Only the early development of B-lymphocytes occurs in bone marrow. The development of T-lymphocytes occurs in the thymus (T-lymphocytes). The immunologic "maturation" of B- and T-cells occurs in other lymphocytic organs such as the spleen, lymph nodes, or tonsils (B-lymphocytes). Lymphocytes constitute about of nucleated bone marrow cells (myelogram).
Bone marrow lymphocytes are not morphologically different from those of peripheral blood. They have a dense, coarse nucleus and minimal basophilic cytoplasm. In the bone marrow, aggregates of lymphocytes are occasionally found that are also known as lymphoid follicles.