The Cells Responsible for the Early Stages of Endochondral Ossification

The Cells Responsible for the Early Stages of Endochondral Ossification.


The process of bone germination is called osteogenesis or ossification. Afterward progenitor cells class osteoblastic lines, they proceed with iii stages of development of cell differentiation, chosen proliferation, maturation of matrix, and mineralization. Based on its embryological origin, there are ii types of ossification, called intramembranous ossification that occurs in mesenchymal cells that differentiate into osteoblast in the ossification center directly without prior cartilage formation and endochondral ossification in which bone tissue mineralization is formed through cartilage formation first. In intramembranous ossification, bone evolution occurs direct. In this process, mesenchymal cells proliferate into areas that have loftier vascularization in embryonic connective tissue in the formation of cell condensation or principal ossification centers. This cell volition synthesize bone matrix in the periphery and the mesenchymal cells continue to differentiate into osteoblasts. After that, the bone volition be reshaped and replaced by mature lamellar os. Endochondral ossification will form the center of primary ossification, and the cartilage extends by proliferation of chondrocytes and degradation of cartilage matrix. Later this formation, chondrocytes in the central region of the cartilage start to proceed with maturation into hypertrophic chondrocytes. After the main ossification center is formed, the marrow cavity begins to expand toward the epiphysis. Then the subsequent stages of endochondral ossification will take place in several zones of the bone.


  • osteogenesis
  • ossification
  • bone germination
  • intramembranous ossification
  • endochondral ossification

1. Introduction

Os is living tissue that is the hardest amidst other connective tissues in the body, consists of 50% h2o. The solid part remainder consisting of various minerals, peculiarly 76% of calcium salt and 33% of cellular material. Bone has vascular tissue and cellular activity products, especially during growth which is very dependent on the blood supply every bit bones source and hormones that greatly regulate this growth process. Bone-forming cells, osteoblasts, osteoclast play an important part in determining bone growth, thickness of the cortical layer and structural arrangement of the lamellae.

Bone continues to modify its internal structure to reach the functional needs and these changes occur through the activeness of osteoclasts and osteoblasts. The os seen from its evolution can be divided into ii processes: commencement is the intramembranous ossification in which bones form directly in the grade of primitive mesenchymal connective tissue, such as the mandible, maxilla and skull basic. 2nd is the endochondral ossification in which bone tissue replaces a preexisting hyaline cartilage, for instance during skull base formation. The same formative cells form ii types of bone formation and the concluding structure is non much dissimilar.

Os growth depends on genetic and environmental factors, including hormonal effects, nutrition and mechanical factors. The growth rate is not e’er the same in all parts, for example, faster in the proximal finish than the distal humerus because the internal design of the spongiosum depends on the direction of bone pressure. The direction of bone germination in the epiphysis plane is determined by the management and distribution of the pressure level line. Increased thickness or width of the bone is caused by deposition of new bone in the grade of circumferential lamellae under the periosteum. If os growth continues, the lamella will be embedded behind the new bone surface and be replaced by the haversian culvert system.


two. Bone cells and matrix

Os is a tissue in which the extracellular matrix has been hardened to accommodate a supporting part. The fundamental components of bone, like all connective tissues, are cells and matrix. Although bone cells compose a small amount of the os volume, they are crucial to the part of bones. Iv types of cells are plant within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts. They each unique functions and are derived from two unlike cell lines (Effigy 1 and Table 1) [one, 2, 3, 4, 5, half-dozen, 7].

  • Osteoblast synthesizes the bone matrix and are responsible for its mineralization. They are derived from osteoprogenitor cells, a mesenchymal stem cell line.

  • Osteocytes are inactive osteoblasts that have go trapped within the bone they accept formed.

  • Osteoclasts break downwards os matrix through phagocytosis. Predictably, they ruffled border, and the space between the osteoblast and the bone is known as Howship’s lacuna.

Figure 1.

Development of bone precursor cells. Bone forerunner cells are divided into developmental stages, which are i. mesenchymal stalk jail cell, two. pre-osteoblast, 3. osteoblast, and 4. mature osteocytes, and v. osteoclast.

