6.5 Bone Formation during infancy-adolescence

As a new bone tissue is being deposited on the other surface of bone

the bone tissue lining the medullary cavity is destroyed by osteoclasts in the endosteum

step 4 of growth in thickness

As an osteon is forming, osteoblasts under the periosteum deposit new circumferential lamellae Which future increases the thickness of the bone As additional periosteal blood vessels become enclosed as in step 1, , the growth process continues

Step 1 of bone growth in thickness summed up

Ridges in periosteum create groove for periosteal blood vessel

Zone of proliferating cartilage

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

Growth in thickness of a bone is determined by the

diameter of a bone and only by appositional growth

Activity of the epiphyseal plate is the only way that the

diaphysis can increase in length

Closure for the epiphyseal plate is a

gradual process

growth in thickness summed up

1. Bone growth in thickness (diameter) is called appositional growth 2. Osteoblasts add new bone to the outer surface of the bone, while osteoclasts destroy bone tissue lining the medullary cavity 3. In the periosteum, osteoprogenitor cells develop into osteoblasts, which secrete bone ECM and become osteocytes

growth in length summed up

1. In long bones, endochondral ossification continues at the epiphyseal plate 2. On the side of the plate nearest the epiphysis, new cartilage is constantly added (interstitial growth of cartilage) 3. On the side of the plate nearest the diaphysis, osteoblasts continuously invade the cartilage and replace it with bone 4. The end result is an increase in the length of the bone

4 zones of epiphyseal growth plate

1.Zone of resting cartilage 2. Zone of proliferating cartilage 3.zone of hypertrophic cartilage 4. Zone of calcified cartilage

Step one of growth in thickness

At the bone surface, periosteal cells differentiate into osteoblasts, which secrete he collagen fibers and other organic molecules that form bone ECM The osteoblasts become surrounded by ECM and velour into osteocytes This process forms bone ridges on either side of a periosteal blood vessel The ridges slowly enlarge and create a groove for the periosteal blood vessel

Step 2 of growth in thickness

Eventually the ridges fold together and fuse the groove becomes a tunnel that encloses the blood vessel The former periosteum now becomes the endosteum that lines the tunnel

The growth in length of long bones involves the following two major events

Interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate Replacement of cartilage on the diaphsyseal of the epiphyseal plate with bone by endochorndral ossification

Zone of calcified cartilage

The final zone of the epiphyseal plate is only a few cells thick an consists of mostly chondrocytes that are dead bc lof the ECM around them has calcified Osteoclasts dissolve the calcified cartilage, and osteoblasts and capillaries from the diaphysis invade the are Osteoblasts lay down one ECM, replacing the calcified cartilage by the process of endochondral ossification (placement of cartilage with bone) zone of calcified cartilage becomes the new diaphysis that is firmly cemented to the rest of the diaphysis of the bone

Zone of hypertrophic cartilage

This layer consists of large, maturing chondrocytes arranged in columns

Zone of resting cartilage

This layer isnearet the epiphysis and consist of small, scattered condrocyytvs the term resting is used because the cells do not function in bone growth Rather they actor the epiphyseal plate to the epiphysis of the bone

an open epiphyseal plate indicates

a younger person

a partially closed epiphyseal plate or a completely closed epiphyseal plate indicates

an older person

what is the epiphyseal plate helpful in determining

bone age, predicting adult height, and establishing age at death from skeletal remains

step 4 of bone growth in thickness summed up

bone grows outward as osteoblasts in periosteum build new circumferential lamallae. osteon formation repeats as new periosteal ridges fold over blood vessel

the appearance of the epiphyseal line shows that

bone growth in length stop completely

the epiphyseal plate fades due to the end of adolescence, leaving a

bony structure called the epiphyseal line

The epiphyseal growth plate is a

layer of hyaline cartilage in the epiphyses of a growing bone that consists of 4 zones

As a bone grows, chondrocytes

multiply on the epiphyseal slide of the plate

New chondrocytes replace

older ones , which are destroyed by calcification

step 3 of growth growth in thickness summed up

osteoblasts in endosteumubuild new concentric lamellae inward toward center of tunnel, forming a new osteon

step 3 of growth in thickness

osteoblasts in the endosteum deposit bone ECM , forming new concentric lamellae The formation oaf additional concentric lamellae proceeds inward toward the periosteal blood vessel In this way, the tunnel fills in and a new osteon is created

Step 2 of bone growth thickness summed up

periosteal ridges fuse,forming an endosteum lined tunnel

Thus, the new cartilage is

replaced by bone on the diaphyseal side of the plate

the epiphyseal plates closing mean that the epiphyseal cartilage cells

stop dividing and bone replaces all the remaining cartilage

Because of a shorter adult bone stature,

the cartilage is damaged and (as we know) cartilage is avascular, and that accolades the closure of the epiphyseal plate due to the cessation of cartilage cell division, thus inhibiting lengthwise growth of the bone

in the way that new cartilage is replaced by bone on the diaphyseal side of the plate, the thickness of

the epiphyseal plate remains relatively constant, but the bone on the diaphyseal side increases in length

When adolescent comes to an end

the epiphyseal plates close

If a bone fracture damages the epiphyseal plate,

the fractured bone my be shorter than no normal once adult state is reached

In the way that bone tissue lines the medullary cavity and destroyed by osteoclasts in the endosteum

the medullary cavity enlarges as the bone increases in thickness

During infancy, childhood, and adolescence, bones through the body grow in

thickness by appositional growth, and long bones lengthen by the addition of bone material on the diaphysial side of the epiphyseal plate by interstitial growth