A fibrocartilage callus is a temporary formation of fibroblast

Fibroblast

A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...

s and chondroblast

Chondroblast

A chondroblast is a cell which originates from a mesenchymal stem cell and forms chondrocytes, commonly known as cartilage cells. Chondroblasts that become embedded in the matrix are called chondrocytes. They lie in the space or lacunae present in the groups of two or more. The groups are formed by...

s which forms at the area of a bone fracture

Bone fracture

A bone fracture is a medical condition in which there is a break in the continuity of the bone...

 as the bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 attempts to heal

Healing

Physiological healing is the restoration of damaged living tissue, organs and biological system to normal function. It is the process by which the cells in the body regenerate and repair to reduce the size of a damaged or necrotic area....

 itself. The cells eventually dissipate and become dormant, lying in the resulting extracellular matrix

Extracellular matrix

In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...

that is the new bone.
The callus is the first sign of union visible on x-rays,usually 3 weeks after the fracture.Callus formation is slower in adults than in children, and in cortical bones than in cancellous bones.