A secondary ossification center is the spot where bone formation occurs after primary ossification, says The Free Dictionary. A secondary ossification center is usually at the expanded end of a long bone, and during growth, it is either made of cartilage or separated from the bone by a cartilage disk.
Bone formation is called ossification and involves two different processes that occur in the primary and secondary ossification centers; both processes replace preexisting support tissue, explains the Histology Laboratory Manual from the Columbia Center for New Media Teaching and Learning. Secondary centers of ossification begin to form at each end of a long bone with penetration by blood vessels. Once secondary ossification centers develop at the ends, the growth of cartilage causes them to get larger. The shaft of a long bone is compact with marrow inside and the ends are made of spongy bone covered by a thin shell of compact bone. When the ends of long bones stop growing, the cartilage disk is replaced by spongy bone and marrow and the bones close.
Primary and secondary ossification centers appear at different times, notes Slippery Rock University School of Physical Therapy. Primary centers can appear as soon as five weeks or as late as two-to-three months. Secondary centers begin as early as three-to-seven years and as late as 20-to-25 years. All the bones of the extremities and the weight-bearing skeleton follow this development, but flat bones in the head and collarbone develop differently.