The nutrient artery passes through the nutrient canal and sends branches into the central Haversian canals to provide for osteocytes. The artery continues into the medullae to supply blood for the marrow and osteocells via the epiphyseal artery. The periosteal arteries pass through Volkmans' canals to a multitude of vessels that supply the outer compact bone region. Nerves follow vessels into bone tissue and the periosteum where they sense damage and transmit pain messages. The process begins during the sixth or seventh week of embryonic life and continues throughout adulthood as two types of ossification, intramembranous and endochondral, involve the replacement of a preexisting connective tissue with bone.
Intramembranous ossification occurs within fibrous membranes of the embryo and the adult.
Flat and irregular bones form in this process. The primary ossification center of a long bone is in the diaphysis. Cartilage degenerates, leaving cavities that merge to form the medullary cavity. Osteoblasts lay down bone. Secondary ossification centers develop in the epiphyses, where bone replaces cartilage, except for the epiphyseal plate.