Table one.

Bone cells, their function, and locations [i, 2, 3, 4, v, 6, vii].

The remainder between osteoblast and osteoclast activity governs bone turnover and ensures that bone is neither overproduced nor overdegraded. These cells build upwards and interruption down os matrix, which is composed of:

  • Osteoid, which is the unmineralized matrix composed of type I collagen and gylcosaminoglycans (GAGs).

  • Calcium hydroxyapatite, a calcium common salt crystal that give bone its strength and rigidity.

Os is divided into two types that are different structurally and functionally. About bones of the torso consist of both types of bone tissue (Figure two) [1, 2, viii, ix]:

  • Compact os, or cortical os, mainly serves a mechanical function. This is the area of bone to which ligaments and tendons adhere. It is thick and dense.

  • Trabecular bone, also known as cancellous os or spongy bone, mainly serves a metabolic part. This type of bone is located between layers of compact bone and is thin porous. Location within the trabeculae is the os marrow.

Figure ii.

Structure of a long os.


3. Bone structure

3.i Macroscopic os structure

Long bones are composed of both cortical and cancellous bone tissue. They consist of several areas (Figure 3) [iii, 4]:

  • The epiphysis is located at the cease of the long os and is the parts of the bone that participate in joint surfaces.

  • The diaphysis is the shaft of the bone and has walls of cortical os and an underlying network of trabecular bone.

  • The epiphyseal growth plate lies at the interface between the shaft and the epiphysis and is the region in which cartilage proliferates to cause the elongation of the bone.

  • The metaphysis is the area in which the shaft of the bone joins the epiphyseal growth plate.

Figure three.

Os macrostructure. (a) Growing long bone showing epiphyses, epiphyseal plates, metaphysis and diaphysis. (b) Mature long os showing epiphyseal lines.

Unlike areas of the bone are covered by different tissue [four]:

  • The epiphysis is lined by a layer of articular cartilage, a specialized form of hyaline cartilage, which serves as protection confronting friction in the joints.

  • The outside of the diaphysis is lined by periosteum, a fibrous external layer onto which muscles, ligaments, and tendons attach.

  • The inside of the diaphysis, at the edge between the cortical and cancellous bone and lining the trabeculae, is lined by endosteum.

3.2 Microscopic os structure

Meaty bone is organized as parallel columns, known as Haversian systems, which run lengthwise down the axis of long basic. These columns are equanimous of lamellae, concentric rings of bone, surrounding a central channel, or Haversian canal, that contains the fretfulness, blood vessels, and lymphatic system of the bone. The parallel Haversian canals are connected to one another by the perpendicular Volkmann’s canals.

The lamellae of the Haversian systems are created by osteoblasts. As these cells secrete matrix, they become trapped in spaces called lacunae and become known as osteocytes. Osteocytes communicate with the Haversian canal through cytoplasmic extensions that run through canaliculi, small interconnecting canals (Figure 4) [1, 2, eight, ix]:

Figure 4.

Os microstructure. Compact and spongy bone structures.

The layers of a long bone, offset at the external surface, are therefore:

  • Periosteal surface of compact bone

  • Outer circumferential lamellae

  • Compact bone (Haversian systems)

  • Inner circumferential lamellae

  • Endosteal surface of compact bone

  • Trabecular os


4. Os formation

Bone evolution begins with the replacement of collagenous mesenchymal tissue past bone. This results in the formation of woven bone, a primitive form of bone with randomly organized collagen fibers that is farther remodeled into mature lamellar os, which possesses regular parallel rings of collagen. Lamellar bone is then constantly remodeled by osteoclasts and osteoblasts. Based on the development of bone formation can be divided into ii parts, called endochondral and intramembranous bone formation/ossification [1, 2, iii, viii].

four.1 Intramembranous bone formation

During intramembranous os germination, the connective tissue membrane of undifferentiated mesenchymal cells changes into bone and matrix bone cells [10]. In the craniofacial cartilage basic, intramembranous ossification originates from nerve crest cells. The primeval evidence of intramembranous bone germination of the skull occurs in the mandible during the 6th prenatal week. In the eighth week, reinforcement center appears in the calvarial and facial areas in areas where in that location is a mild stress strength [11].

