Bones: Growth and Regrowth

Bone Development

• The process of bone development is similar to the process of bone healing• Specifically, one of the two types of bone

development

• Bones can be developed in a fetus in one of two ways:• Intramembranous ossification• Endochondral ossification

• What can you deduce from the names?

Intramembranous Ossification

• Unpack the name!• Intra=within• Ossify=to become a bone• “To become a bone within a membrane”

• A membrane forms, and then within an ossification center, osteogenic cells become osteoblasts and convert the membrane into a flat bone• Only flat bones are developed this way

Intramembranous Ossification pt2• Once the ossification has

initiated, the connective tissue membrane converts into a flat bone from the center outwards• This is NOT how bone heals since

it is converting fibrous connective tissue directly to bone• Only takes place during fetal

development of flat bones

Endochondral Ossification

• Endochondral ossification is the conversion of cartilage to bone• Process begins when chondroblasts form

new cartilage in the shape of the bone• Chondroblasts that become trapped in the

cartilage become chondrocytes

• Cartilage develops an ossification center that spreads outwards• Due to what cells?

Endochondral Ossification

Bone collarforms aroundhyaline cartilagemodel.

Cartilage in thecenter of thediaphysis calcifiesand then developscavities.

The periostealbud inavades theinternal cavitiesand spongy bonebegins to form.

The diaphysis elongatesand a medullary cavityforms as ossificationcontinues. Secondaryossification centers appearin the epiphyses inpreparation for stage 5.

The epiphysesossify. Whencompleted, hyalinecartilage remains onlyin the epiphysealplates and articularcartilages.

Hyalinecartilage

Area ofdeterioratingcartilage matrix

Epiphysealblood vessel

Spongyboneformation

Epiphysealplatecartilage

Secondaryossificationcenter

Bloodvessel ofperiostealbud

Medullarycavity

Articularcartilage

Childhood toadolescence

BirthWeek 9 Month 3

Spongybone

BonecollarPrimaryossificationcenter

1 2 3 4 5

Further Notes

• Endochondral ossification is how most bones in the body are formed• Some cartilage remains at the joints, creating

the articular cartilage that pads the bones at the joints• If this cartilage gets worn away, arthritis will result

• Long bones have three initial ossification centers (diaphysis and each epiphysis) so the growth plate is the cartilage that remains

Bone Healing

• Bones may break (suffer a fracture) and need to be healed• The process of healing closely resembles

endochondral ossification• Chondroblasts and fibroblasts form a

fibrocartilaginous callus• Osteoclasts clean away rough edges of

broken bone• The cartilage callus essentially undergoes

ossification by osteoblasts• Becomes a bony callus and eventually

reincorporated into bone

Fracture Healing

• The most important part of treating a broken bone is to:

1. Set the bone in the correct position2. Use a cast or equivalent to keep it from

slipping out of place

• Why? Because the bone will fuse back into place whether you want it to or not• Pictured: archaeological finding of a tibia

that healed without being set Oh sweet Jesus that must have hurt

Bone Growth/Decay• All bone growth and decay is

managed by two competing types of cells• Osteoblasts build new bone tissue• Osteoclasts degrade/remove bone

tissue

• Bone tissue is constantly being worn down and rebuilt• Osteoblasts absorb blood calcium

to build bone, osteoclasts release calcium back into the blood

Osteoblast Activity• Osteoblast activity is stimulated by

increased blood calcium and by force/stress on bone• Bones thicken when stressed• Forensic analysis can determine

whether someone was right- or left-handed!

• Bone density increases with increased force applied to the bones• What exercises are good/bad for bone

density?• Bone density decreases with age, so

build it up when you’re young!

Osteoclast Activity• Osteoclasts break down bone material• Release calcium into bloodstream

when active• Low blood calcium stimulates osteoclasts

to break down bones to replace calcium

• Chronic low bone density can lead to osteoporosis in old age• This is due to osteoclast activity• Don’t blame them, you were the one

who didn’t drink milk and then put stress on your bones when you had the chance!

Calcium Homeostasis

• Osteoclasts are stimulated to wear down bone by a hormone, PTH• Parathyroid hormone• PTH also stimulates calcium absorption from the

kidneys and intestines• PTH is released by the Parathyroid gland when

blood calcium becomes low• When blood calcium becomes high, PTH is not

released, and further absorption of calcium is not possible• Also causes you to urinate out excess calcium• Calcitonin is released from the thyroid as well to

further block osteoclast activity (not a key factor)

Osteoclastsdegrade bonematrix and release Ca2+

into blood.

Parathyroidglands

Thyroidgland

Parathyroidglands releaseparathyroidhormone (PTH).

StimulusFalling bloodCa2+ levels

PTH

Calcium homeostasis of blood: 9–11 mg/100 mlBALANCEBALANCE

Bone Density

• Bone density is increased by the following behaviors:• Regular calcium intake (also vitamins)• Heavy forces applied to bones• Higher metabolism• Being male (testosterone activity)

• Bone density is decreased by the following behaviors:• Lack of weight-bearing exercise• Lack of regular calcium and vitamins• Aging

That’s our show!

• Friday: Joints! (Chapter 7)