6. BONE FORMATION

6. BONE FORMATION

• The process by which bone forms is called ossification (ossi- bone; -fication making) or osteogenesis

 Bone formation occurs in four principal situations: 

(1) the initial formation of bones in an embryo and fetus, 

(2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, 

(3) the remodeling of bone (replacement of old bone by new bone tissue throughout life), and 

(4) the repair of fractures (breaks in bones) throughout life.


Initial Bone Formation in an Embryo and Fetus

• The embryonic “skeleton,” initially composed of mesenchyme shaped like bones, 
• is the site where cartilage formation and ossification occur during the sixth week of embryonic development. 

Bone formation follows one of two patterns.

• The two methods of bone formation, which both involve the replacement of a preexisting connective tissue with bone, 

• do not lead to differences in the structure of mature bones, 
• but are simply different methods of bone development.

1.  In the first type of ossification, called intramembranous ossification ( intra- within; membran- membrane),

• bone forms directly within mesenchyme arranged in sheet like layers that resemble membranes. 

2.  In the second type, endochondral ossification ( endo- within; -chondral cartilage), 

• bone forms within hyaline cartilage that develops from mesenchyme.

Intramembranous Ossification

• Intramembranous ossification is the simpler of the two methods of bone formation. 
• The flat bones of the skull and mandible (lower jawbone) are formed in this way. 
• Also, the “soft spots” that help the fetal skull pass through the birth canal later harden as they undergo intramembranous ossification, which occurs as follows

1.  Development of the ossification center. 

At the site where the bone will develop, 

• specific chemical messages cause the mesenchymal cells to cluster together and differentiate, 
• first into osteogenic cells 
• and then into osteoblasts. 
• ( mesenchyme is the tissue from which almost all other connective tissues arise.) 
• 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. 
• Blood vessels grow into the spaces between the trabeculae. 
• Connective tissue that is 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.

Endochondral Ossification

• The replacement of cartilage by bone is called endochondral ossification. 
• Although most bones of the body are formed in this way, the process is best observed in a long bone. 

It proceeds as follows 

1. Development of the cartilage model. 

• At the site where the bone is going to form, specific chemical messages cause the mesenchymal cells to crowd together in the shape of the future bone, 
• and then develop into chondroblasts. 
• The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model 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 by continual cell division of chondrocytes accompanied by further secretion of the cartilage extracellular matrix. 

• This type of growth is termed interstitial growth (growth from within) 
• and results in an increase in length. 

• The cartilage model also grows by the addition of more extracellular matrix material to the periphery of the model by new chondroblasts that develop from the perichondrium. 
• This growth pattern is called appositional growth (growth at the outer surface) 
• and results in an increase in thickness. 

• As the cartilage model continues to grow, chondrocytes in its mid-region 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, lacunae form and eventually merge into small cavities.

3. Development of the primary ossification center. 

• A nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient foramen in the mid-region of the cartilage model, 
• stimulating cells in the perichondrium to differentiate into osteoblasts instead of chondroblasts.

• 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 toward both ends of the cartilage model.

4. Development 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.

5. Development 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 that in primary ossification centers. 
• One difference, however, is that spongy bone remains in the interior of the epiphyses 
• (no medullary cavities are formed here).

6. Formation of articular cartilage and the epiphyseal 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, 
• which is responsible for the lengthwise growth of long bones 

Bone Growth During Infancy, Childhood, and Adolescence

• During infancy, childhood, and adolescence, long bones grow in length and bones throughout the body grow in thickness.

Growth in Length

• The growth in length of long bones involves 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.

The structure of the epiphyseal plate

•  The epiphyseal (growth) plate  is a layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones 

1. 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.

2. 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.

3. Zone of hypertrophic cartilage 

• This layer consists of large, maturing chondrocytes arranged in columns.

4. 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.

•  endochondral ossification is the replacement of cartilage with bone. 

