Schematic diagram for development of the IPFP, synovial membrane and ScAT

By sharing the same origin in development, the (A) IPFP and (B) synovium derive from a condensation of mesenchymal stem cells (MSCs), which have been used to model differentiating mesoderm [18] within the intermediate layer, called the interzonal mesenchyme [19]. With the upregulation of unique molecules, cavitation appears within the interzone [20] and then synovium joint formation under appropriate developmental cues. (C) ScAT is generally regarded as having a mesodermal origin [18]. Although the exact number of intermediate stages between the mesoderm/MSCs and mature adipocytes is uncertain, it is believed that the MSC gives rise to common early pre-adipocytes, which could differentiate into mature adipocytes under appropriate stimulatory conditions [21, 22]. It is notable that fat tissue present in the cephalic region has a neuro-ectodermal origin [23].