intensely stained.

The thymus is a highly lobulated organ that consists of a cortex and medulla. Cortex, intensely basophilic due to the large number of T cells present, is the site of T cell maturation and selection. Migration of mature T cells from the thymus occurs in the medulla, which is less intensely stained.