Hemopoiesis in ectopically implanted bone marrow.

Ectopically implanted bits of marrow undergo a regenerative process that recapitulates the marrow ontogeny. This process is possible only because marrow tissue has considerable angiogenic potential. The regenerative process originates from marrow stroma, leading to the formation of primitive mesenchyme, osteoid bone, reconstitution of marrow organization including its distinctive sinusoidal system, and repopulation with circulating hemopoietic stem cells. Expansion of hemopoiesis is then associated with bone resorption. Also, few adipose cells develop and they are interspersed with hemopoiesis. The final product is a hemopoietic nodule surrounded by a shell of bone. A similar process occurs within the marrow cavity after ablation of the marrow tissue. In yellow marrow implants, the subsequent development of adipose tissue replaces entirely the hemopoietic tissue. Splenic implants can also regenerate in an analogous fashion despite their lack of significant angiogenic potential. As a model system, ectopic implantation of marrow has been the forerunner of long-term marrow culture and has provided important information on the relationship between hemopoietic cells and their supporting stroma. It has also led us to further understanding of the relationship between the marrow and its surrounding bone. Moreover, it has been an excellent system to study the relationship between red and yellow marrow and their interconversion. The full potential of this model system has not yet been fully realized. In application, for example, the conversion of yellow to red marrow can be exploited to reactivate the areas of hemopoietically inactive marrow in the limbs. Such exploitation may permit more liberal use of ablative radiotherapy in malignant diseases, particularly those of the lymphoreticular system. In basic research, in conjunction with long-term bone marrow culture, ectopic marrow implantation can yet provide considerable information on the role of stroma and bone in hemopoiesis.