Not surprisingly, mammal bones differ from those of other vertebrates. Such differences are not only in number and proportions, but also in the frequent presence of epiphyses. No, an epiphysis isn't a religious experience—it's a separate center of bone formation. In most of the load-bearing bones of a mammal, the embryonic construction is of cartilage. This has the advantage of being able to grow by expansion rather than becoming large solely by adding superficial layers. In time, centers of ossification—bone formation—appear and transform the cartilage.
ln many of such bones, there are more than one such centers; usually,
one in the center of the bone's shaft and one at each end. As ossification
progresses, the main part of the shaft turns to bone, separated from the end
ossifications, known as epiphyses, by a bridge of cartilage which continues to expand.
Eventually, the bone material overtakes the cartilage, and growth in length is
complete. The overall process allows both growth and joint stability. In fact, you
might say the combined efforts of cartilage and bone are a joint endeavor.
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.