Tissue chondrification and ossification in keloids with primary report of five cases

Keloid is commonly regarded as a benign skin tumour. Some keloids clinically exhibit hard tissue texture similar to that of cartilage or bone. We hypothesized that the keloid pathological niche environment is likely to induce keloid MSCs towards chondrogenic or osteogenic differentiation and leads to cartilage or bone‐like tissue formation. The differences in tissue ossification, histology, mechanical properties, abnormal extracellular matrices and chondrogenic/osteogenic gene expression among sclerous keloids (SKs), regular keloids (RKs) and normal skins (NKs) were carefully examined. The sporadic ossified islets existed in SK group whereas no ossified/chondrified islet was found in other groups by micro‐CT reconstruction. H&E, Masson trichrome and safranin O staining revealed lacuna‐like structures in SKs, which were featured as bone/cartilage histology. Immunohistochemical staining showed overproduction of osteoprotegerin, type I and III collagen in SK group but similar production level of aggrecan among three groups. The biomechanical analysis demonstrated the weakest compliance of SK tissues. In addition, SK fibroblasts exhibited a relatively slower proliferation rate but higher expression levels of osteogenic and chondrogenic genes among all three groups. These cell populations also showed the strongest potential for lineage transformation. In conclusion, we first reported the presence of ossified and chondrified matrices in some extremely hard keloids in the present study.