Intramembranous os formation is found in the growth of the skull and is besides found in the sphenoid and mandible even though it consists of endochondral elements, where the endochondral and intramembranous growth process occurs in the same bone. The basis for either bone formation or os resorption is the aforementioned, regardless of the type of membrane involved.

Sometimes co-ordinate to where the formation of bone tissue is classified equally “periosteal” or “endosteal”. Periosteal os always originates from intramembranous, merely endosteal bone can originate from intramembranous as well as endochondral ossification, depending on the location and the way it is formed [iii, 12].

4.1.1 The stage of intramembranous os germination

The statement beneath is the stage of intramembrane bone formation (Figure v) [3, 4, xi, 12]:

  1. An ossification center appears in the fibrous connective tissue membrane. Mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into specialized cells. Some of these cells differentiate into capillaries, while others will get osteogenic cells and osteoblasts, then forming an ossification center.

  2. Bone matrix (osteoid) is secreted inside the fibrous membrane. Osteoblasts produce osteoid tissue, past means of differentiating osteoblasts from the ectomesenchyme condensation center and producing bone fibrous matrix (osteoid). And then osteoid is mineralized within a few days and trapped osteoblast become osteocytes.

  3. Woven bone and periosteum form. The encapsulation of cells and blood vessels occur. When osteoid deposition by osteoblasts continues, the encased cells develop into osteocytes. Accumulating osteoid is laid downward between embryonic blood vessels, which class a random network (instead of lamellae) of trabecular. Vascularized mesenchyme condenses on external face of the woven bone and becomes the periosteum.

  4. Production of osteoid tissue by membrane cells: osteocytes lose their power to contribute directly to an increase in bone size, but osteoblasts on the periosteum surface produce more osteoid tissue that thickens the tissue layer on the existing os surface (for example, appositional bone growth). Formation of a woven os collar that is later on replaced by mature lamellar bone. Spongy bone (diploe), consisting of distinct trabeculae, persists internally and its vascular tissue becomes ruby marrow.

  5. Osteoid calcification: The occurrence of bone matrix mineralization makes bones relatively impermeable to nutrients and metabolic waste. Trapped blood vessels function to supply nutrients to osteocytes likewise as bone tissue and eliminate waste products.

  6. The germination of an essential membrane of bone which includes a membrane outside the bone called the bone endosteum. Os endosteum is very important for bone survival. Disruption of the membrane or its vascular tissue can cause bone cell death and bone loss. Bones are very sensitive to pressure level. The calcified bones are hard and relatively inflexible.

Figure v.

The stage of intramembranous ossification. The following stages are (a) Mesenchymal cells group into clusters, and ossification centers form. (b) Secreted osteoid traps osteoblasts, which so go osteocytes. (c) Trabecular matrix and periosteum course. (d) Compact bone develops superficial to the trabecular os, and crowded blood vessels condense into cerise marrow.

The matrix or intercellular substance of the bone becomes calcified and becomes a os in the cease. Bone tissue that is constitute in the periosteum, endosteum, suture, and periodontal membrane (ligaments) is an example of intramembranous bone germination [3, thirteen].

Intramembranous bone germination occurs in ii types of bone: bundle bone and lamellar bone. The os packet develops direct in connective tissue that has non been calcified. Osteoblasts, which are differentiated from the mesenchyme, secrete an intercellular substance containing collagen fibrils. This osteoid matrix calcifies by precipitating apatite crystals. Main ossification centers only show minimal bone calcification density. The apatite crystal deposits are mostly irregular and structured like nets that are contained in the medullary and cortical regions. Mineralization occurs very speedily (several tens of thousands of millimeters per 24-hour interval) and can occur simultaneously in big areas. These apatite deposits increase with time. Bone tissue is only considered mature when the crystalized area is arranged in the same direction equally collagen fibrils.

