The Job of the Bone Marrow. Bone marrow is where B-cells are selected, developed and matured in a process called hematopoiesis. Two types of stem cells are made in the bone marrow: stromal cells, which make fat, cartilage and bone, and hematopoietic stem cells, which make blood cells.he bone marrow is one of two primary lymphoid organs, along with the thymus. Bone marrow is a sponge-like tissue found at the center of the body's large, flat bones.