Sheep Fat Bingo: A comparison of subcutaneous adipose-derived mesenchymal stem cells with infrapatellar adipose-derived mesenchymal stem cells with regard to their chondrogenic ability in a sheep model of osteochondral defect repair.

Abstract

Articular cartilage is an avascular, aneural, alymphatic tissue that has a very low capability for intrinsic repair. Injuries to the tissue often lead to progressive degeneration, leading to painful joint disease. The most successful biologically-based treatments for articular cartilage lesions are cell-based therapies that introduce new cells to the lesion with the intention that they produce and replace lost extracellular matrix. Adipose-derived mesenchymal stem cells (ASC) are an attractive source of graft cells for their ease of collection, ease of expansion, and ability to differentiate into the chondrogenic phenotype and produce cartilage-specific extracellular matrix. While many studies report the inferiority of ASC with regard to their chondrogenic ability, the vast majority of these authors isolate ASC from subcutaneous (SC) fat harvested during lipoaspiration procedures. Few studies have studied the chondrogenic potential of ASC isolated from the infrapatellar (IP) fat pad and none have compared the chondrogenic potential of ASC from these two sources.

This study compared ASC isolated from IP and SC fat in an ovine model. Adipose tissue was harvested surgically from both SC and IP fat sources. ASC were isolated from each source and expanded by monolayer culture in vitro. Monolayer cultures were induced to pellet formations using a chondrogenic medium and then cultured in the mechanically-loading environment of a three-dimensional rotary cell culture system. Neocartilage pellets from each source were compared for their ability to produce the phenotypic cartilage extracellular matrix proteins collagen type II and aggrecan as well as pericellular collagen type VI. Qualitative analysis was performed with immunohistochemical evaluation of pellets by stain with antibodies against these matrix proteins. Quantitative assays for total collagen, total glycosaminoglycan (GAG), and DNA content of pellets were performed to confirm qualitative results. Pellets were then autologously implanted into osteochondral defects surgically created in the articular surfaces of sheep knees. After four and twelve weeks of recovery, post-mortem examination of defect repair was performed and defects removed by dissection for fixation and subsequent processing for histopathology and immunohistochemical analysis.

Qualitative comparison of SC and IP pellets revealed that IP pellets produced greater relative amounts of all extracellular matrix proteins studied than SC pellets. IP pellets were significantly larger than SC pellets and contained more total collagen and total GAG per unit of DNA than SC pellets. When implanted, none of the pellets contributed substantially to defect repair, but IP pellets remained within the defects at twelve weeks while SC pellets were absent.

In conclusion, this study shows that IP ASC have greater chondrogenic ability than SC ASC, suggesting potential for the use of IP ASC in tissue-engineered constructs for repair of articular cartilage injury.