Bone tissue is divided into ii, called the outer cortical and medullary regions, these two areas are destroyed by the resorption process; which goes forth with farther bone formation. The surrounding connective tissue will differentiate into the periosteum. The lining in the periosteum is rich in cells, has osteogenic function and contributes to the formation of thick bones as in the endosteum.

In adults, the parcel bone is usually only formed during rapid bone remodeling. This is reinforced by the presence of lamellar bone. Dissimilar package os formation, lamellar os development occurs only in mineralized matrix (e.g., cartilage that has calcified or packet bone spicules). The nets in the bone package are filled to strengthen the lamellar bone, until compact bone is formed. Osteoblasts appear in the mineralized matrix, which so form a circle with intercellular thing surrounding the central vessels in several layers (Haversian arrangement). Lamella bone is formed from 0.vii to 1.5 microns per day. The network is formed from complex cobweb arrangements, responsible for its mechanical properties. The arrangement of apatites in the concentric layer of fibrils finally meets functional requirements. Lamellar bone depends on ongoing degradation and resorption which tin exist influenced past environmental factors, one of this which is orthodontic treatment.

4.1.2 Factors that influence intramembranous bone formation

Intramembranous bone formation from desmocranium (suture and periosteum) is mediated past mesenchymal skeletogenetic structures and is accomplished through bone deposition and resorption [8]. This development is almost entirely controlled through local epigenetic factors and local environmental factors (i.east. past muscle strength, external local pressure, encephalon, optics, tongue, nerves, and indirectly by endochondral ossification). Genetic factors just have a nonspecific morphogenetic effect on intramembranous bone formation and only determine external limits and increase the number of growth periods. Bibelot disorder (specially genetically produced) can affect endochondral os formation, so local epigenetic factors and local environmental factors, including steps of orthodontic therapy, tin directly impact intramembranous bone formation [3, eleven].

four.2 Endochondral os formation

During endochondral ossification, the tissue that will become bone is firstly formed from cartilage, separated from the joint and epiphysis, surrounded by perichondrium which and so forms the periosteum [11]. Based on the location of mineralization, it can exist divided into: Perichondral Ossification and Endochondral Ossification. Both types of ossification play an essential part in the formation of long bones where but endochondral ossification takes place in short bones. Perichondral ossification begins in the perichondrium. Mesenchymal cells from the tissue differentiate into osteoblasts, which surround bony diaphyseal before endochondral ossification, indirectly touch on its direction [3, 8, 12]. Cartilage is transformed into os is craniofacial os that forms at the eigth prenatal week. Just bone on the cranial base and part of the skull bone derived from endochondral bone formation. Regarding to differentiate endochondral os germination from chondrogenesis and intramembranous os formation, v sequences of bone formation steps were adamant [3].

4.two.ane The stages of endochondral os formation

The statements beneath are the stages of endochondral bone germination (Figure 6) [iv, 12]:

  1. Mesenchymal cells group to form a shape template of the future bone.

  2. Mesenchymal cells differentiate into chondrocytes (cartilage cells).

  3. Hypertrophy of chondrocytes and calcified matrix with calcified central cartilage primordium matrix formed. Chondrocytes show hypertrophic changes and calcification from the cartilage matrix continues.

  4. Entry of claret vessels and connective tissue cells. The nutrient artery supplies the perichondrium, breaks through the nutrient foramen at the mid-region and stimulates the osteoprogenitor cells in the perichondrium to produce osteoblasts, which changes the perichondrium to the periosteum and starts the formation of ossification centers.

  5. The periosteum continues its development and the division of cells (chondrocytes) continues also, thereby increasing matrix production (this helps produce more length of bone).

  6. The perichondrial membrane surrounds the surface and develops new chondroblasts.

  7. Chondroblasts produce growth in width (appositional growth).

  8. Cells at the centre of the cartilage lyse (break apart) triggers calcification.

Figure half dozen.

