Histomorphological and proliferative characterization of developing periosteal neochondrocytes in vitro

Periosteal chondrogenesis is relevant to cartilage repair and fracture healing. Cell proliferation is a limiting factor of cartilage production. We used an in vitro organ culture model to test the hypothesis that proliferative activity correlates with cell morphology. One hundred and four periosteal explants from 26 two-month old New Zealand rabbits were cultured for up to 42 days. They were analyzed histomorphologically, and immunohistochemically with proliferative cell nuclear antigen (PCNA). The periosteal neocartilage displayed a consistent zonal pattern of chondrocyte cell shapes. The flat cell zone from day 7 to 21, consisted of uniform-sized small spindle-shaped cells. The round cell zone, which appeared on day 14, consisted of variable-sized round cells averaging 510 +/- 250 microm2 in area. They were subdivided into small round (<510 microm2) and large round cells (>510 microm2). The proliferative index was highest in the small round cell group (32 +/- 6%), intermediate in the flat cell group (27 +/- 6%), and lowest in the large round cell group (20 +/- 7%) (P < 0.001). Furthermore, the proliferative indices in the round cell group were inversely proportional to cell size. Therefore, (1) there is a sequential progression of cell morphology during periosteal chondrogenesis, (2) cell differentiation is arrested prior to terminal differentiation for some cells and not for others, and (3) proliferative activity is strongly related to cell morphology. This organ culture model provides us with opportunities to study the regulation of terminal chondrocyte differentiation and the control of cell proliferation. This will contribute to our understanding of cartilage repair, fracture healing and growth plate physiology.