Day 14: The Skeleton

The Growth of Bone

This image is a light microscopic view of the humerus (upper arm bone) of a three and a half month old dog. The purple line across the section is the cartilaginous growth plate. The growth plate remains active through puberty and is responsible for lengthening the long bone. New cartilage forms at the top of the growth plate while new bone forms on older cartilage fragments at the bottom of the plate. Spongy bone has formed where cartilage used to be in the primary and secondary centers of ossification. Cartilage remains on the head of the humerus to form a smooth gliding surface for the shoulder joint.

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