Slide DMS 072 [Fetal long bone]. This is a cross section of the mid-shaft region of a developing fetal long bone. This slide illustrates well the processes of circumferential growth and modeling of this region of a long bone. You should understand that in the developing long bone the transformation of the cancellous periosteal bone collar into a compact bone shaft begins simultaneously with the initiation of endochondral ossification at the epiphyseal-metaphyseal complex.

Survey the section using low and high power. Note the highly irregular periosteal (outer) and endosteal (inner) surfaces of the shaft primary spongiosa (composed of immature woven bone). The periosteum is very distinct and its inner osteogenic and outer fibrogenic zones easily discerned. The latter provides attachment for the tendons of the developing muscles surrounding the shaft. Centrally, note the developing marrow cavity and the abundance of active hemopoietic tissue (blood forming cells).

The alteration of the size and shape of bones by bone formation and resorption at different surfaces and rates during the growth process is known as modeling. Modeling serves primarily to alter the amount of bone that is present and to determine the final form of the bone. During normal growth of a long bone the rates of external apposition of new bone and internal resorption are coordinated so that the cylindrical shaft expands markedly in diameter while the thickness of its wall and the marrow cavity increase slowly.

Now study the section with high power. Where do you find the greatest number of bone forming cells (osteoblasts)? Where do you find the greatest number of bone resorbing cells (osteoclasts)? Do your observations support the concept of shaft modeling as explained above? Where in the section is the process of compaction (primary osteon formation) occurring? What is the significance of this process?