In a long bone, for example, at about 6 to 8 weeks after conception, some of the mesenchymal cells differentiate into chondroblasts cartilage cells that form the hyaline cartilaginous skeletal precursor of the bones Figure 6. This cartilage is a flexible, semi-solid matrix produced by chondroblasts and consists of hyaluronic acid, chondroitin sulfate, collagen fibers, and water.
As the matrix surrounds and isolates chondroblasts, they are called chondrocytes. Unlike most connective tissues, cartilage is avascular, meaning that it has no blood vessels supplying nutrients and removing metabolic wastes. All of these functions are carried on by diffusion through the matrix from vessels in the surrounding perichondrium , a membrane that covers the cartilage, a.
As more and more matrix is produced, the cartilaginous model grow in size. Blood vessels in the perichondrium bring osteoblasts to the edges of the structure and these arriving osteoblasts deposit bone in a ring around the diaphysis — this is called a bone collar Figure 6. The bony edges of the developing structure prevent nutrients from diffusing into the center of the hyaline cartilage. This results in chondrocyte death and disintegration in the center of the structure.
Without cartilage inhibiting blood vessel invasion, blood vessels penetrate the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many of which will become osteoblasts.
These enlarging spaces eventually combine to become the medullary cavity. Bone is now deposited within the structure creating the primary ossification center Figure 6. This continued growth is accompanied by remodeling inside the medullary cavity osteoclasts were also brought with invading blood vessels and overall lengthening of the structure Figure 6. By the time the fetal skeleton is fully formed, cartilage remains at the epiphyses and at the joint surface as articular cartilage.
After birth, this same sequence of events matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum, and seeding with osteogenic cells that become osteoblasts occurs in the epiphyseal regions, and each of these centers of activity is referred to as a secondary ossification center Figure 6.
Throughout childhood and adolescence, there remains a thin plate of hyaline cartilage between the diaphysis and epiphysis known as the growth or epiphyseal plate Figure 6. 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. By the second or third month of fetal life, bone cell development and ossification ramps up and creates the primary ossification center , a region deep in the periosteal collar where ossification begins Figure c.
By the time the fetal skeleton is fully formed, cartilage only remains at the joint surface as articular cartilage and between the diaphysis and epiphysis as the epiphyseal plate, the latter of which is responsible for the longitudinal growth of bones.
After birth, this same sequence of events matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum, and seeding with osteogenic cells that become osteoblasts occurs in the epiphyseal regions, and each of these centers of activity is referred to as a secondary ossification center Figure e.
The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length.
The epiphyseal plate is composed of four zones of cells and activity Figure. 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 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. 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. 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 the cartilage, longitudinal growth stops.
All that remains of the epiphyseal plate is the epiphyseal line Figure. How Bones Grow in Diameter While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases.
This is called appositional growth. Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum.
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. This process is called modeling. The process in which matrix is resorbed on one surface of a bone and deposited on another is known as bone modeling. However, in adult life, bone undergoes 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.
Skeletal System Osteogenesis imperfecta OI is a genetic disease in which bones do not form properly and therefore are fragile and break easily.
It is also called brittle bone disease. The disease is present from birth and affects a person throughout life. The severity of the disease can range from mild to severe. Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone. This forms a primary ossification center. Ossification continues from this center toward the ends of the bones. After spongy bone is formed in the diaphysis, osteoclasts break down the newly formed bone to open up the medullary cavity.
The cartilage in the epiphyses continues to grow so the developing bone increases in length. Later, usually after birth, secondary ossification centers form in the epiphyses. Ossification in the epiphyses is similar to that in the diaphysis except that the spongy bone is retained instead of being broken down to form a medullary cavity.
When secondary ossification is complete, the hyaline cartilage is totally replaced by bone except in two areas.
A region of hyaline cartilage remains over the surface of the epiphysis as the articular cartilage and another area of cartilage remains between the epiphysis and diaphysis. This is the epiphyseal plate or growth region. The resorbed matrix is reduced to thin septa between the chondrocytes. The calcified cartilage zone: chondrocytes undergo apoptosis, the thin septa of cartilage matrix become calcified.
The ossification zone: endochondral bone tissue appears. Blood capillaries and osteoprogenitor cells from the periosteum invade the cavities left by the chondrocytes. The osteoprogenitor cells form osteoblasts, which deposit bone matrix over the three-dimensional calcified cartilage matrix.
Epiphyseal plate growth. Five zones of epiphyseal growth plate includes: 1. When bones are increasing in length, they are also increasing in diameter; diameter growth can continue even after longitudinal growth stops. This is called appositional growth. The bone is absorbed on the endosteal surface and added to the periosteal surface.
Osteoblasts and osteoclasts play an essential role in appositional bone growth where osteoblasts secrete a bone matrix to the external bone surface from diaphysis, while osteoclasts on the diaphysis endosteal surface remove bone from the internal surface of diaphysis. The more bone around the medullary cavity is destroyed, the more yellow marrow moves into empty space and fills space.
