Hematopoiesis is the process of forming new blood cells from hematopoietic stem cells. It produces the different lineages of red blood cells, white blood cells, and platelets that the body needs to function. Where it takes place in bones:

• In adults, hematopoiesis primarily occurs in the bone marrow, especially in the porous, spongy (trabecular) areas of certain bones. The marrow cavities of the pelvis, vertebrae, sternum, skull, ribs, and proximal ends of long bones (like the femur and humerus) are major sites. A large portion of red marrow is concentrated in these regions, with the distribution varying by age and individual health.
• In children, hematopoiesis is more broadly distributed in long bones and in many of the same axial sites, but the marrow activity is higher overall as growth and development require more blood cell production.
• Within the bone marrow, hematopoietic tissue forms islands or niches among the trabecular bone and vascular sinuses. Hematopoietic stem and progenitor cells reside in these niches and differentiate into mature blood cells that then enter the circulation through the marrow’s venous system.