Anatomy 6.5 Bone Formation

Zone of proliferating cartilage

Slightly larger chondrocytes in this zone are arranged like stacks of coins. These chondrocytes undergo interstitial growth as they divide and secrete extracellular matrix. The chondrocytes in this zone divide to replace those that die at the diaphyseal side of the epiphyseal plate

Zone of calcified cartilage

The final zone of the epiphyseal plate is only a few cells thick and consists mostly of chondrocytes that are dead because the extracellular matrix around them has calcified. Osteoclasts dissolve the calcified cartilage, and osteoblasts and capillaries from the diaphysis invade the area. The osteoblasts lay down bone extracellular matrix, replacing the calcified cartilage by the process of endochondral ossification. Recall that endochondral ossification is the replacement of cartilage with bone.

Growth in Thickness

bone can grow in thickness (diameter) only by appositional growth Bone grows in diameter as a result of interstitial and appositional addition of new bone tissue by osteoblasts around the outer surface of the bone and to a lesser extent internal bone dissolution by osteoclasts in the bone cavit

intramembranous ossification

bone forms directly within mesenchyme, which is arranged in sheetlike layers that resemble membranes

Endochondral ossification

formation of bone from hyaline cartilage models

epiphyseal (growth) plate

layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones the zone of resting cartilage, zone of proliferation cartilage, zone of hypertrophic cartilage, and zone of calcified cartilage The activity of the epiphyseal plate is the only means by which the diaphysis can increase in length. When the epiphyseal plate closes, is replaced by bone, the epiphyseal line appears and indicates the bone has completed its growth in length.

bone remodeling

ongoing replacement of old bone tissue by new bone tissue. It involves bone resorption, the removal of minerals and collagen fibers from bone by osteoclasts, bone deposition, the addition of minerals and collagen fibers to bone by osteoblasts

ossification

process of bone formation

factors affecting bone growth

1.Minerals. Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller amounts of magnesium, fluoride, and manganese. These minerals are also necessary during bone remodeling. 2.Vitamins. Vitamin A stimulates activity of osteoblasts. Vitamin C is needed for synthesis of collagen, the main bone protein. As you will soon learn, vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract into the blood. Vitamins K and B12 are also needed for synthesis of bone proteins. 3.Hormones. During childhood, the hormones most important to bone growth are the insulin‐like growth factors (IGFs), which are produced by the liver and bone tissue . IGFs stimulate osteoblasts, promote cell division at the epiphyseal plate and in the periosteum, and enhance synthesis of the proteins needed to build new bone. IGFs are produced in response to the secretion of growth hormone (GH) from the anterior lobe of the pituitary gland Thyroid hormones (T3 and T4) from the thyroid gland also promote bone growth by stimulating osteoblasts. In addition, the hormone insulin from the pancreas promotes bone growth by increasing the synthesis of bone proteins

Intramembranous Ossification cont.

The flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone) are formed in this way 1 Development of the ossification center. At the site where the bone will develop, specific chemical messages cause the cells of the mesenchyme to cluster together and differentiate, first into osteoprogenitor cells and then into osteoblasts. The site of such a cluster is called an ossification center. Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it. 2 Calcification. Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification). 3 Formation of trabeculae. As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue associated with the blood vessels in the trabeculae differentiates into red bone marrow. 4 Development of the periosteum. In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into its adult size and shape

growth in length

The growth in length of long bones involves the following two major events: (1) interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate and (2) replacement of cartilage on the diaphyseal side of the epiphyseal plate with bone by endochondral ossification

Zone of hypertrophic cartilage

This layer consists of large, maturing chondrocytes arranged in columns

Zone of resting cartilage

This layer is nearest the epiphysis and consists of small, scattered chondrocytes. The term "resting" is used because the cells do not function in bone growth. Rather, they anchor the epiphyseal plate to the epiphysis of the bone

Endochondral ossification cont.

involves replacement of cartilage by bone and forms most of the bones of the body 1 Development of the cartilage model. At the site where the bone is going to form, specific chemical messages cause the cells in mesenchyme to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model (future diaphysis) consisting of hyaline cartilage. A covering called the perichondrium develops around the cartilage model. 2 Growth of the cartilage model. Once chondroblasts become deeply buried in the cartilage extracellular matrix, they are called chondrocytes. The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix. This type of cartilaginous growth, called interstitial (endogenous) growth (growth from within), results in an increase in length. In contrast, growth of the cartilage in thickness is due mainly to the deposition of extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium. This process is called appositional (exogenous) growth As the cartilage model continues to grow, chondrocytes in its midregion hypertrophy (increase in size) and the surrounding cartilage extracellular matrix begins to calcify. Other chondrocytes within the calcifying cartilage die because nutrients can no longer diffuse quickly enough through the extracellular matrix. As these chondrocytes die, the spaces left behind by dead chondrocytes merge into small cavities called lacunae. 3 Development of the primary ossification center. Primary ossification proceeds inward from the external surface of the bone. A nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient foramen in the midregion of the cartilage model, stimulating osteoprogenitor cells in the perichondrium to differentiate into osteoblasts. Once the perichondrium starts to form bone, it is known as the periosteum. Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, inducing growth of a primary ossification center, a region where bone tissue will replace most of the cartilage. Osteoblasts then begin to deposit bone extracellular matrix over the remnants of calcified cartilage, forming spongy bone trabeculae. Primary ossification spreads from this central location toward both ends of the cartilage model. 4Development of the medullary (marrow) cavity. As the primary ossification center grows toward the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabeculae. This activity leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft). Eventually, most of the wall of the diaphysis is replaced by compact bone. 5Development of the secondary ossification centers. When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop, usually around the time of birth. Bone formation is similar to what occurs in primary ossification centers. However, in the secondary ossification centers spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here). In contrast to primary ossification, secondary ossification proceeds outward from the center of the epiphysis toward the outer surface of the bone. 6Formation of articular cartilage and the epiphyseal (growth) plate. The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones that you will learn about next.