Final answer:
Primary bone is found in early bone development during fetal growth and healing, seen in intramembranous ossification of flat bones and endochondral ossification of the rest of the skeleton, beginning as early as the fifth week of fetal development and continuing until around age 25.
Step-by-step explanation:
Primary bone, also known as immature or woven bone, is observed during the early stages of bone development and healing. This type of bone is characterized by a haphazard organization of collagen fibers and is typically replaced by more organized secondary or lamellar bone as it matures. You can see primary bone during two key processes: intramembranous ossification, which is responsible for the formation of flat bones like those of the face, cranial bones, and the clavicles, and during the endochondral ossification, which is the process for forming the rest of the skeleton from a hyaline cartilage model. Intramembranous ossification begins during the fetal stage, with the clavicle beginning to ossify as early as the fifth week of development. Endochondral ossification also starts during the fetal stage but continues until around age 25, when the growth in the length of bones generally ceases. During bone repair, primary bone is formed initially and is subsequently remodeled into secondary bone.