Early in development a human embryo’s skeleton develops first as hyaline cartilage and sheets of dense irregular connective tissue, later these get converted into bone. The hyaline cartilage that makes up much of the skeleton undergoes a process called endochondral ossification. Intermembranous ossification involves the formation of bone in an area without cartilage, usually an area containing dense irregular connective tissue. I’ll discuss what happens in a long bone as an example of what occurs during endochondral ossification.
In the hyaline cartilage of an embryonic long bone a primary ossification center develops in the middle of the bone (along the diaphysis), this area is changed from cartilage into bone on the outside as the perichondrium (membrane that surrounds cartilage tissue) transforms into periosteum (outer membrane of a bone) and osteoblasts that develop there start forming bone matrix, this creates a collar of bone around the outside of the diaphysis of the long bone. At almost the same time, deep inside the same area of the bone, the cells in the cartilage enlarge and start secreting substances that encourage bone formation. Osteoblasts then start forming bone matrix and as the area of bone tissue increases osteoclast cells start to break down the center area of bone matrix ultimately creating a medullary cavity.
Secondary ossification centers develop in the epiphyses (the ends) of the long bone and turn the inside of the epiphyses into bone. Once the primary ossification center reaches a secondary ossification center a boarder of hyaline cartilage known as an epiphyseal plate or growth plate forms between the two, these growth plates are responsible for the long bone growing longer. The growth plates are thin lines of hyaline cartilage that continually add bone matrix away from the epiphysis as the bone grows. The growth plates will remain active until adulthood is reached and then they will turn completely into bone. Here is an image of a growth plate under a microscope…
Appositional growth of a long bone refers to the bone getting wider as it grows, this occurs as osteoblast cells add bone matrix to the outside of the bone (under the periosteum) while osteoclast cells remove bone matrix inside the bone. This basic process continues until the bone is at its adult width.
It is important to know that although our bones stop growing when we reach adult size they never stop remodeling themselves. Bone tissue is a dynamic ever changing tissue that is continuously renewing itself and changing based on calcium needs of the body, stresses placed on the bones by muscles and gravity, and other factors.
Intermembranous ossification refers to the formation of bone without cartilage present. The skull plates, the clavicles, and other bones of the fetal skeleton develop using the process of intermembranous ossification. The process starts with mesenchymal stem cells dividing and grouping together in string like formations. The mesenchymal cells then differentiating into osteoblasts and start to secrete bone matrix (collagen proteins and calcium phosphate) which hardens. Some of the osteoblasts become trapped in the matrix and become osteocytes. In this way the stringy collections of mesenchymal cells becomes spicules of bone tissue. These spicules grow and merge with each other to form compact bone on the outer surfaces of the bone (cortical bone) or stay as spicules inside the bone (spongy bone).
Intermembranous ossification is important for the formation of especially flat bones, but it is also important during the healing of a fractured bone.