Slide 22 - Decalcified Bone cut in both longitudinal and transverse sections.  This bone is further advanced than the others, but still immature.

The marrow cavity is in the center.  Skeletal muscle and dense collagenous periosteum and ligaments may be seen attached at the outer surface of the bone.  The bone itself may be compact, arranged in Haversian systems, or it may still appear as spongy bone, with large vascular areas of unorganized trabeculae (or spicules) of bone.  In either  event, notice that the matrix is deposited in layers (lamellae), with osteocytes caught in lacunae between the layers.  Can you see canaliculi?  Note while here the possible presence of osteoblasts and osteoclasts in spongy bone.  At this stage of development, they are most likely to be along the outermost (under the periosteum) or innermost (next to the bone marrow) surfaces of the bone; look at both the cross-cut and long sections.  Remember that osteoclasts are giant cells and multinucleated; osteoblasts are smaller, have single, often eccentrically placed nuclei, and are lined up along the surfaces where they are depositing osteoid.

Notice along the periosteal edge of the cross-cut bone that there are many irregularities where the inner cellular, vascular layer of periosteum extends into the bone.  At first these spaces are quite wide, making an irregular, spongy bone surface.  The osteoblasts in these spaces lay down layers of bony matrix; the spaces thus become smaller and eventually become remodeled into the Haversian canals of compact bone (central vascular channels surrounded by layers of bony matrix). Undifferentiated osteoprogenitor cells always remain in the cellular layer of periosteum adjacent to mature osteoblasts, and in the lining of Haversian canals and are thus in position to rapidly become osteoblasts in case of needed remodeling or repair.  In what ways is the repair of fractured bone similar to endochondral ossification?