On the right is a cross-sectional “donut” of compact, decalcified bone forming the diaphysis of a growing long bone. Skeletal muscle surrounds the donut. Red marrow fills the internal “hole” and will be replaced by yellow marrow in the adult. Periosteum covers the external surface; endosteum lines the marrow surface and any other internal spaces. 100x, 40x, 10x
Two rules apply to bone deposition, leading to bone growth. Bone always grows appositionally; the rigid matrix of bone prohibits interstitial growth. Second, with one exception, bone is always deposited by endosteum or periosteum on a framework of pre-existing bone or cartilage. Consequently, new bone will assume the shape of the framework on which it is deposited.
If periosteum deposits bone, layers are formed along the outer circumference of the bone and are called outer circumferential lamellae. The first lamella formed is deposited on the donut of the original bone. Thus, the diaphysis increases in thickness by appositional growth as the periosteum apposes new lamellae of bone around the periphery.
If endosteum at the marrow surface deposits bone, layers are formed along the inner circumference of the bone and are called, unsurprisingly, inner circumferential lamellae. The first lamella formed is deposited on the donut of pre-existing bone. Thus, the diaphysis increases in thickness by the endosteum apposing new lamellae around the inner circumference of the bone, and the diameter of the marrow space is reduced proportionally.
Next Image >
The next image is similar to the area outlined by the rectangle and demonstrates the inner and outer circumferential lamellae at higher magnification.