Identification of important interactions between subchondral bone ...

CHAPTER 3: Overview of OA models
3.2 Advantages/disadvantages of pre-clinical models
3.2.1 In vitro
Cultures of isolated chondrocytes are the easiest and most convenient way to investigate
chondrogenesis in vitro; inexpensive, easy to setup and handle. However, several limitations must
be considered. Two-dimensional monolayer cultures support the proliferation of articular
chondrocytes, but change their phenotypic appearance to de-differentiated fibroblast-like cells,
which synthesize collagen type I (a marker of osteoblasts) 1,2 . Consequently, cultures of isolated
chondrocytes are not suitable for investigating cartilage degradation or for studying mechanisms
involved in cartilage matrix assembly, but are rather used for investigating proteoglycan and
collagen biosynthesis in response to various stimuli (Table 2) 3 . Pellet culture systems were
originally described as a method for preventing the phenotypic de-differentiation of chondrocytes
in vitro 4 . This culture system allows three-dimensional cell-cell interaction and is investigated by
the same techniques as for isolated chondrocytes in monolayer, and additionally by
histology/immunohistochemistry (Table 1) 5 .
When investigating bone cells (osteoclasts and osteoblasts), the cells are not
isolated from the bone matrix, but differentiated from their progenitor cells (e.g. isolated from
blood) and cultured on e.g. bone or plastic 6 . Differently from isolated chondrocytes, osteoclasts
and osteoblasts are able to resorb and deposit bone, respectively, which is their natural task in
vivo. Thus, in vitro models for bone cells are very close to imitating in vivo conditions 6 . The
osteoclast and osteoblast cell cultures have been used to evaluate the direct effect of
glucocorticoids in this thesis (PAPER II).
3.2.2 Ex vivo
The more complicated and technically advanced ex vivo explants model offers the ability to
investigate the biology of the tissue, including both ECM and cells. This model also offers the
ability to investigate how the tissue is affected by different compounds 7 . The cells are embedded
in their natural matrix where the immediate environment is preserved, including macromolecules
and cell-binding proteins. Additionally, the explants model allows preservation of the cell
phenotype; e.g. the chondrocytes in cartilage explants maintain their spherical appearance (Table
2) 8 . Several studies have used the cartilage explants model to resemble the pathological events in
OA cartilage 9,10,11 . The pioneering work on porcine cartilage organ cultures were done by Fell et
al. in 1973 12,13 . Since then, the cartilage explants model is widely used as a degradation assay that
allows investigation of cartilage metabolism in response to different stimuli and agents 14,15,16 . A
prominent example is that of Sondergaard et al., who showed that calcitonin abrogated the CTX-
II release from catabolically induced cartilage explants 9 . Other studies investigated the proteolytic
pathways in other cartilage ex vivo studies 10,17 . The advantage of studying cartilage metabolism in
the cartilage explants model is the unique in vivo likeness. In combination with biochemical
markers, this cartilage explants model is considered to be the best non-in vivo model in the study
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