Bone development begins with the replacement of collagenous mesenchymal tissue by bone. Generally, bone is formed by endochondral or intramembranous ossification. Intramembranous ossification is essential in bones such as the skull, facial bones, and pelvis, where MSCs directly differentiate into osteoblasts. Endochondral ossification involves an initial phase of cartilage formation followed by bone remodeling. This process occurs primarily in long bones such as those found in the arms and legs. During this phase, blood vessels invade the cartilage template that has been created by MSCs, causing it to break down and be replaced by bone.
Bones are constantly being broken down (resorbed) and rebuilt (formed). The cells that carry out these tasks are called osteoblasts for bones that are growing and chondroblasts for cartilage. Osteoblasts produce a protein called osteoid which is the material from which new bone is made. Chondroblasts make up the bulk of the cell mass in cartilage but can also produce osteoid. Cartilage is more dense than bone and provides support for the weight of muscles and organs while also serving as a protective shield around them. When fractures occur, blood vessels invade the damaged area and cells from the surrounding tissue migrate in to fill the gap. New bone is then produced by these cells and grows together into a single piece.
Intramembranous ossification is the formation of bone from fibrous membranes. It helps to build the flat bones of the skull, the mandible, and the clavicles. As mesenchymal cells establish a pattern for future bone, ossification starts. Cells on the outer surface of these membranes begin to calcify, forming small plates of bone that will one day become the nose, cheeks, or ears. This process continues as more bone forms over the remaining living tissue.
Endochondral ossification is the formation of bone by the direct transformation of cartilage into bone. It is how long bones are grown when they are not fully developed at birth. Endochondral ossification begins with blood vessels supplying nutrients to the area where new bone is growing. At this stage, the bone is called "cartilage." The body uses protein molecules called collagens to make up much of the cartilage matrix. These proteins provide a framework within which other cells can grow. As more collagen is deposited, more bone is formed. When the supply of blood vessels reaches the end of the cartilage rod, then it dies and is replaced by bone.
The process of intramembranous ossification can also occur in the flat bones of the skull, but it is usually much slower than its counterpart. These bones develop thick layers of hard, dense bone that protect the brain inside the skull.
It also helps build the curved bones of the body, such as those of the chest and the spine. The word "ossify" means "to become a bone." Thus, intramembranous ossification means that certain soft tissues in the body are converted into bone.
In humans, intramembranous ossification occurs during embryonic development when bone cells called osteoblasts produce new bone directly from existing soft tissue. This process does not require an intermediate stage called cartilage. Instead, osteoblasts deposit their own protein matrix, which becomes calcified to form bone. Intramembranous ossification is limited to developing bones that will become flat or curved. In contrast, endochondral ossification involves the formation of bone through a cartilage template. Adult bones undergo endochondral ossification when blood vessels invade the cartilage template and stimulate osteoblast activity. Endochondral ossification is required for bones to grow longer than they are wide. For example, when we bend over, our spinal columns ossify primarily via endochondral ossification because it allows for greater mobility of the spine.
Compact and spongy bone grows directly from sheets of mesenchymal (undifferentiated) connective tissue during intramembranous ossification. Intramembranous ossification forms the flat bones of the face, the majority of the cranial bones, and the clavicles (collarbones). The skull is formed primarily by membranous bone during embryonic development, but later on it will be replaced by bone marrow and connective tissue.
Membranous bone is strong but light, and is used to cover and protect important organs such as the brain and the heart. The skull consists mainly of membranous bone; only a small fraction is made up of cortical bone around large blood vessels. Most of the bone in your body is membranous, including some of the bones in your hands and feet. The breastbone (sternum) and the pelvis are exceptions; they're mostly made up of cortical bone to provide support for the heart and lungs.
Intramembranous ossification is responsible for the growth of the skeleton during childhood. Because this type of bone is so thin, it's very susceptible to trauma. However, it also has an advantage because it's very flexible and can absorb impacts that might damage more solid bones.
During adulthood, most bone loss occurs through normal wear-and-tear processes rather than through active cell death.
Intramembranous ossification: the production of bone tissue in the absence of cartilage; and endochondral ossification: the process by which cartilage transforms into bone tissue. These two processes combine to form a complete skeleton. As intramembranous ossification occurs, the word "bone" is used instead of "cartilage." For example, a newborn baby has no true cartilage, just dense bundles of collagen fibers. However, within a few months, these fibers will calcify and become bone.
During skeletal development, cells called chondrocytes produce large amounts of collagen II and IX, along with smaller amounts of other collagens such as XI and XIV. The resulting matrix is rich in proteoglycans that provide nutrients for chondrocyte growth and help retain water. Chondrocytes also release signaling molecules (growth factors) that attract blood vessels to the site and stimulate cell division among other things. This allows the chondrocytes to produce sufficient collagen to support their own growth as well as induce new chondrocytes to replace those that die off. Eventually, the growth plate closes up and the bones reach their final size and shape.
Intramembranous ossification begins with clusters of chondrocytes forming hollow centers within the bone tissue.