The effect of heparin and the LMWH, Tinzaparin, on the percentage of cancellous bone surface length occupied by either osteoblasts or osteoid. Rats were injected with either unfractionated heparin or LMWH, and the cancellous bone surface, proximal to the epiphyseal growth plate, characterized as being lined with either osteoblasts (A) or osteoid (B). Data are expressed as mean ± SEM. ^a P < .005 when compared with either baseline or control values.