Which sequence describes endochondral ossification?

• A. Hyaline cartilage models develop; chondrocyte hypertrophy and calcification occur; periosteal bud invades; primary ossification center forms; growth continues with secondary centers in the epiphyses.
• B. Cartilage is replaced by bone directly without a cartilage stage.
• C. Osteoblasts differentiate from mesenchyme and form bone directly in a connective tissue framework.
• D. Osteoid is secreted by osteoblasts without a cartilage precursor.

Answer: (C)

Endochondral ossification builds bone from a cartilage template. It starts with a hyaline cartilage model that shapes the future bone. Chondrocytes enlarge and the cartilage matrix becomes calcified, which sets up a scaffold for bone to replace. Then a periosteal bud—blood vessels and osteogenic cells—invades the cartilage, and a primary ossification center forms in the shaft. Growth continues as secondary ossification centers appear in the ends of the bone (epiphyses), enabling lengthwise growth from the growth plates. This sequence—cartilage model, chondrocyte hypertrophy and calcification, invasion by vessels, primary center, and later secondary centers—is the hallmark of endochondral ossification.

The other main pathway involves bone forming directly from mesenchyme without a cartilage stage, which is intramembranous ossification. In that process, osteoblasts differentiate within a connective tissue framework to lay down bone, rather than replacing a cartilage scaffold.