The stage of endochondral ossification. The following stages are: (a) Mesenchymal cells differentiate into chondrocytes. (b) The cartilage model of the hereafter bony skeleton and the perichondrium course. (c) Capillaries penetrate cartilage. Perichondrium transforms into periosteum. Periosteal collar develops. Primary ossification centre develops. (d) Cartilage and chondrocytes continue to abound at ends of the bone. (e) Secondary ossification centers develop. (f) Cartilage remains at epiphyseal (growth) plate and at articulation surface every bit articular cartilage.

During endochondral os formation, mesenchymal tissue firstly differentiates into cartilage tissue. Endochondral bone formation is morphogenetic adaptation (normal organ development) which produces continuous bone in sure areas that are prominently stressed. Therefore, this endochondral os formation tin be constitute in the bones associated with articulation movements and some parts of the skull base. In hypertrophic cartilage cells, the matrix calcifies and the cells undergo degeneration. In cranial synchondrosis, in that location is proliferation in the formation of basic on both sides of the bone plate, this is distinguished past the formation of long bone epiphyses which only occurs on one side only [ii, xiv].

Every bit the cartilage grows, capillaries penetrate it. This penetration initiates the transformation of the perichondrium into the bone-producing periosteum. Here, the osteoblasts form a periosteal neckband of meaty os effectually the cartilage of the diaphysis. Past the second or third month of fetal life, bone cell development and ossification ramps up and creates the

primary ossification heart
, a region deep in the periosteal collar where ossification begins [4, 10].

While these deep changes occur, chondrocytes and cartilage continue to grow at the ends of the os (the future epiphyses), which increment the os length and at the same fourth dimension bone as well replaces cartilage in the diaphysis. By the time the fetal skeleton is fully formed, cartilage only remains at the joint surface every bit articular cartilage and between the diaphysis and epiphysis as the epiphyseal plate, the latter of which is responsible for the longitudinal growth of basic. 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) occur in the epiphyseal regions, and each of these centers of activity is referred to as a

secondary ossification heart

[iv, 8, 10].

There are iv important things about cartilage in endochondral bone germination:

  1. Cartilage has a rigid and firm structure, but not usually calcified nature, giving three basic functions of growth (a) its flexibility tin support an appropriate network structure (nose), (b) force per unit area tolerance in a particular place where compression occurs, (c) the location of growth in conjunction with enlarging bone (synchondrosis of the skull base and condyle cartilage).

  2. Cartilage grows in two next places (past the action of the chondrogenic membrane) and grows in the tissues (chondrocyte cell partition and the addition of its intercellular matrix).

  3. Bone tissue is not the same as cartilage in terms of its tension adaptation and cannot grow directly in areas of high pinch because its growth depends on the vascularization of os formation covering the membrane.

  4. Cartilage growth arises where linear growth is required toward the force per unit area direction, which allows the bone to lengthen to the area of strength and has not yet grown elsewhere by membrane ossification in conjunction with all periosteal and endosteal surfaces.

four.2.2 Factors that influence endochondral ossification

Membrane disorders or vascular supply problem of these essential membranes tin directly result in bone cell death and ultimately bone damage. Calcified bones are more often than not hard and relatively inflexible and sensitive to pressure [12].

Cranial synchondrosis (e.g., spheno ethmoidal and spheno occipital growth) and endochondral ossification are further determined by chondrogenesis. Chondrogenesis is mainly influenced by genetic factors, similar to facial mesenchymal growth during initial embryogenesis to the differentiation phase of cartilage and cranial bone tissue.

This procedure is but slightly afflicted past local epigenetic and environmental factors. This tin can explain the fact that the cranial base is more than resistant to deformation than desmocranium. Local epigenetic and ecology factors cannot trigger or inhibit the amount of cartilage formation. Both of these have trivial effect on the shape and direction of endochondral ossification. This has been analyzed especially during mandibular condyle growth.

