What is true about endochondral ossification?

• A. Hyaline cartilage models develop.
• B. Direct differentiation of mesenchyme into bone without cartilage.
• C. It occurs only in the skull.
• D. It does not involve growth plates.

Answer: (A)

Endochondral ossification starts with a hyaline cartilage model that will be replaced by bone. In this process, mesenchymal cells first condense and differentiate into chondrocytes to form that cartilage template. This template is then gradually replaced as the cartilage at key sites hypertrophies, calcifies, and is invaded by blood vessels and osteoblasts, which lay down bone matrix. In long bones, the primary ossification center forms in the shaft and secondary centers appear in the ends; growth plates keep adding length as long as they stay open, allowing joint bones to lengthen during development and adolescence.

The statement that hyaline cartilage models develop captures this essential sequence, making it the best description of endochondral ossification. The idea that mesenchyme differentiates directly into bone describes intramembranous ossification, not endochondral. Saying it happens only in the skull is incorrect because endochondral ossification forms most of the bones of the body, especially long bones, while some skull bones arise intramembranously. And growth plates are a central feature of endochondral ossification for longitudinal growth, so the claim that it does not involve growth plates is false.