Describe how long bones grow longer

Eventually, this hyaline cartilage will be removed and replaced by bone to become the epiphyseal line. The epiphyseal plate is the area of elongation in a long bone. It includes a layer of hyaline cartilage where ossification can continue to occur in immature bones. We can divide the epiphyseal plate into a diaphyseal side closer to the diaphysis and an epiphyseal side closer to the epiphysis. On the epiphyseal side of the epiphyseal plate, hyaline cartilage cells are active and are dividing and producing hyaline cartilage matrix.

On the diaphyseal side of the growth plate, cartilage calcifies and dies, then is replaced by bone figure 6. As cartilage grows, the entire structure grows in length and then is turned into bone.

Once cartilage cannot grow further, the structure cannot elongate more. The epiphyseal plate is composed of five zones of cells and activity Figure 6. The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix.

These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the overlying osseous tissue of the epiphysis.

The proliferative zone is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes. It makes new chondrocytes via mitosis to replace those that die at the diaphyseal end of the plate. Chondrocytes in the next layer, the zone of maturation and hypertrophy , are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. The longitudinal growth of bone is a result of cellular division in the proliferative zone and the maturation of cells in the zone of maturation and hypertrophy.

This growth within a tissue is called interstitial growth. Most of the chondrocytes in the zone of calcified matrix , the zone closest to the diaphysis, are dead because the matrix around them has calcified, restricting nutrient diffusion.

Capillaries and osteoblasts from the diaphysis penetrate this zone, and the osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus, the zone of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows in length when osseous tissue is added to the diaphysis. Bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones, which will be discussed later. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces all the cartilage, longitudinal growth stops.

All that remains of the epiphyseal plate is the ossified epiphyseal line Figure 6. While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases. This growth by adding to the free surface of bone is called appositional growth. Appositional growth can occur at the endosteum or peristeum where osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts produce new bone tissue.

The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis but also increase the diameter of the medullary cavity. However, in adult life, bone undergoes constant remodeling, in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed.

Injury, exercise, and other activities lead to remodeling. Those influences are discussed later in the chapter, but even without injury or exercise, about 5 to 10 percent of the skeleton is remodeled annually just by destroying old bone and renewing it with fresh bone.

The severity of the disease can range from mild to severe. Those with the most severe forms of the disease sustain many more fractures than those with a mild form. Frequent and multiple fractures typically lead to bone deformities and short stature. Bowing of the long bones and curvature of the spine are also common in people afflicted with OI.

Curvature of the spine makes breathing difficult because the lungs are compressed. Because collagen is such an important structural protein in many parts of the body, people with OI may also experience fragile skin, weak muscles, loose joints, easy bruising, frequent nosebleeds, brittle teeth, blue sclera, and hearing loss.

There is no known cure for OI. As shown in Figure below , ossification of long bones, which are found in the arms and legs, begins at the center of the bones and continues toward the ends. By birth, several areas of cartilage remain in the skeleton, including growth plates at the ends of the long bones. This cartilage grows as the long bones grow, so the bones can keep increasing in length during childhood.

Long bones ossify and get longer as they grow and develop. These bones grow from their ends, known as the epiphysis, and the presence of a growth plate, or epiphyseal line, signifies that the bone is still growing.

In the late teens or early twenties, a person reaches skeletal maturity. By then, all of the cartilage has been replaced by bone, so no further growth in bone length is possible. However, bones can still increase in thickness.

This may occur in response to increased muscle activity, such as weight training. How do bones grow? Growth and Development of Bones Early in the development of a human fetus, the skeleton is made entirely of cartilage. In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify.

Secondary ossification centers form in the epiphyses as blood vessels and osteoblasts enter these areas and convert hyaline cartilage into spongy bone. Until adolescence, hyaline cartilage persists at the epiphyseal plate growth plate , which is the region between the diaphysis and epiphysis that is responsible for the lengthwise growth of long bones Figure 1.

Figure 1. The periosteum is the connective tissue on the outside of bone that acts as the interface between bone, blood vessels, tendons, and ligaments. Long bones continue to lengthen, potentially until adolescence, through the addition of bone tissue at the epiphyseal plate. They also increase in width through appositional growth. Chondrocytes on the epiphyseal side of the epiphyseal plate divide; one cell remains undifferentiated near the epiphysis, and one cell moves toward the diaphysis.

The cells, which are pushed from the epiphysis, mature and are destroyed by calcification. This process replaces cartilage with bone on the diaphyseal side of the plate, resulting in a lengthening of the bone. Long bones stop growing at around the age of 18 in females and the age of 21 in males in a process called epiphyseal plate closure. During this process, cartilage cells stop dividing and all of the cartilage is replaced by bone. The epiphyseal plate fades, leaving a structure called the epiphyseal line or epiphyseal remnant, and the epiphysis and diaphysis fuse.

Appositional growth is the increase in the diameter of bones by the addition of bony tissue at the surface of bones. Osteoblasts at the bone surface secrete bone matrix, and osteoclasts on the inner surface break down bone. The osteoblasts differentiate into osteocytes. A balance between these two processes allows the bone to thicken without becoming too heavy.

Bone renewal continues after birth into adulthood. Bone remodeling is the replacement of old bone tissue by new bone tissue. It involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts. Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorous, and magnesium.

Hormones such as parathyroid hormone, growth hormone, and calcitonin are also required for proper bone growth and maintenance. Bone turnover rates are quite high, with five to seven percent of bone mass being recycled every week.

Differences in turnover rate exist in different areas of the skeleton and in different areas of a bone. For example, the bone in the head of the femur may be fully replaced every six months, whereas the bone along the shaft is altered much more slowly. Figure 2. After this bone is set, a callus will knit the two ends together.