Types of Ossification Centers
There are two types of ossification centers - primary and secondary.
A primary ossification center is the first area of a bone to start ossifying. It usually appears during prenatal development in the central part of each developing bone. In long bones the primary centers occur in the diaphysis/shaft and in irregular bones the primary centers occur usually in the body of the bone. Most bones have only one primary center (e.g. all long bones) but some irregular bones such as the os coxa (hip) and vertebrae have multiple primary centers.
A secondary ossification center is the area of ossification that appears after the primary ossification center has already appeared - most of which appear during the postnatal and adolescent years. Most bones have more than one secondary ossification center. In long bones, the secondary centres appear in the epiphyses.
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