Local epigenetics and environmental factors only bear upon the shape and direction of cartilage formation during endochondral ossification Considering the fact that condyle cartilage is a secondary cartilage, it is assumed that local factors provide a greater influence on the growth of mandibular condyle.

iv.two.3 Chondrogenesis

Chondrogenesis is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondrocytes and begins secreting the molecules that form the extracellular matrix [5, 14].

The statement below is five steps of chondrogenesis [eight, 14]:

  1. Chondroblasts produce a matrix: the extracellular matrix produced by cartilage cells, which is firm but flexible and capable of providing a rigid back up.

  2. Cells become embed in a matrix: when the chondroblast changes to be completely embed in its ain matrix material, cartilage cells turn into chondrocytes. The new chondroblasts are distinguished from the membrane surface (perichondrium), this will consequence in the add-on of cartilage size (cartilage can increase in size through apposition growth).

  3. Chondrocytes enlarge, split and produce a matrix. Cell growth continues and produces a matrix, which causes an increment in the size of cartilage mass from within. Growth that causes size increase from the within is called interstitial growth.

  4. The matrix remains uncalcified: cartilage matrix is rich of chondroitin sulfate which is associated with non-collagen proteins. Nutrition and metabolic waste material are discharged directly through the soft matrix to and from the cell. Therefore, blood vessels aren’t needed in cartilage.

  5. The membrane covers the surface but is not essential: cartilage has a closed membrane vascularization chosen perichondrium, but cartilage can be without any of these. This belongings makes cartilage able to grow and adapt where it needs pressure (in the joints), so that cartilage can receive pressure.

Endochondral ossification begins with characteristic changes in cartilage bone cells (hypertrophic cartilage) and the surround of the intercellular matrix (calcium laying), the formation which is called as primary spongiosa. Blood vessels and mesenchymal tissues and then penetrate into this surface area from the perichondrium. The binding tissue cells then differentiate into osteoblasts and cells. Chondroblasts erode cartilage in a cavern-like pattern (cavity). The remnants of mineralized cartilage the central part of laying the lamellar bone layer.

The osteoid layer is deposited on the calcified spicules remaining from the cartilage and and so mineralized to form spongiosa bone, with fine reticular structures that resemble nets that possess cartilage fragments between the spicular bones. Spongy bones can turn into meaty bones by filling empty cavities. Both endochondral and perichondral bone growth both take place toward epiphyses and joints. In the bone lengthening process during endochondral ossification depends on the growth of epiphyseal cartilage. When the epiphyseal line has been airtight, the bone volition not increment in length. Unlike bone, cartilage bone growth is based on apposition and interstitial growth. In areas where cartilage bone is covered by os, various variations of zone characteristics, based on the developmental stages of each private, can differentiate which then continuously merge with each other during the conversion procedure. Environmental influences (co: mechanism of orthopedic functional tools) accept a strong effect on condylar cartilage because the bone is located more superficially [5].


5. Os growth

Cartilage bone height development occurs during the third calendar month of intra uterine life. Cartilage plate extends from the nasal bone capsule posteriorly to the foramen magnum at the base of the skull. It should be noted that cartilages which close to avascular tissue accept internal cells obtained from the diffusion procedure from the outermost layer. This means that the cartilage must exist flatter. In the early stages of development, the size of a very small embryo can grade a chondroskeleton easily in which the further growth preparation occurs without internal blood supply [i].

During the fourth month in the uterus, the development of vascular elements to various points of the chondrocranium (and other parts of the early cartilage skeleton) becomes an ossification center, where the cartilage changes into an ossification center, and bone forms effectually the cartilage. Cartilage continues to grow rapidly but information technology is replaced by bone, resulting in the rapid increment of bone amount. Finally, the former chondrocranium amount will decrease in the area of cartilage and big portions of bone, causeless to exist typical in ethmoid, sphenoid, and basioccipital basic. The cartilage growth in relation to skeletal bone is similar every bit the growth of the limbs [1, 3].

Longitudinal os growth is accompanied by remodeling which includes appositional growth to thicken the os. This process consists of bone formation and reabsorption. Bone growth stops around the age of 21 for males and the age of 18 for females when the epiphyses and diaphysis have fused (epiphyseal plate closure).

