(428j) Development of a Mathematical Model for a 3-D Perfused Bone Marrow Culture System

Bone marrow (BM), in adults, is the site of blood formation (haematopoiesis). The human marrow has an intricate three dimensional (3-D) architecture that is highly vascularised, consisting of the central sinus, the sinusoid network, nutrient arteries, and the radial arteries, collectively known as the intra-vascular space. Haematopoiesis, which takes place in the extra-vascular space, is an intricate process that is controlled precisely by the 3-D bone marrow microenvironment consisting of the spatially arranged cells, the extracellular matrix they produce, and the cytokines they secrete [1]. Malfunctions of either the intra/extra-vascular space can lead to the several BM diseases, such as leukaemia. Bone marrow transplantation is a widely practiced clinical procedure for BM diseases. The lack of suitable donors and the complications of allogeneic transplants have led the research community to focus on developing ex vivo expansion culture systems, which may hold great promise for clinical use [2]. However, there are several questions that remain unanswered and new methodologies that are needed for the well-controlled and reproducible culture of clinically relevant populations of haematopoietic stem cells (HSC).

Traditionally, ex vivo bone marrow culture systems are two dimensional (2-D) involving the use of flask cultures. The static nature of these systems does not facilitate mass transfer of nutrients and metabolites [3]. In addition, they can only support monolayer growth which does not provide appropriate cell-cell contact as in the 3-D BM microenvironment [4], despite the fact that it is well established that the 3-D BM microenvironment is critical in the regulation of the stem cell self-renewal, proliferation, and differentiation.

Bioreactor systems such as stirred, perfusion and airlift bioreactors have been developed and used for the growth of HSC cultures to enable better control of the culture parameters, enhance the mass transport and support high density growth [5-8]. Nevertheless, these systems have their drawbacks such as high shear stress and lack of 3-D growth environment. Porous scaffolds have been used to provide a mean for the HSC cells to grow in a 3-D orientation; however, most of the studies were performed under static conditions and suffer for limited mass transport.

In our work, we have selected to use a porous scaffold embedded in the Rotating Wall Perfused Bioreactor (RWPB-Synthecon). The scaffold will provide the 3-D growth environment that will simulate the bone marrow architecture. Furthermore, the perfused RWPB will provide efficient mass transport of nutrients and metabolites as well as low mechanical stress necessary for cellular growth [9].

To model the operation of the 3-D perfused bioreactor and to determine the optimal features of the scaffold, we have developed a 3-D computer model that, as a first approximation, captures the vascular arrangement qualitatively (Fig. 1), incorporates the transport and utilisation of the growth nutrients and the accounts for the stem cell differentiation into different cell lineages. The model is implemented within a CFD environment (CFX 4.4, CFX-Ansys), which combines porous media characteristics with the features described above.

Simulations have been performed for the fluid transport, oxygen and glucose utilization and stem cell growth inside a RWPB. The flow is assumed to be laminar, incompressible and Newtonian. The physical properties of the scaffold, which represents the extra-vascular region of the BM, are assumed to be isotropic and homogenous, and the dimensions of the scaffold do not change with time (i.e. there is no flow blockage or deposition within the scaffold). The developed computer model will be used to choose design parameters for the scaffold and to optimize the fluid dynamics and mass transfer environment for cell growth.

References

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