Discussion:

The behavior of human mesenchymal stem cells in 3D fibrin clots: dependence on fibrinogen concentration and clot structure

Natural biopolymers such as human fibrin are appealing to tissue engineers, because fibrin is biocompatible, bioresorbable, and essential in normal wound healing. There have been numerous studies to date to develop a fibrin-based injectable cell delivery system, albeit with varying success. We propose that the outcome of fibrin cell delivery can, in part, be attributed to the relative concentrations of fibrinogen and thrombin solutions (i.e., formulations) and the structure of the final 3D fibrin clot. Formulation-dependent proliferation of human mesenchymal stem cells (hMSCs) within 3D fibrin clots was investigated in vitro. Our results indicate that hMSCs are viable in all fibrin sealant formulations investigated, and proliferation rates vary with fibrin formulations. Furthermore, the fibrinogen solution, not thrombin, was found to have a more dominant role on hMSC proliferation, with dilute fibrinogen solutions promoting greater hMSC proliferation. Confocal and electron microscopy reveal formulation dependence on 3D fibrin clot structure, with dilute fibrinogen solutions yielding more open, homogeneous microstructures. This study suggests that the concentrations of fibrinogen and thrombin solutions must be carefully considered for cell delivery because they affect 3D fibrin clot structure and cell proliferation.

Periosteal cells in bone tissue engineering

In 1742, H.L. Duhamel published a report in which the osteogenic function of periosteum was described. In 1932 H.B. Fell was the first to successfully culture periosteum; Fell concluded that this tissue might have the capability to form mineralized tissue in vitro. In the 1990s the research group of A.L. Caplan pioneered work exploring the osteogenic potential of periosteal cells in the field of bone engineering. On the basis of these studies a number of research groups have developed hard tissue generation concepts that aim to repeat the clinical success of bone autografts by culturing cells from periosteum and seeding a sufficient quantity of those cells into scaffolds made of biomaterials of natural and synthetic origin. The highly porous matrices support the induction of bone regeneration by creating and maintaining a space that facilitates progenitor cell migration, proliferation, and differentiation as well as graft revascularization. In this way, a host tissue-scaffold cell interphase might be created that allows reproduction of the intrinsic properties of autogenous bone, including the ability to be incorporated into the surrounding host bone and to continue normal bone-remodeling processes. This review discusses the history and state of the art of bone tissue engineering from a periosteum and periosteal cell source point of view and attempts to indicate future research directions.