Bone tissue engineering and spinal fusion: the potential of hybrid constructs by combining osteoprogenitor cells and scaffolds

In this paper, we discuss the current knowledge and achievements on bone tissue engineering with regard to spinal fusion and highlight the technique that employs hybrid constructs of porous scaffolds with bone marrow stromal cells. These hybrid constructs potentially function in a way comparable to the present golden standard, the autologous bone graft, which comprises besides many other factors, a construct of an optimal biological scaffold with osteoprogenitor cells. However, little is known about the role of the cells in autologous grafts, and especially survival of these cells is questionable. Therefore, more research will be needed to establish a level of functioning of hybrid constructs to equal the autologous bone graft. Spinal fusion models are relevant because of the increasing demand for graft material related to this procedure. Furthermore, they offer a very challenging environment to further investigate the technique. Anterior and posterolateral animal models of spinal fusion are discussed together with recommendations on design and assessment of outcome parameters.

Hematological effects in dogs after sequential irradiation of the upper and lower part of the body with single myeloablative doses

The compensating mechanisms determining the tolerance of the hemopoietic system to sequential hemibody irradiation (HBI) with large single doses, the regeneration of the irradiated bone marrow and the long-term effects of such treatment were studied in dogs. The main emphasis was laid on the determination of the granulocyte/macrophage progenitor cells (GM-CFC) in the bone marrow and blood. The general pattern of events in the GM-CFC compartment after each exposure was similar. Irradiation with a dose of 11.7 Gy of the upper body (UBI), that involved the abrogation of approximately 70% of the total active marrow, was followed by an immediate increase in the proliferation and differentiation of GM-CFC in the protected bone marrow. Repopulation of the GM-CFC in the irradiated sites most probably due to seeding of hemopoietic cells from the protected marrow already became evident at day 7 after UBI. At day 56 after UBI, when the irradiation of the lower body (LBI) was performed, the GM-CFC had recovered to between 30 and 40% of their pre-treatment values. Despite this incomplete regeneration, the GM-CFC compartment responded to LBI in a similar way as the GM-CFC had in the protected (normal) marrow after UBI, i.e. by an increased proliferation for at least 21 days. Already at day 7, the bone marrow of the iliac crest that had been exposed to LBI showed a considerable number of GM-CFC. Within no more than 370 days all the bone marrow sites irradiated during either the first or the second treatment had regained their normal GM-CFC values.

Acute and long-term alterations in the granulocyte/macrophage progenitor cell (GM-CFC) compartment of dogs after partial-body irradiation: irradiation of the upper body with a single myeloablative dose

The acute and long-term effects of a single dose of partial-body irradiation on the granulocyte/macrophage progenitor cell compartment were studied in dogs. A myeloablative dose of 11.7 Gy (dose rate 6.5 cGy/min) was given to the upper body which contains approximately 70% of the total bone marrow mass. The lower part of the body (pelvis, lower extremities and tail) was shielded by a lead box. In the non-irradiated bone marrow, the concentration of the GM-CFC/10(5) mononuclear cells was slightly decreased within the first 7 days and showed some fluctuations around the normal value for several weeks thereafter. In the irradiated bone marrow, virtually no GM-CFC could be detected on day 1 after exposure. Beginning on day 7, a continuous increase took place up to day 21 when the GM-CFC concentration reached between 25% (sternum) and 43% (humerus) of the initial value. No further increase took place up to day 80. Between day 120 and 380 a secondary increase was observed which reached near-normal bone marrow GM-CFC concentrations. The blood GM-CFC concentration first showed a strong depression followed by a transient increase between day 10 and 30. This coincided with GM-CFC normalization in the protected bone marrow as well as with the initial phase of regeneration in the irradiated sites. A prolonged secondary long-lasting depression between day 33 and 120 amounted to 20 to 50% of normal values. This depression was closely related to the stagnation in the GM-CFC recovery in the irradiated bone marrow sites. The GM-CFC concentration in the blood was found to be supranormal at day 380 when the bone marrow GM-CFC had recovered. The colony stimulating activity in the serum showed an increase within the first 20 days after exposure, that is, within the same interval the bone marrow GM-CFC concentration experienced the strongest alterations, and was inversely related to the changes in the blood granulocyte values.

Decrease in hematopoietic stem cell domains as a delayed effect of x-irradiation

Although the hematopoietic integrity of locally X-irradiated sites can be restored for a time even after fairly large doses, a secondary aplasia often occurs some months later. To gain further insight into this delayed effect within the framework of the stem cell regulatory domain hypothesis, we characterized the growth kinetics of spleen colony forming units (CFU-S) in WBB6FI-+/+ bone marrow transplanted into WBB6FI-W/WV mice in which one leg had been exposed to 10-30 Gy of X rays 4-5 months previously. Compared to unirradiated contralateral marrow, fewer CFU-S either reached the previously irradiated marrow or were seeded into sites that could support growth. The initial exponential growth of effectively seeded CFU-S was unchanged, but growth deceleration (inflection point) occurred at a lower level of CFU-S in marrow previously irradiated with 20-30 Gy. This change in the inflection point indicates a radiation dose-dependent decrease consistent with the decrease in bone marrow cellularity. The decrease in effective stem cell domains after 20 Gy was calculated to be about 35%. We interpret these results to reflect the highly localized nature of delayed radiation damage to the marrow microenvironment.

Functional changes in the vascularity of the irradiated rat femur. Implications for late effects

Early and late changes in the extraction of 125I antipyrine was investigated in the rat femur after local irradiation with single doses of 5 to 25 Gy. The extraction of antipyrine by bone was generally reduced after irradiation, with the greatest effect being found 3 months after treatment. However, the effect was transient and by 7 months after less than or equal to 20 Gy, antipyrine extraction was similar to that in the normal femur. Extraction was still reduced after 25 Gy. The first significant reduction in dry bone weight occurred 7 months after 15, 20 and 25 Gy. The fall in the calcium and phosphorus content of bone was similar to that of the dry bone weight. Calcium/phosphorus ratio was not modified in irradiated bone. The likely role of the vascular changes in the subsequent development of bone atrophy is discussed.