CHAPTER 6-2CHAPTER 6-2

Bone Development, Growth Bone Development, Growth and Remodelingand Remodeling

ObjectivesObjectives

1.1. Functions of the skeletal systemFunctions of the skeletal system

2.2. Classification of bones based on shapeClassification of bones based on shape

3.3. General features of boneGeneral features of bone

4.4. Bone cells and matrix of boneBone cells and matrix of bone

5.5. Compact and spongy boneCompact and spongy bone

6.6. Bone marrowsBone marrows

7.7. Bone developmentBone development

8.8. Overview of bone growth and Overview of bone growth and remodelingremodeling

Bone developmentBone development

Ossification or osteogenesisOssification or osteogenesis

- is the process of forming new bone- is the process of forming new bone

Two methods of ossification:Two methods of ossification:1.1. Endochondral ossificationEndochondral ossification

2.2. Intramembranous ossificationIntramembranous ossification

Classification based on Classification based on formationformation

1.1. Endochondral ossification = The process Endochondral ossification = The process by which bone is formed from hyaline by which bone is formed from hyaline cartilage.cartilage.

2.2. Intramembranous ossification = Bone Intramembranous ossification = Bone formed by replacing a fibrous formed by replacing a fibrous membrane and not from cartilage. membrane and not from cartilage. (ex. (ex. Clavicle and skull bones).Clavicle and skull bones).

Endochondral ossificationEndochondral ossification The process by which bone is formed from The process by which bone is formed from

hyaline cartilagehyaline cartilage

Most bones in the body are formed by this Most bones in the body are formed by this method (including the vertebrae, pelvic bones method (including the vertebrae, pelvic bones and limb bones).and limb bones).

Consists of 3 sites of ossification.Consists of 3 sites of ossification.1.1. The primary ossification centerThe primary ossification center2.2. The metaphysisThe metaphysis3.3. The secondary ossification centerThe secondary ossification center

Endochondral ossificationEndochondral ossification

During the first 8 weeks of fetal development, During the first 8 weeks of fetal development, hyaline cartilage forms a model of future hyaline cartilage forms a model of future bone formation. The center (diaphysis) of the bone formation. The center (diaphysis) of the cartilage is “primary ossification site” which cartilage is “primary ossification site” which contains numerous chondrocytes in lacunae. contains numerous chondrocytes in lacunae. Surrounding the cartilage model is an outer Surrounding the cartilage model is an outer layer chondrocytes called the layer chondrocytes called the “perichondrium” which differentiate into “perichondrium” which differentiate into “osteoblast cells” and begin to lay down a “osteoblast cells” and begin to lay down a bony collar around the site.bony collar around the site.

Once the collar is formed the perichondrium Once the collar is formed the perichondrium becomes periosteum.becomes periosteum.

Endochondral ossificationEndochondral ossification Buds of connective tissue grow from the Buds of connective tissue grow from the

periosteum into the caritlage and transform the periosteum into the caritlage and transform the primary ossification site into the primary marrow primary ossification site into the primary marrow space. This space is lined with spongy bone.space. This space is lined with spongy bone.

The metaphysis forms between the marrow The metaphysis forms between the marrow space and the cartilaginous epiphyseal end.space and the cartilaginous epiphyseal end.-It is considered as a transitional zone where -It is considered as a transitional zone where cartilage is formed into bone at each end of the cartilage is formed into bone at each end of the diaphysis. The metaphysis is just beneath the diaphysis. The metaphysis is just beneath the epiphyseal plate where bone growth continues epiphyseal plate where bone growth continues until after adulthood.until after adulthood.

The secondary ossification The secondary ossification centercenter

Begins at the time of birth.Begins at the time of birth. Forms in the epiphysis and develops Forms in the epiphysis and develops

similarly to the primary ossification center.similarly to the primary ossification center. The bone formed in the secondary The bone formed in the secondary

ossification site persists as spongy bone ossification site persists as spongy bone and growth occurs beneath the outer and growth occurs beneath the outer covering of hyaline cartilage which persists covering of hyaline cartilage which persists as articular cartilage within the joint cavity as articular cartilage within the joint cavity on each end of the epiphysis.on each end of the epiphysis.

Endochondral ossification

Endochondral ossificationEndochondral ossification

Intramembranous Intramembranous ossificationossification

Bone formed by replacing a fibrous Bone formed by replacing a fibrous membrane and not from cartilage. membrane and not from cartilage. Ex. Skull Ex. Skull and clavicleand clavicle

Basic Overview of ProcessBasic Overview of Process– During the first 8 wks of embryonic development, During the first 8 wks of embryonic development,

fibrous connective tissue membranes form in the fibrous connective tissue membranes form in the areas of future flat bones.areas of future flat bones.

