Articular cartilage is the tissue covering the articulating surfaces in diarthrodial joints. It is a smooth, glistening tissue that supports near frictionless motion and mechanical loads within the joint. Articular cartilage is avascular and aneural, and its major components are water, collagen, proteoglycans (PGs), and cells; water accounts for 60-85% of tissue wet weight, collagen for 10-30%, PGs for 3-10%, and cells for less than 5%. The main type of collagen in articular cartilage is type II collagen (Col2). Col2 is a homotrimer of three α1(II) protein chains that form a helix, and many Col2 molecules polymerize to form a fibril network with the assistance of cartilage oligomeric matrix protein (COMP) and type XI collagen (Col11). Minor collagens in the fibril network include types III (Col3), VI (Col6), IX (Col9), XII (Col12), and XIV (Col14). The primary PG in articular cartilage is aggrecan, which consists of a core protein modified along its length with the sulfated glycosaminoglycans (GAGs) chondroitin sulfate and keratan sulfate. Many aggrecan molecules bind to hyaluronan with link protein to form large aggregates embedded with Col2 fibrils. Other matrix PGs include biglycan, decorin, fibromodulin, lumican, perlecan, and versican. The cells in articular cartilage are chondrocytes, which are isolated from each other in lacunae throughout the tissue and are responsible for maintenance of the surrounding tissue. Adult chondrocytes are relatively inactive metabolically, but they produce and secrete the collagens and PGs of the extracellular matrix and the enzymes required to remodel healthy tissue and degrade damaged tissue.
The organization of its various matrix components varies through the depth of articular cartilage. Col2 content decreases from the superficial zone at the joint surface through the middle zone and deep zone, while aggrecan increases through these layers. In the superficial zone, thin collagen fibrils are arranged parallel to the joint surface with very little aggrecan and higher levels of Col1. The chondrocytes in this zone are flattened with a fibroblastic appearance and secrete lubricin to facilitate joint articulation. In the middle zone, round chondrocytes are sparsely embedded in the extracellular matrix. Aggrecan content is increased and the Col2 fibrils are thicker and randomly oriented. In the deep zone, aggrecan content increases even more and the thickest Col2 fibrils are oriented perpendicular to the articular surface. The chondrocytes are still round and are arranged in columns. Below the deep zone, the tidemark delineates the beginning of the calcified cartilage. In the calcified cartilage the structural organization remains the same, but with depleted aggrecan levels and with a mineral content of 65%. The chondrocytes in the calcified cartilage exist in uncalcified lacunae and express type X collagen (Col10) and alkaline phosphatase (ALP). Blood vessels from the subchondral bone invade the calcified cartilage, but the mineral prevents diffusion of soluble factors from the subchondral bone across the tidemark.
Chondrocytes first appear during embryonic skeletal development. Limb formation begins with the clustering of prechondrogenic mesenchymal cells into a precartilage mesenchymal condensation with high numbers of cell-cell contacts. The mesenchymal cells then differentiate into chondrocytes secreting the characteristic extracellular matrix of Col2 and aggrecan. As the matrix is elaborated around them, individual chondrocytes are pushed away from each other to yield the skeletal anlagen, a cartilaginous structure containing discrete chondrocytes that serves as a template for the developing skeleton. Ultimately, the anlagen is lengthened by chondrocyte maturation and turned into bone through the process of endochondral ossification. Chondrocytes remain only in the articulating surfaces at the ends of the bone and, temporarily, in the growth plate where they continue the process of skeletal elongation until the growth plate fuses during puberty. While most of the chondrocytes in the anlagen die during the process of endochondral ossification, the mechanisms that control their initial differentiation are essential to understanding chondrocyte regulation and MSC chondrogenesis.
Developmental chondrogenesis is characterized by alterations in cellular morphology, extracellular matrix content, and the gene ex
The role of mesenchymal condensation in the subsequent chondrogenesis of mesenchymal cells has been studied in micromass cultures of embryonic limb bud cells that recapitulate the condensation-chondrogenesis sequence observed in vivo. Increasing the density of the micromass correlates with greater overall chondrogenesis. Enhancement or disruption of micromass condensation via modulation of cell adhesion proteins results in improved or inhibited chondrogenesis, respectively. The simplest explanation is that a larger condensation represents a greater number of cells committed to a chondrogenic lineage, but condensation may also aid lineage commitment and/or chondrogenic differentiation by allowing cells to directly communicate with each other. Gap junctions, membrane-bound channels that directly link neighboring cells, increase during mesenchymal condensation and persist through chondrogenesis until the cells detach from one another and accumulate extracellular matrix between them. In micromass culture, transfer of gap junction-permeable dye is observed exclusively in regions of chondrogenic differentiation, suggesting a role for cell coupling in the spatial coordination of chondrogenesis, whereas blocking gap junction communication inhibits chondrogenesis.
Lineage commitment and condensation alone are not sufficient to initiate chondrogenic differentiation. Extracellular matrix interactions, cell-cell interactions, secreted factors, and transcription factors have all been implicated as factors that regulate developmental chondrogenesis following condensation. Since MSCs in pellet culture physically recapitulate the dense packing of the precartilage condensation, the factors regulating MSC chondrogenesis are likely to overlap with those that are known to regulate developmental chondrogenesis. Some of the key regulators of developmental chondrogenesis will be explored in more detail below to clarify which signals might be involved in the regulation of MSC chondrogenesis.