Normal os growth is dependent on proper dietary intake of protein, minerals and vitamins. A deficiency of vitamin D prevents calcium absorption from the GI tract resulting in rickets (children) or osteomalacia (adults). Osteoid is produced but calcium salts are not deposited, so bones soften and weaken.

five.1 Oppositional bone growth

At the length of the long bones, the reinforcement airplane appears in the middle and at the end of the bone, finally produces the central axis that is called the diaphysis and the bony cap at the end of the bone is called the epiphysis. Between epiphyses and diaphysis is a calcified area that is non calcified chosen the epiphyseal plate. Epiphyseal plate of the long os cartilage is a major center for growth, and in fact, this cartilage is responsible for near all the long growths of the bones. This is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, the cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis and so grows in length. The epiphyseal plate is composed of five zones of cells and activity [iii, 4].

Near the outer end of each epiphyseal plate is the active zone dividing the cartilage cells. Some of them, pushed toward diaphysis with proliferative activity, develop hypertrophy, secrete an extracellular matrix, and finally the matrix begins to fill up with minerals so is speedily replaced by bone. Every bit long equally cartilage cells multiply growth will go along. Finally, toward the stop of the normal growth flow, the rate of maturation exceeds the proliferation level, the latter of the cartilage is replaced by bone, and the epiphyseal plate disappears. At that time, bone growth is consummate, except for surface changes in thickness, which can be produced by the periosteum [4]. Bones go along to grow in length until early on adulthood. The lengthening is stopped in the end of adolescence which chondrocytes stop mitosis and plate thins out and replaced past os, so diaphysis and epiphyses fuse to be 1 os (Figure 7). The rate of growth is controlled by hormones. When the chondrocytes in the epiphyseal plate cease their proliferation and os replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line. Epiphyseal plate closure will occur in xviii-year former females or 21-year old males.

Figure 7.

Oppositional os growth and remodeling. The epiphyseal plate is responsible for longitudinal os growth.

5.ane.1 Epiphyseal plate growth

The cartilage found in the epiphyseal gap has a defined hierarchical structure, directly beneath the secondary ossification center of the epiphysis. By close examination of the epiphyseal plate, it appears to be divided into five zones (starting from the epiphysis side) (Figure 8) [4]:

  1. The resting zone: it contains hyaline cartilage with few chondrocytes, which means no morphological changes in the cells.

  2. The proliferative zone: chondrocytes with a higher number of cells divide speedily and form columns of stacked cells parallel to the long axis of the os.

  3. The hypertrophic cartilage zone: information technology contains big chondrocytes with cells increasing in volume and modifying the matrix, effectively elongating bone whose cytoplasm has accumulated glycogen. The resorbed matrix is reduced to thin septa between the chondrocytes.

  4. The calcified cartilage zone: chondrocytes undergo apoptosis, the thin septa of cartilage matrix become calcified.

  5. 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 eolith bone matrix over the three-dimensional calcified cartilage matrix.

Figure 8.

Epiphyseal plate growth. Five zones of epiphyseal growth plate includes: 1. resting zone, 2. proliferation zone, three. hypertrophic cartilage zone, iv. calcified cartilage zone, and 5. ossification zone.

5.2 Appositional bone growth

When basic are increasing in length, they are also increasing in diameter; diameter growth can go along even subsequently longitudinal growth stops. This is called appositional growth. The bone is captivated 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 effectually 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 profile. The erosion of onetime bone along the medullary cavity and new bone deposition nether the periosteum non only increases the bore of the diaphysis simply too increases the bore of the medullary cavity. This process is called modeling (Figure 9) [3, 4, fifteen].

Figure nine.

Appositional bone growth. Bone eolith by osteoblast as os resorption by osteoclast.