• As a result, the zone of calcified cartilage becomes “new diaphysis” that is firmly cemented to the rest of the diaphysis of the bone.
• The activity of the epiphyseal (growth) plate is the only way that the diaphysis can increase in length. 

As a bone grows, 

• new chondrocytes are formed on the epiphyseal side of the plate,
• while old chondrocytes on the diaphyseal side of the plate are replaced by bone. 
• In this way the thickness of the epiphyseal (growth) plate remains relatively constant, 
• but the bone on the diaphyseal side increases in length 

• At about age 18 in females and 21 in males, the epiphyseal (growth) plates close; the epiphyseal cartilage cells stop dividing, and bone replaces all the cartilage. 
• The epiphyseal plate fades, leaving a bony structure called the epiphyseal line. 
• The appearance of the epiphyseal line signifies that the bone has stopped growing in length. 

• The clavicle is the last bone to stop growing. 

• If a bone fracture damages the epiphyseal (growth) plate, the fractured bone may be shorter than normal once adult stature is reached. 
• This is because damage to cartilage accelerates closure of the epiphyseal plate,
•  thus inhibiting lengthwise growth of the bone.

Growth in Thickness

• Bones grow in thickness by appositional growth (growth at the outer surface). 
• At the bone surface, cells in the periosteum differentiate into osteoblasts, which secrete bone extracellular matrix.
• Then the osteoblasts develop into osteocytes, lamellae are added to the surface of the bone, and new osteons of compact bone tissue are formed. 
• At the same time, osteoclasts in the endosteum destroy the bone tissue lining the medullary cavity.

• Bone destruction on the inside of the bone by osteoclasts occurs at a slower rate than bone formation on the outside of the bone.
• Thus the medullary cavity enlarges as the bone increases in thickness.

Remodeling of Bone

• Like skin, bone forms before birth but continually renews itself thereafter. 
• Bone remodeling is the ongoing replacement of old bone tissue by new bone tissue. 
• It involves 

1. bone resorption, the removal of minerals and collagen fibers from bone by osteoclasts,
2. and bone deposition, the addition of minerals and collagen fibers to bone by osteoblasts. 

• Thus, bone resorption results in the destruction of bone extracellular matrix, 
• while bone deposition results in the formation of bone extracellular matrix. 

• At any given time, about 5% of the total bone mass in the body is being remodeled. 
• The renewal rate for compact bone tissue is about 4% per year, and 
• for spongy bone tissue it is about 20% per year. 

• Remodeling also takes place at different rates in different regions of the body. 
• The distal portion of the femur is replaced about every four months. 
• By contrast, bone in certain areas of the shaft of the femur will not be replaced completely during an individual’s life. 

• Even after bones have reached their adult shapes and sizes, old bone is continually destroyed and new bone is formed in its place. 
• Remodeling also removes injured bone, replacing it with new bone tissue. 
• Remodeling may be triggered by factors such as 

1. exercise, 
2. sedentary lifestyle,
3. and changes in diet.

• Remodeling has several other benefits. 

Since the strength of bone is related to the degree to which it is stressed,

•  if newly formed bone is subjected to heavy loads, it will grow thicker and therefore be stronger than the old bone. 
• Also, the shape of a bone can be altered for proper support based on the stress patterns experienced during the remodeling process. 
• Finally, new bone is more resistant to fracture than old bone.

•  During the process of bone resorption, an osteoclast attaches tightly to the bone surface at the endosteum or periosteum
• and forms a leak proof seal at the edges of its ruffled border
•  Then it releases protein-digesting lysosomal enzymes and several acids into the sealed pocket. 
• The enzymes digest collagen fibers and other organic substances while the acids dissolve the bone minerals. 

• Working together, several osteoclasts carve out a small tunnel in the old bone. 
• The degraded bone proteins and extracellular matrix minerals, mainly calcium and phosphorus, 
• enter an osteoclast by endocytosis,
• cross the cell in vesicles, 
• and undergo exocytosis on the side opposite the ruffled border. 

