Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA.
J Orthop Res. 2010 Sep;28(9):1184-92. doi: 10.1002/jor.21113.
The formation of cartilaginous tissue by chondroprogenitor cells, whether in vivo or in vitro, appears to require a critical initial stage of "condensation" in which intercellular space is reduced through an aggregation of cells, leading to development of cell-to-cell junctions followed by chondrocytic differentiation. The objective of this study was to investigate the association of aggregation (condensation) of mesenchymal stem cell (MSCs) and chondrogenesis in vitro. Previous work with chondrocytes indicated that the cross-link density and related cell-mediated contraction of collagen scaffolds significantly affects cartilaginous tissue formation within the cell-seeded construct. Based on this finding, we hypothesized that the cell-aggregating effect of the contraction of MSC-seeded collagen scaffolds of lower cross-link density favors chondrogenesis; scaffolds of higher cross-link density, which resist cell-mediated contraction, would demonstrate a lower cell number density (i.e., subcritical packing density) and less cartilage formation. Type II collagen-GAG scaffolds, chemically cross-linked to achieve a range of cross-link densities, were seeded with caprine MSCs and cultured for 4 weeks. Constructs with low cross-link densities experienced cell-mediated contraction, increased cell number densities, and a greater degree of chondrogenesis (indicated by the chondrocytic morphology of cells, and synthesis of GAG and type II collagen) compared to more highly cross-linked scaffolds that resisted cellular contraction. These results provide a foundation for further investigation of the mechanisms by which condensation of mesenchymal cells induces chondrogenesis in this in vitro model, and may inform cross-linking protocols for collagen scaffolds for use in cartilage tissue engineering.
软骨祖细胞在体内或体外形成软骨组织，似乎需要一个关键的初始“凝聚”阶段，在此阶段，细胞间空间通过细胞聚集而减少，导致细胞间连接的发展，随后是软骨细胞分化。本研究的目的是研究间质干细胞（MSCs）聚集（凝聚）与体外软骨生成的相关性。以前对软骨细胞的研究表明，胶原支架的交联密度和相关的细胞介导收缩显著影响细胞接种构建体内的软骨组织形成。基于这一发现，我们假设较低交联密度的 MSC 接种胶原支架的细胞聚集效应有利于软骨生成；交联密度较高的支架抵抗细胞介导的收缩，将表现出较低的细胞数密度（即亚临界堆积密度）和较少的软骨形成。用化学交联的 II 型胶原-GAG 支架达到一系列交联密度，用山羊 MSCs 接种并培养 4 周。与抵抗细胞收缩的交联密度更高的支架相比，低交联密度的支架经历了细胞介导的收缩，增加了细胞数密度，并在更大程度上促进了软骨生成（通过细胞的软骨细胞形态和 GAG 和 II 型胶原的合成来指示）。这些结果为进一步研究在该体外模型中，间充质细胞凝聚诱导软骨生成的机制提供了基础，并可能为用于软骨组织工程的胶原支架的交联方案提供信息。