6. The role of mesenchymal stem cell migration and differentiation in bone formation

Recent enquiry reported that bone microstructure is also the principle of bone part, which regulates its mechanical function. Bone tissue function influenced past many factors, such equally hormones, growth factors, and mechanical loading. The microstructure of os tissue is distribution and alignment of biological apatite (BAp) crystallites. This is adamant past the direction of bone cell behavior, for example cell migration and jail cell regulation. Ozasa et al. found that bogus command the direction of mesenchymal stem cell (MSCs) migration and osteoblast alignment can reconstruct bone microstructure, which guide an appropriate bone formation during bone remodeling and regeneration [sixteen].

Bone evolution begins with the replacement of collagenous mesenchymal tissue by bone. More often than not, os is formed by endochondral or intramembranous ossification. Intramembranous ossification is essential in the bone such equally skull, facial bones, and pelvis which MSCs direct differentiate to osteoblasts. While, endochondral ossification plays an important role in well-nigh bones in the human being skeleton, including long, short, and irregular bones, which MSCs firstly experience to condensate and and then differentiate into chondrocytes to form the cartilage growth plate and the growth plate is then gradually replaced past new bone tissue [3, 8, 12].

MSC migration and differentiation are two important physiological processes in os formation. MSCs migration raise every bit an essential step of bone formation because MSCs initially need to migrate to the bone surface and then contribute in bone germination 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 os formation [17]. Bone marrow and periosteum are the chief sources of MSCs that participate in bone formation [xviii].

In the intramembranous ossification, MSCs undergo proliferation and differentiation along the osteoblastic lineage to class os 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 and then form osteocytes which embedded in an extracellular matrix (ECM). Other factors as well regulate the intramembranous ossification of MSCs such as Runx2, special AT-rich sequence binding poly peptide 2 (SATB 2), and Osterix likewise as pathways, like the wnt/β-catenin pathway and bone morphogenetic protein (BMP) pathway [17, 19].

In the endochondral ossification, MSCs are outset condensed to initiate cartilage model formation. The process is mediated by BMPs through phosphorylating and activating receptor SMADs to transduce signals. During condensation, the cardinal 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 go maturation, accompanied with an invasion of hypertrophic cartilage by the vasculature, followed past differentiation of osteoblasts inside the perichondrium and marrow cavity. The inner perichondrium cells differentiate into osteoblasts, which secrete bone matrix to form the os collar after vascularization in the hypertrophic cartilage. Many factors that regulate endochondral ossification are growth factors (GFs), transforming growth cistron-β (TGF-β), Sry-related high-mobility group box ix (Sox9) and Cell-to-cell interaction [17, 19].


vii. Conclusions

  • Osteogenesis/ossification is the process in which new layers of bone tissue are placed by osteoblasts.

  • During bone formation, woven bone (haphazard organisation 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 (more often than not only one layer of prison 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.

  • The process of os formation occurs through two basic mechanisms:

    • Intramembranous os germination occurs when os forms within 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 simply occurs during development likewise every bit the fracture repair.

    • Endochondral bone formation occurs when hyaline cartilage is used equally a precursor to bone germination, then bone replaces hyaline cartilage, forms and grows all other bones, occurs during evolution 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:

    • Zone of resting.

    • Zone of proliferation.

    • Zone of hypertrophy.

    • Zone of calcification.

    • Zone of ossification and resorption.

  • During appositional growth, osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new os tissue below the periosteum.

  • Mesenchymal stem cell migration and differentiation are two important physiological processes in bone formation.



The writer is grateful to Zahrona Kusuma Dewi for assistance with grooming of the manuscript.


Disharmonize of interest

The authors declare that there is no conflict of interests regarding the publication of this paper.


Acronyms and abbreviations


alkaline phosphatase


biological apatite


bone morphogenetic protein


extracellular matrix


growth factors


mesenchymal stem cells


runt-related transcription cistron ii


special AT-rich sequence bounden protein two


sry-related high-mobility grouping box 9


transforming growth factor-β


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Written By

Rosy Setiawati and Paulus Rahardjo

Submitted: September twelfth, 2018
Reviewed: November 8th, 2018
Published: December 14th, 2018

The Cells Responsible for the Early Stages of Endochondral Ossification