• Now in the interstitial fluid, the products of bone resorption diffuse into nearby blood capillaries. 
• Once a small area of bone has been resorbed,
• osteoclasts depart and
• osteoblasts move in to rebuild the bone in that area.

• A delicate balance exists between the actions of osteoclasts and osteoblasts. 

• Should too much new tissue be formed, the bones become abnormally thick and heavy. 
• If too much mineral material is deposited in the bone, the surplus may form thick bumps, called spurs, on the bone that interfere with movement at joints. 
• Excessive loss of calcium or tissue weakens the bones, and they may break, as occurs in osteoporosis, 
• or they may become too flexible, as in rickets and osteomalacia. 
• Abnormal acceleration of the remodeling process results in a condition called Paget’s disease, in which the newly formed bone, especially that of the pelvis, limbs, lower vertebrae, and skull, becomes hard and brittle and fractures easily.

CLINICAL CONNECTION 

Remodeling and Orthodontics

• Orthodontics  is the branch of dentistry concerned with the prevention and correction of poorly aligned teeth. 

• The movement of teeth by braces places a stress on the bone that forms the sockets that anchor the teeth. 
• In response to this artificial stress, osteoclasts and osteoblasts remodel the sockets so that the teeth align properly.

Factors Affecting Bone Growth and Bone Remodeling

• Normal bone metabolism—growth in the young and bone remodeling in the adult—depends on several factors. 

These include 

• adequate dietary intake of minerals and vitamins, 
• as well as sufficient levels of several hormones.

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. 
•  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 insulinlike 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 human growth hormone (hGH) 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.

• At puberty, the secretion of hormones known as sex hormones causes a dramatic effect on bone growth. 

The sex hormones include 

• estrogens (produced by the ovaries) and 
• androgens such as testosterone (produced by the testes).

• Although females have much higher levels of estrogens and males have higher levels of androgens, females also have low levels of androgens, and males have low levels of estrogens. 

• The adrenal glands of both sexes produce androgens, 
• and other tissues, such as adipose tissue, can convert androgens to estrogens. 

These hormones are responsible for 

• increased osteoblast activity 
• and synthesis of bone extracellular matrix 
• and the sudden “growth spurt” that occurs during the teenage years.

•  Estrogens also promote changes in the skeleton that are typical of females, such as widening of the pelvis. 

Ultimately sex hormones, especially estrogens in both sexes, 

• shut down growth at epiphyseal (growth) plates, 
• causing elongation of the bones to cease.

• Lengthwise growth of bones typically ends earlier in females than in males due to their higher levels of estrogens.

• During adulthood, sex hormones contribute to bone remodeling by slowing resorption of old bone and promoting deposition of new bone. 
• One way that estrogens slow resorption is by promoting apoptosis (programmed death) of osteoclasts. 

•  parathyroid hormone, calcitriol (the active form of vitamin D), and calcitonin are other hormones that can affect bone remodeling. 

CLINICAL CONNECTION 

Hormonal Abnormalities That Affect Height

• Excessive or deficient secretion of hormones that normally control bone growth can cause a person to be abnormally tall or short 
•  Oversecretion of hGH during childhood produces giantism, in which a person becomes much taller and heavier than normal.
• Undersecretion of hGH produces pituitary dwarfism, in which aperson has short stature. 
• (A dwarf has a normal-sized head and torso but small limbs; a midget has a proportioned head, torso, and limbs.)

• Because estrogens terminate growth at the epiphyseal (growth) plates, both men and women who lack estrogens or receptors for estrogens grow taller than normal. 

• Oversecretion of hGH during adulthood is called acromegaly 

• Although hGH cannot produce further lengthening of the long bones because the epiphyseal (growth) plates are already closed, 
• the bones of the hands, feet, and jaws thicken 
• and other tissues enlarge. 
• In addition, the eyelids, lips, tongue, and nose enlarge, 
• and the skin thickens and develops furrows, especially on the forehead and soles. 

• hGH has been used to induce growth in short-statured children.