Endochondral ossification begins with the formation of what?

Endochondral ossification is a critical process in the formation of long bones and begins with the establishment of a cartilage model. This model serves as a precursor to bone, where the initial framework for the future bony structure is created.

During this process, mesenchymal cells differentiate into chondrocytes, forming a hyaline cartilage model that mimics the shape of the eventual bone. This cartilage is then gradually replaced by bone tissue in a series of stages involving the growth of the cartilage, its calcification, and subsequent invasion by bone-forming cells. The significance of the cartilage model lies in its ability to support the formation of bone at various stages, particularly in areas that undergo growth, such as the growth plates in the long bones.

The other choices do not accurately represent the initial step in endochondral ossification. A bone model refers to an advanced stage where bone has already formed, while a connective tissue model does not exist in this context, as the process specifically begins with cartilage. A bony callus model pertains to the healing of fractures and occurs later in the healing process, not during the initial development of bone.