In between these two fragments is an intermediate zone consisting largely of fibroblasts and poorly differentiated osteoblasts. Fibroblasts proliferate in this area, arising from marrow cells with fibroblastic potential.[34] From the 1st to the 3rd week following injury, regenerated bone begins to fill in the gap between the two bony fragments. The first osteons begin to appear within the depths of the growth zone and there are numerous hypertrophied vessels.