Questions: Which of the following events does not happen during endochondral ossification? Secondary ossification centers develop. Osteoblasts replace the calcified cartilage with early spongy bone. Mesenchymal cells differentiate into osteoblasts. Osteoclasts form a medullary cavity.

The event that does not happen during endochondral ossification is:

Mesenchymal cells differentiate into osteoblasts.

Explanation for each option:

1. Secondary ossification centers develop.

   • This is a correct event in endochondral ossification. Secondary ossification centers form in the epiphyses of long bones after birth, contributing to the growth of bones.

2. Osteoblasts replace the calcified cartilage with early spongy bone.

   • This is a correct event. During endochondral ossification, osteoblasts lay down bone matrix on the calcified cartilage, forming early spongy bone.

3. Mesenchymal cells differentiate into osteoblasts.

   • This event is characteristic of intramembranous ossification, not endochondral ossification. In endochondral ossification, mesenchymal cells first differentiate into chondrocytes to form cartilage, which is later replaced by bone.

4. Osteoclasts form a medullary cavity.

   • This is a correct event. Osteoclasts resorb bone tissue to create a medullary cavity in the diaphysis of long bones during endochondral ossification.

In summary, the event that does not occur during endochondral ossification is the differentiation of mesenchymal cells directly into osteoblasts.