Cartilage does not develop into bone. Instead, cartilage acts as a blueprint for new bone to totally replace. The process of endochondral ossification takes substantially longer than that of intramembranous ossification. Endochondral ossification is the process through which bones near the base of the skull and long bones develop. It begins with an aggregation of chondrocytes below the perichondrium that produces a mass of cartilage called a physis. As this mass of cartilage grows larger, vascular channels penetrate its center where blood vessels invade the cartilage forming a growth plate. Chondrocytes in the proliferative zone of the growth plate divide rapidly and intensively. They also differentiate into hypertrophic chondrocytes, which are large and round. These cells eventually die by apoptosis, but their remains are incorporated into the matrix of the growing bone. In contrast, chondrocytes in the resting zone of the growth plate do not divide nor mature further. They instead produce large amounts of collagen II, which forms the bulk of the cartilage matrix.
Endochondral ossification can be divided into three stages: proliferation, maturation, and remodeling. During the proliferation stage, chondrocytes in the growth plate undergo rapid division and differentiation. This phase lasts from approximately 10-18 months old until skeletal maturity. At the end of this period, all growth has stopped and the skeleton reaches its maximum size.
Endochondral ossification is the process through which cartilage tissue is formed from aggregated mesenchymal cells and then replaced by bone (Horton 1990). The endochondral ossification process is separated into five phases (Figure 14.13). In the first phase, called activation, resting chondrocytes are stimulated to divide and differentiate. In the second phase, called proliferation, chondrocytes undergo mitosis as they migrate toward the center of the callus where they undergo hypertrophy. In the third phase, called matrix deposition, chondrocytes produce an extracellular matrix that will become calcified later in order to provide support during skeletal growth. In the fourth phase, called mineralization, calcium carbonate replaces the protein matrix of the callus resulting in a stable structure capable of withstanding compressive forces. In the final phase, called remodeling, the previous four stages are repeated as chondrocytes degrade their own extracellular matrix before being replaced by new cells and tissues.
Endochondral ossification is important for long bones to grow. Without this process, bones would not be able to expand during puberty when calcium is absorbed from the body into the skeleton. Endochondral ossification also occurs in skulls, where it plays a role in forming teeth and their supporting structures.
In the developing skeleton, cartilage may grow quicker than bone. When the adult skeleton is destroyed, cartilage regenerates quicker than bone. Bone tissue has a low water content as compared to cartilage tissue, which has a high water content. This difference in water content causes the bone to shrink when it dies back to form the marrow inside our bones. The cartilage on the other hand remains solid even after losing water, causing it to increase in size.
Cartilage is generally divided into two types: elastic and rigid. Elastic cartilage can be found in your ears, nose, and throat. It allows sound to enter your body but does not break down hard objects like bone would. Rigid cartilage forms the base of your skull and acts as a protective shield for your brain. It cannot bend without breaking.
During childhood, cartilage grows more quickly than bone Masses of elastic cartilage will increase in size and shape during adolescence and early adulthood. These increases are due to the growth of new cartilage cells that replace some of the old cells lost through use. However, once you reach skeletal maturity, growth of this tissue stops. Elongation of bone tissue occurs because bones are constantly remodeled; old bone cells die and new ones take their place. Increases in length occur because bones get longer rather than thicker.
Cartilage, once formed, lacks lymphatic or blood supply, and waste and nourishment are moved mostly by diffusion to and from surrounding tissues. Cartilage, like bone, is surrounded by a fibrous membrane that resembles the perichondrium. The term "synovium" is used to describe both the true synovial membrane and its replacement by fibrous tissue.
The blood supply to cartilage comes only from the capillaries within the joint space. In addition, some researchers believe there is a small population of slow-growing cells within cartilage called chondrocytes that continue to produce new vessels (angiogenesis). However, since all vascularization occurs outside of the body, this system is not capable of supplying nutrients to replace those used up by the growing organism.
Thus, cartilage has no direct connection with the bloodstream. It does, however, share a border with bone, which does have a direct connection with the bloodstream. Some investigators believe that molecules released into the joint space by damaged cartilage may stimulate blood vessel growth. This process, known as angiogenesis, would provide new routes by which nutrients could reach the injured area. Angiogenesis may also help remove toxic products such as ammonia produced when protein in urine breaks down.
As your baby develops within you, bones begin to replace cartilage in a process known as ossification. This replacement occurs primarily for three reasons: 1 to support the growing body 2 to protect delicate tissue beneath the skin and 3 to maintain the shape of the skeleton. Ossification is often visible as a white line against the black skin of a newborn. This line marks where cartilage will eventually develop into bone.
Bones are classified according to their appearance and function. Sesamoid bones are small, irregularly shaped bones that form in pairs at the ends of some long bones (such as the bones in your foot). They are composed mainly of collagen and calcified tissue. A sesamoid bone cannot be removed from the body without causing pain or damage to surrounding tissues. Because of this, they have no vital functions; rather, they act as supportive structures for larger bones.
Sutures are anatomical openings through which organs, tissues, or bones fuse together to form a single hollow organ or skeletal structure. In humans, these openings usually close up by themselves when no longer needed. However, if they do not close, then surgery is required to repair them. Sutures can be open or closed.
Long-distance diffusion from adjacent capillaries in the perichondrium nourishes cartilage. As a result, cartilage can never get particularly thick because diffusion is insufficient to give nutrients and oxygen to the cartilage. (This is in contrast to bone, which has an excellent blood supply.)
Cartilage does have some small blood vessels that run through it; however, these are sparsely distributed and don't reach very far into the tissue. The few cells at the end of each cartilage fiber are connected together by a web of collagen fibers; they lack true nuclei and therefore cannot reproduce themselves. They do, however, contain many large vacuoles that are filled with fluid that provides some cushioning effect when the foot strikes the ground.
Thus, cartilage lacks the ability to grow on its own and instead relies on repeated injury to stimulate new cartilage formation. When this occurs in adults, it is called osteoarthritis. Adults also retain the ability to form cartilage in certain areas of their bodies, such as in the ball of the thumb and the ears. This ability disappears after puberty because there's no need for more cartilage in adults. However, young people can regenerate cartilage in the ear during development if they're born with a defect in the hearing mechanism caused by lack of proper stimulation during infancy. This phenomenon is called epiphyseal growth plate chondrodysplasia.