Osteoclasts resorb the old bone lining the medullary cavity, while osteoblasts through intramembrane ossification produce new bone tissue beneath the periosteum. Periosteum on the bone surface also plays an important role in increasing thickness and in reshaping the external contour. The erosion of old bone along the medullary cavity and new bone deposition under the periosteum not only increases the diameter of the diaphysis but also increases the diameter of the medullary cavity.
This process is called modeling Figure 9 [ 3 , 4 , 15 ]. Appositional bone growth. Bone deposit by osteoblast as bone resorption by osteoclast. Recent research reported that bone microstructure is also the principle of bone function, which regulates its mechanical function. Bone tissue function influenced by many factors, such as hormones, growth factors, and mechanical loading. The microstructure of bone tissue is distribution and alignment of biological apatite BAp crystallites.
This is determined by the direction of bone cell behavior, for example cell migration and cell regulation. Ozasa et al. Generally, bone is formed by endochondral or intramembranous ossification. Intramembranous ossification is essential in the bone such as skull, facial bones, and pelvis which MSCs directly differentiate to osteoblasts. While, endochondral ossification plays an important role in most bones in the human skeleton, including long, short, and irregular bones, which MSCs firstly experience to condensate and then differentiate into chondrocytes to form the cartilage growth plate and the growth plate is then gradually replaced by new bone tissue [ 3 , 8 , 12 ].
MSC migration and differentiation are two important physiological processes in bone formation. MSCs migration raise as an essential step of bone formation because MSCs initially need to migrate to the bone surface and then contribute in bone formation process, although MSCs differentiation into osteogenic cells is also crucial. MSC migration during bone formation has attracted more attention.
Some studies show that MSC migration to the bone surface is crucial for bone formation [ 17 ]. Bone marrow and periosteum are the main sources of MSCs that participate in bone formation [ 18 ].
In the intramembranous ossification, MSCs undergo proliferation and differentiation along the osteoblastic lineage to form bone directly without first forming cartilage. MSC and preosteoblast migration is involved in this process and are mediated by plentiful factors in vivo and in vitro. MSCs initially differentiate into preosteoblasts which proliferate near the bone surface and secrete ALP. Then they become mature osteoblasts and then form osteocytes which embedded in an extracellular matrix ECM.
In the endochondral ossification, MSCs are first condensed to initiate cartilage model formation. During condensation, the central part of MSCs differentiates into chondrocytes and secretes cartilage matrix. While, other cells in the periphery, form the perichondrium that continues expressing type I collagen and other important factors, such as proteoglycans and ALP.
Chondrocytes undergo rapid proliferation. Chondrocytes in the center become maturation, accompanied with an invasion of hypertrophic cartilage by the vasculature, followed by differentiation of osteoblasts within the perichondrium and marrow cavity. The inner perichondrium cells differentiate into osteoblasts, which secrete bone matrix to form the bone collar after vascularization in the hypertrophic cartilage.
During bone formation, woven bone haphazard arrangement of collagen fibers is remodeled into lamellar bones parallel bundles of collagen in a layer known as lamellae.
Periosteum is a connective tissue layer on the outer surface of the bone; the endosteum is a thin layer generally only one layer of cell that coats all the internal surfaces of the bone.
Major cell of bone include: osteoblasts from osteoprogenitor cells, forming osteoid that allow matrix mineralization to occur , osteocytes from osteoblasts; closed to lacunae and retaining the matrix and osteoclasts from hemopoietic lineages; locally erodes matrix during bone formation and remodeling. Intramembranous bone formation occurs when bone forms inside the mesenchymal membrane. Bone tissue is directly laid on primitive connective tissue referred to mesenchyma without intermediate cartilage involvement.
It forms bone of the skull and jaw; especially only occurs during development as well as the fracture repair. Endochondral bone formation occurs when hyaline cartilage is used as a precursor to bone formation, then bone replaces hyaline cartilage, forms and grows all other bones, occurs during development and throughout life.
During interstitial epiphyseal growth elongation of the bone , the growth plate with zonal organization of endochondral ossification, allows bone to lengthen without epiphyseal growth plates enlarging zones include:. During appositional growth, osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum.
Mesenchymal stem cell migration and differentiation are two important physiological processes in bone formation. The author is grateful to Zahrona Kusuma Dewi for assistance with preparation of the manuscript. The authors declare that there is no conflict of interests regarding the publication of this paper. Licensee IntechOpen.
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Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Abstract The process of bone formation is called osteogenesis or ossification. Keywords osteogenesis ossification bone formation intramembranous ossification endochondral ossification. Table 1. Mesenchymal cells differentiate into chondrocytes cartilage cells. The perichondrial membrane surrounds the surface and develops new chondroblasts.
Chondroblasts produce growth in width appositional growth. Cells at the center of the cartilage lyse break apart triggers calcification. More Print chapter. How to cite and reference Link to this chapter Copy to clipboard.
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