The diaphysis of a long bone shows how growth in width occurs because of bone deposition on the inner and outer bony surfaces. Skeletal muscle can be seen in the upper right corner, and a small amount of bone marrow is present at the bottom of the image. 40x
Pre-existing bone >
The pre-existing bone is represented by the arrow.
The periosteum deposits lamellae around the outer circumference of the diaphysis, depositing the first lamella on the pre-existing bone. These layers are called the outer circumferential lamellae.
The endosteum at the marrow surface also participates in growth in width of the diaphysis by depositing lamellae around the inner surface of the diaphysis, forming the inner circumferential lamellae. Thus, the diameter of the marrow cavity decreases in size.
1st lamellae >
Since bone must be deposited on a pre-existing layer of bone or cartilage, the first layer of bone deposited by the periosteum is located at X, and the first layer of bone deposited by the endosteum is located at Y.