Permanence of proliferating cells in developing, juvenile and adult knee epiphyses of lizards in relation to bone growth and regeneration

Abstract

AbstractThe distribution of long‐labelling‐retaining cells, putative progenitor or stem cells, in the developing knees of embryo, juvenile and adult lizards has been analysed using H3‐thymidine autoradiography and 5BrdU immunohistochemistry. Proliferating cells are present in developing cartilaginous femur and tibia, especially in the epiphyses where a higher cell multiplication likely determines their typical enlarged shape in comparison with the diaphyses where chondroblast proliferation is low to absent. Sparse 5BrdU‐labelled cells remain in the articular and growth plate cartilages of the epiphyses in older stages of development and are still detected in developing epiphyses 13 days after injection of 5BrdU. This indicates they are slow‐cycling cells, a typical characteristic for progenitor or stem cells. Long retaining 5BrdU‐labelled cells remain in the articular surface also during adult life where they likely sustain the growth of long bones. Adult epiphyses show secondary ossification centres where the articular cartilage is partially or largely replaced by bone trabeculae. The damage in the epiphysis of lizards stimulates the proliferation of progenitor cells for the regeneration of new cartilaginous epiphyses. The localization of cells capable of proliferation in the epiphyses of adult femur and tibia pre‐adapts these lizards to cartilage regeneration in case of injury.