Correctly order the following key steps in the process of endochondral ossification. A) Chondrocytes die. B) Osteoblasts build the bone collar on the external surface of the bone. C) Perichondrium is invaded by blood vessels. In the primary ossification center, osteoblasts replace calcified cartilage with early spongy bone. D) Perichondrial cells differentiate into osteogenic cells then into osteoblasts.

The following key steps in the process of endochondral ossification, in the correct order is:

1. Letter B

2. Letter A

3. Letter C

4. Letter D

Explanation:

1. Osteoblasts build the bone collar on the external surface of the bone (bone collar formation)

2. Cavitation, which means that chondrocytes die. They die because the calcified hyalin cartilage is impermeable to the difusion nutrients, so they cannot receive any nutrients.

3. Periosteal bud invasion, the perichondriume is invaded by blood vessels. It can be compared with a group of robbers who invaded the graves where fallen chondrocytes uesd to rest.

4. Perichondrial cells differentiate into osteogenic cells then into osteoblasts, the bone ends develop (secondary) ossification centers.