Abbreviations

risk of immunological rejection and malignant transformation [65]. Therefore, the use of MSCs to produce exosomes for drug delivery is the subject of the day [66]. Recently, liposomes are preferred drug delivery systems. It is a synthetic vesicle with a phospholipid membrane that has the ability to self-assemble into various sizes and shapes in an aqueous environment [67]. Another morphological feature detected is pseudopodia-like structures extending from the cell membrane (Figures 7 and 10). This might explain the capacity of the cells for migration within the receiving tissue. The main role of these structures is to transmit the produced material from one cell into another by extending the pseudopodia and communicating cells with each other as well as in cell signaling [68]. Interestingly, one of the most striking features during differentiation is the observation of finger-like extensions of the plasma membrane known as fibripositors (Figure 11). These fibripositors were located at the side of the cell and protrude into the spaces between cells. These fibripositors are the site where collagen fibrils were located. It was reported that the initial stage of extracellular matrix deposition results in arrays of short collagen fibrils completely enclosed within these fibripositors. These fibrils are then

Figure 11. Transmission electron micrograph of cultured human bone marrow derived stromal cells. The cell surface of shows an open fibripositors (short arrow) with large amounts of secretory product (S) is observed. Note the euchromatic

It was reported that fibrils leaving the fibripositors were seen to run along the external surface of the cell. Tracking of fibrils revealed that the collagen fibrils in fibripositors were shorter than those extracellularly. Thus, these data suggested that fibripositors might be a place of fibril assembly. They determined that short fibrils become longer inside closed fibripositors, then protruding fibripositors (open), often project into the matrix, releasing fibrils extracellularly where individual fibrils then coalesce into bundles. Thus, fibripositors are specialized sites not

Another study declared that the fibripositors are dynamic structures and their formation and stabilization depend on the actin cytoskeleton [72]. This might explain the existence of the cytoskeletal filaments in the differentiating cells [38]. Accordingly, these cytoskeletal structures might be actin filaments. It is possible that fibripositors have been involved in the alignment of

only of fibril assembly, but also share in fibril transport extracellularly [71].

extracellular collagen fibrils in a parallel arrangement [73].

subsequently deposited extracellularly [69, 70].

nucleus (N) [38]. Scale bar 0.5 μm.

38 Stromal Cells - Structure, Function, and Therapeutic Implications

