Slide #DMS 071 [Mature compact bone, Decalcified, H&E]. 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?

The outer border of an osteon is marked by a cement line, a region of collagen-poor bone matrix. These cement lines are actually seen to best advantage in those sections that do have some staining artifacts where they are visualized as thin dark lines delimiting an osteon. Cement lines represent weak, fracture prone areas of the bone, also evidenced in those slides with sectioning artifacts.

Closing down the iris diaphragm on your microscope, one can better appreciate the lamellar arrangement of the bone matrix, as well as some of the details of the canalicular system connecting neighboring osteocytes and the haversion canal. The canalicular architecture will be seen to best effect in a subsequent slide.

Identify some interstitial lamella found amidst the osteons. These lamellae are remnants of former osteons, the other parts of which were resorbed during a previous remodeling cycle.

Identify resorption cavities which are evidence of current remodeling taking place even in this adult bone. Remember, bone is not a static tissue! The cellular preservation of this specimen is not so good as to allow positive identification of the cells within these cavities. How could you tell which phase of remodeling (resorption, formation) was occurring in a particular cavity? Think about the type of cells you would find during each process.

Turn now to the marrow cavity and identify the trabecular bone (if any is present in your slide). Being mature bone, its organization is also lamellar, though this may be more difficult to appreciate because of the numerous oblique sections resulting from the 3-dimensional, sponge-like arrangement of the trabeculae (try closing down your iris diaphragm to observe the matrix). While occasional concentric arrangements of bone tissue may be found in a trabecula, these are not true osteons. Trabeculae are thin enough to derive their nourishment from surrounding vessels and so lack the osteon architecture that characterizes mature compact bone. The trabeculae of bone, found bridging the marrow cavity, do not have Haversian systems.

This is a very low power view of a section through a mature bone. In this sample, both compact, cortical bone as well as some mature cancellous bone, in the marrow space, may be seen.

In this low power view, the characteristic architecture of mature, compact bone can be observed. The majority of the bone tissue is organized into osteons, but the free surfaces show layers of longitudinally-disposed lamella known as the inner circumferential lamellae and outer circumferential lamellae. Although this bone is mature, the continuous process of bone remodeling is occurring as evidenced by resorption cavities.

At this higher power, one may appreciate the orientation of circumferential lamellae compared to the rest of the bone which is organized into osteons and remnants of former osteons, the interstitial lamellae (seen to better effect in the next image). Note the resorption cavity as evidence of bone remodeling.

In this medium power image, identify some of the numerous osteons in the field, the centrally-place haversion canal, and the concentric arrangement of lamellae characteristic of mature, compact bone.

A high power micrograph of an osteon shows the centrally-located haversion canal, concentric lamellae of bone with lacunae housing the shrunken remains of osteocytes. A hint of the canalicular network interconnecting osteocytes to the haversion canal is seen as a network of radial striations in the matrix.

In some sections of this specimen that show more preparation artifacts, a dark ring around the circumference of the osteons marks the cement line, a region of collagen-poor matrix that defines the borders between neighboring osteons.

Some sections of this specimen also contain some mature cancellous bone forming a trabecular network in the marrow cavity. In some of these trabeculae, the lamellar architecture of this mature bone is evident, but in other spicules, this matrix organization is difficult to appreciate because of the three-dimensional, sponge-like architecture of cancellous bone.

One may occasionally see small, concentric arrangements of lamellae in cancellous bone that resemble the osteon architecture of compact bone. Although they are more typical of compact bone, osteons can be found in large pieces of trabecular bone.