Bone Development and Clinical Application

What do all skeletal features start as?

Either fibrous connective tissue or hyaline cartilage

When does the formation of the skeleton begin?

Around week 8 in-utero (in the womb)

What are the two types of bone formation?

Endochondral ossification and intramembranous ossification

What happens during endochondral ossification?

Bone replaces cartilag

What happens during intramembranous ossification?

Bone develops from fibrous connective tissue

Which bones form through intramembranous ossification?

Flat bones of the skull, parts of the clavicle, and mandible

What is the first step of intramembranous ossification?

Mesenchymal cells cluster around a blood vessel and become osteoblasts

What happens after mesenchymal cells become osteoblasts?

Osteoblasts secrete osteoid, forming an ossification center in fibrous connective tissue

What happens as osteoblasts continue to grow and secrete osteoid?

Some osteoblasts become trapped and turn into osteocytes

What forms as osteoid continues to develop around blood vessels?

Immature spongy bone forms

What does the mesenchyme on the bone’s outer surface become?

The periosteum

What happens to immature spongy bone near the periosteum?

It is remodeled and replaced with compact bone

What happens to the central immature spongy bone?

It matures into spongy bone

What does endochondral ossification form from?

A hyaline cartilage model

What is the first step of endochondral ossification?

Mesenchymal cells specialize into osteoblasts

What does the perichondrium become during endochondral ossification?

The periosteum

What do osteoblasts secrete to create a bone collar?

Osteoid

What happens to chondrocytes left within the cartilage model?

They enlarge (hypertrophy) and form the primary ossification center

What happens at the primary ossification center?

Chondrocytes ossify the surrounding cartilage, then die as the matrix dissolves

What forms after chondrocytes die and the matrix dissolves?

Cavities within the cartilage model

What happens to the remaining cartilage at the ends of the model?

It stays healthy and grows quickly to lengthen the bone

What invades the cavities during endochondral ossification?

The periosteal bud (containing blood vessels, nerves, red marrow, osteoprogenitor cells, and osteoclasts)

What do osteoclasts do when the periosteal bud enters the cavity?

They partially erode the cartilage matrix

What do osteoprogenitor cells become after the periosteal bud enters?

Osteoblasts, which form spongy bone

What do osteoclasts do after spongy bone forms?

They break down some of the spongy bone to create the medullary (marrow) cavity

What remains as cartilage through birth?

The epiphyses (ends of the bone)

When do secondary ossification centers develop?

After birth, in the epiphyses

Which bones only have a primary ossification center?

Short bones

Which bones have multiple ossification centers?

Irregular bones

What happens during secondary ossification?

It mimics primary ossification, but spongy bone stays and no cavity forms

What is the space between the diaphysis and epiphysis called?

The epiphyseal plate (made of hyaline cartilage)

What are the zones of postnatal bone growth in length (epiphyseal plate)?

• Resting zone

• Proliferation zone

• Hypertrophic zone

• Calcification zone

• Ossification zone

What happens to chondroblasts as puberty ends?

They divide less and are replaced by bone tissue

How is postnatal bone growth in width completed?

Through appositional growth

What do osteoblasts in the periosteum do during appositional growth?

They secrete bone matrix on the external surface of the bone

What do osteoclasts on the endosteal surface do?

Remove bone

Which activity is typically greater, osteoblast or osteoclast?

Osteoblast activity

What determines if and when bones are remodeled?

Hormones

What determines where bones are remodeled?

Mechanical stress

What is the role of calcium in the body?

Maintaining homeostasis for resting membrane potentials

How much of the body’s calcium is stored in the bone matrix?

99%

What is the normal calcium level in the blood?

9–11 mg/dL

Which hormones help maintain blood calcium levels?

Parathyroid hormone, calcitriol, and calcitonin

What is hypercalcemia?

High calcium levels in the blood

What is hypocalcemia?

Low calcium levels in the blood

Where is parathyroid hormone (PTH) secreted from?

The parathyroid glands

What does PTH stimulate osteoclasts to do?

Promote bone resorption

How does PTH affect the kidneys?

It promotes calcium reabsorption and the final step of calcitriol synthesis

How does PTH affect osteoblasts?

It inhibits collagen synthesis by osteoblasts

What is Wolff’s Law?

The body adapts to the forces placed upon it

What is a non-displaced fracture?

The bone ends stay in their normal position

What is a displaced fracture?

The bone ends are out of alignment

What is an incomplete fracture?

The bone is not broken all the way through

What is a comminuted fracture?

The bone is broken into three or more pieces

What are the four stages of traumatic (acute) fracture healing?

1. Hematoma formation 

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

What happens in osteoporosis?

Bone breaks down faster than it’s rebuilt, making bones weak and easy to fracture

How common is osteoporosis in women over 50?

1 in 3 women will have a fracture due to osteoporosis

What happens in osteomalacia?

Bones are poorly mineralized, making them weak and soft

What is the childhood version of osteomalacia?

Rickets

What are signs of rickets?

Bowed legs and deformities in the skull, pelvis, and ribs

What happens in osteitis deformans (Paget’s disease)?

Bone is made and broken down in a disorganized way, creating weak, spongy bone

Where does osteitis deformans usually occur?

Spine, pelvis, femur, and skull

What percent of elderly North Americans have osteitis deformans?

About 3%