Abstract
Intra-articular adipose tissue deposits known as articular fat pads (AFPs) are described to exist within synovial joints. Their assumed role in normal joint biomechanics is increasingly objectivized by means of advanced methods of functional imaging. AFPs possess structural similarity with body subcutaneous white adipose tissue (WAT), however, seems to be regulated by independent metabolic loops. AFP dimension are conserved during extreme WAT states: obesity, metabolic syndrome, lipodystrophy, and cachexia. Hoffa fat pad (HFP) in the knee is increasingly recognized as a major player in pathological joint states such as anterior knee pain and osteoarthritis. HFP contains numerous population of mesenchymal and endothelial progenitors; however, the possible role of mature adipocytes in the maintenance of stem cell niche is unknown. We propose that AFP is an active component of the joint organ with multifunctional roles in the maintenance of joint homeostasis. Endowed with a rich network of sensitive nervous fibbers, AFPs may act as a proprioceptive organ. Adipokines and growth factors released by AFP-resident mature adipocytes could participate in the maintenance of progenitor stem cell niche as well as in local immune regulation. AFP metabolism may be locally controlled, correlated with but independent of WAT homeostasis. The identification of AFP role in normal joint turnover and its possible implication in pathological states could deliver diagnostic and therapeutic targets. Drug and/or cell therapies that restore AFP structure and function could become the next step in the design of disease modifying therapies for disabling joint conditions such as osteoarthritis and inflammatory arthritis.
Highlights
White adipose tissue (WAT) is increasingly recognized as a multifunctional, metabolically active organ [1]
We propose that articular fat pads (AFPs) is a specialized tissue of the joint organ contributing to its homeostasis by releasing bioactive molecules implicated in cell and extracellular matrix (ECM) growth, turnover, and repair
Adiponectin, and resistin were reportedly found in synovial fluid (SF) of OA and rheumatoid arthritis (RA) patients at concentrations that differ from blood levels
Summary
White adipose tissue (WAT) is increasingly recognized as a multifunctional, metabolically active organ [1]. AFPs have been mainly mentioned in the context of pathological joint states (such as knee pain and osteoarthritis—OA) [10] or as a source of progenitor and stem cells [11] Their potential roles in maintaining homeostasis in normal joints remains unexplored. It remains obscure if AFP function and metabolic profile is correlated with systemic WAT normal and pathological states or is regulated by potential separate mechanism connected or not to the biomechanical function within the joint. Despite the structural similarity to body-wide WAT, AFPs persist even during advanced states of malnutrition, when all other deposits are depleted This observation raised the hypothesis that such tissue might possess yet unexplored particular ultrastructural and secretory features of potential importance for joint function [23]. Could it be possible that similar to other musculoskeletal structures, AFP possess a proprioceptive role contributing to dynamic alignment of the structures around the joint? Further studies about the presence of such proprioceptive receptors within AFP could elucidate this question
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