In vitro rapid organization of rabbit meniscus fibrochondrocytes into chondro-like tissue structures

We described the behaviour of 120 days rabbit knee-meniscus cells in monolayer culture. The cells were grown forming cellular aggregates resembling true cellular nodules. Three stages of development of these nodules could be observed: formation of the cellular nodules between days 1 and 3; nodular growth, with their maximal at day 5; and nodular regression beginning at day 8. Ultrastructural analysis of the extracellular matrix of these cellular nodules was assessed on days 3, 5 and 8. At the formation stage, we could observe striated collagen fibrils and small bundles of tubular microfibrils either interspersed with very low quantities of amorphous elastin, being morphologically identical to elaunin fibers, or without only trace of elastin, being morphologically identical to oxytalan fibers. By day 5, fibrillar elements with 100 nm periodic ladder-like collagen VI fibrillar aggregates could also be detected. At day 8, the striated collagen fibrils and oxytalan fibers could not be observed. During this same period, there was an increase of a dense matrix comprised of collagen VI and mature elastic fibers. Chondroitin/dermatan sulfate proteoglycans were synthesized and became essential for the arrangement of collagen type VI, since chondroitinase ABC treatment of the culture disrupted collagen VI assembly, associated with the large spaces near the cell surface. In addition, the cells lost their fusiform morphology and changed into rounded cells. The results show that primary cultures of rabbit meniscus fibrochondrocytes maintain their capacity to form chondro-like structures in vitro. The organization process was rapid and uniform throughout the entire culture presuming that the normal signal transduction pathways are maintained intact and that essential factors in some phases of tissue organization are present.