– Beginning around 8 wks, an “ossification center” Beginning around 8 wks, an “ossification center” forms in the membrane. This center is composed forms in the membrane. This center is composed of of osteoblastsosteoblasts..

– The osteoblasts begin to secrete hydroxyapatite.The osteoblasts begin to secrete hydroxyapatite.– The internal spongy bone forms.The internal spongy bone forms.– The external compact bone forms.The external compact bone forms.

Intramembranous Intramembranous ossificationossification

Intramembranous Intramembranous ossificationossification

Bone GrowthBone Growth

Once the cartilage models of embryonic Once the cartilage models of embryonic development are replaced by bone, development are replaced by bone, they must continue to grow through they must continue to grow through infancy, childhood and adolescence. infancy, childhood and adolescence. – Increased lengthIncreased length: : bones continue to bones continue to

lengthen because hyaline cartilage lengthen because hyaline cartilage remaining in the epiphyseal plates remaining in the epiphyseal plates continues to grow. As adulthood continues to grow. As adulthood approaches, this cartilage becomes less approaches, this cartilage becomes less active and is eventually replaced by bone.active and is eventually replaced by bone.

Epiphyseal plate growth Epiphyseal plate growth zoneszones

Consists of five distinct zonesConsists of five distinct zones::

1.1. Zone of resting cartilageZone of resting cartilage

2.2. Zone of cartilage proliferationZone of cartilage proliferation

3.3. Zone of cell hypertrophic Zone of cell hypertrophic cartilagecartilage

4.4. Zone of calcified cartilageZone of calcified cartilage

5.5. Zone of ossificationZone of ossification

Epiphyseal plate growth Epiphyseal plate growth zoneszones

Bone growthBone growth

Appositional growth - Increased WidthAppositional growth - Increased Width:: bones continue to widen as bones continue to widen as osteoblasts form more layers of bone osteoblasts form more layers of bone around the outside and osteoclasts around the outside and osteoclasts break down some of the bony matrix break down some of the bony matrix inside. inside.

Why would bone need to be broken Why would bone need to be broken down inside as it grows outside?down inside as it grows outside?

Appositional growthAppositional growth

Control of Bone GrowthControl of Bone Growth

Bones increase in length and width Bones increase in length and width because of the influence of minerals, because of the influence of minerals, vitamins, and hormones in the body. vitamins, and hormones in the body.

Calcium and phosphate are necessary for Calcium and phosphate are necessary for calcificationcalcification

Vitamins A, C and D promote bone growth.Vitamins A, C and D promote bone growth. The specific hormones which affect growth The specific hormones which affect growth

are are growth hormone (GH), thyroid growth hormone (GH), thyroid hormone (T3 and T4)hormone (T3 and T4), and the , and the sex steroids sex steroids (estrogen and testosterone)(estrogen and testosterone)..

Chemicals important in bone Chemicals important in bone homeostasishomeostasis

Bone remodelingBone remodeling

Even though the bones in an adult do not Even though the bones in an adult do not continue to grow as described above, continue to grow as described above, they are they are constantlyconstantly being being remodeledremodeled. . This means that bone is always being This means that bone is always being broken down by osteoclasts and reformed broken down by osteoclasts and reformed by osteoblasts (really no different from by osteoblasts (really no different from remodeling ones home).remodeling ones home).

Each week we turn over about 5% of our Each week we turn over about 5% of our

bone mass. bone mass.

Control of RemodelingControl of Remodeling

Two Two major factors influence remodeling.major factors influence remodeling. Calcium levelsCalcium levels - - our bodies need a our bodies need a

homeostatic level of calciumhomeostatic level of calcium in the in the blood for all cells to function properly. blood for all cells to function properly.

Mechanical stressMechanical stress - - the varied activities the varied activities of life put different stresses on each of life put different stresses on each bone as we age, which requires slight bone as we age, which requires slight adjustments to compensate for these adjustments to compensate for these stresses. stresses.

Bone blood supplyBone blood supply

Bone is very vascular and well nourished Bone is very vascular and well nourished by blood.by blood.

Bone fracturesBone fractures

Bone fracturesBone fractures Types of bone Types of bone

fracturesfractures

Simple fractures do Simple fractures do not break the skin.not break the skin.

Compound fractures Compound fractures puncture the skin puncture the skin and are at great risk and are at great risk of infection.of infection.

Bone fracture repairBone fracture repair

There are four primary stages for There are four primary stages for bone repair after a fracture.bone repair after a fracture.

Bone fracturesBone fractures