LECTURE 8 Flashcards
- Mesenchyme condenses into a membrane permeated with blood vessels, which gives rise to bone
- Bones of skull, and clavicles form this way
intramembranous ossification
- Mesenchyme gives rise to cartilage, which is replaced by bone
- Most bones form this way - limb bones, hip bones, vertebrae, sternum, and scapulae
endochondral ossification
Describe the steps involved in intramembranous ossification
- Begins between week 8 and 12 of development
- Mesenchymal cells surround the blood vessels and differentiate into osteoblasts
- Osteoblasts begin secreting a precursor to bone matrix called osteoid (contains collagen but no calcium phosphate)
- Osteoid begins calcifying and the osteoblasts become trapped, differentiating into osteocytes
- Mesenchyme forms periosteum around bone tissue
- Spongy bone forms in the middle
- Osteoblasts of periosteum lay down compact bone
- Process continues until a typical flat bone structure is obtained. Spongy bone sandwiched between compact. Red marrow present.
Describe the steps involved in endochondral ossification
- Process starts around week 6 of development
Initial skeleton is hyaline cartilage, and is formed during embryonic development - Cartilage then ossifies, completed at around age 20 for some bones
- Cartilaginous skeleton develops from mesenchyme
- Mesenchymal cells differentiate into chondroblasts which make cartilage to be used as a model for bone formation. Covered with perichondrium
- Cartilage begins to calcify. Chondrocytes at center of cartilage enlarge and begin to die. Matrix becomes porous and begins to ossify. Perichondrium becomes periosteum.
- Osteoblasts in newly formed periosteum form “bone collar” around cartilage model
- Epiphyses fill in with spongy bone during infancy and childhood
- Only remaining cartilage in bones are epiphyseal plates and articular cartilages
- Plates continue to grow new cartilage, which continues to ossify
- Damage to epiphyseal plate can stunt growth in that limb
- Sex hormone decline shuts off cartilage growth. At around age 20, growth plates completely ossified. Dense form of spongy bone (called epiphyseal lines) remains.
Blood and lymphatic vessels, nerves, along with blood and bone cells invade the center of calcified cartilage. Bone cells come in from the blood and infiltrate area
periosteal bud
The point where we start the ossification from the inside out. Osteoclasts eventually hollow out the shaft to form the medullary cavity.
primary ossification center
Form in the epiphyses of long bones. Calcified cartilage begins closer to diaphysis.
secondary ossification center
Define interstitial growth. What age does interstitial growth typically occur until?
Growth in the length of a bone (bone elongation). Early in life, rate of cartilage growth and ossification are equal. When sex hormones decline, cartilage growth slows and ossification speeds up. Up to the point where the epiphyseal plates seal up, ossify, and become epiphyseal lines.
5 zones of epiphyseal plate
- Zone of resting cartilage
- Zone of proliferation
- Zone of hypertrophy
- Zone of calcification
- Zone of ossification
cartilage cells aren’t doing anything, maintaining cartilage matrix but are not replicating
zone of resting cartilage
Chondrocytes divide to produce new cells, active mitosis. Pushes resting zone cartilage away from diaphysis.
zone of proliferation
cells enlarge and die, which leave it susceptible to calcification.
zone of hypertrophy
cartilage matrix on diaphysis side calcifies first, and then ossifies
zone of calcification
closest to diaphysis, marrow cavity is distal to this
zone of ossification
Thickening and widening of bones, changing the girth of bones. Technically a type of intramembranous ossification. Will continue to occur throughout adulthood.
appositional growth
