What connective tissue surrounds the diaphysis of long bones?

The periosteum is the connective tissue that surrounds the diaphysis of long bones. It serves several important functions, including providing a protective layer that encloses the bone, serving as an attachment point for muscles and tendons, and playing a crucial role in bone growth and repair. The periosteum contains a rich supply of blood vessels and nerves, which are essential for the nourishment and sensation of the bone.

In contrast, the endosteum lines the inner surfaces of the bone, including the medullary cavity, rather than surrounding the diaphysis. Hyaline cartilage is found at the ends of long bones, where it forms articular cartilage that covers joint surfaces, allowing smooth movement. Fibrocartilage can be found in structures like intervertebral discs and the menisci, but it does not surround the diaphysis of long bones. Thus, the periosteum is the correct answer as it specifically fulfills the role of encasing the diaphysis.