bonedevo Flashcards
primary bone
woven or immature bone
temporary, formed first
secondary bone
lamellar or mature bone
forms second
What are two processes is bone formed through?
intramembranous ossification
endochondral ossification
What bones are formed through intramembranous ossification?
flat bones of the skull, bones of the face
When does intramembranous ossification occur?
begins to around 8th week of gestation
How does intramembranous ossification proceed?
bone cells (osteoblasts) differentiate directly from mesenchyme, producing osteoid
mesenchyme –> bone
How does endochondral ossification proceed?
Mesenchyme is replaced by a hyaline cartilage model that is eventually eroded and replaced with bone
mesenchyme –> hyaline cartilage –> bone
What bones does endochondral ossification produce?
long and short bones
When does endochondral ossification occur?
hyaline cartilage models visible in 6th week
ossification centers present in all long bones by 12th week
What are the steps of intramembranous ossification?
- Development of ossification center
- Calcification
- Woven bone and periosteum development
- Replacement of woven bone
How do mesenchymal cells change during development of the ossification center?
mesenchymal cells become osteoprogenitor cells
go from elongated to rounded
go from eosinophilic to basophilic
now they are osteoblasts
What are cells that are trapped in the calcifying osteoid called and where do they lie?
osteocytes, lie in lacunae
How is woven bone produced?
small, irregular shaped pieces (called spicules) are increased in size by appositional growth (growth along the surface)
these patches of bone production join
embryonic blood vessels infiltrate spaces between spicules and become red bone marrow
How is the periosteum formed?
mesenchyme at periphery of bone condenses
What is woven bone replaced by?
lamellar bone, both spongy and compact
in flat bones, spongy is between two layers of compact
What is a Howship’s lacuna?
home to osteoclasts, site of bone resorption/breakdown
What are the steps of endochondral ossification?
- Development of fetal cartilage model
- Bone collar forms around diaphysis, cartilage of shaft begins to calcify
- Development of a primary ossification center in diaphysis
- Development of secondary ossification centers
- Retention of hyaline cartilage as articular cartilage and epiphyseal plate
What type of tissue forms the fetal cartilage model?
mesenchymal cells differentiate into chondroblasts, creating hyaline cartilage
How does the fetal cartilage model grow?
both interstitially due to activity of chondrocytes in lacunae and appositionally due to chondroblasts on the surface
What surrounds the fetal cartilage model?
perichondrium
What happens to the perichondrium as endochondral ossification continues?
perichondrium near mid region of the bone has progenitor cells that become osteoblasts–perichondrium now functions as periosteum
forms bone collar around diaphyseal portion of bone
What is the first bone tissue that appears?
bone collar around diaphyseal portion of bone
How does the bone collar promote change?
impedes diffusion of oxygen and nutrients to underlying cartilage
What happens to chondrocytes in mid region of developing bone?
accumulate glycogen, undergo hypertrophy, produce alkaline phosphatase. then they die. now we have a porous 3D structure of hyaline cartilage
these changes compress matrix, signal it to calcify
Is calcified cartilage the same as bone?
No, cartilage is primarily type II fibers while bone is primarily type I
What color does calcified hyaline cartilage stain?
blue/purple (basophilic)
What color does newly formed bone stain?
pink/red (eosinophilic)
What induces the formation of the primary ossification center?
capillaries and osteoprogenitor cells from new periosteum penetrate bone collar, grow into disintegrating calcified cartilage
How is woven bone produced produced near primary ossification center?
osteoprogenitor cells differentiate into osteoblasts
Does calcified cartilage have cells?
NO (but bone does in lacunae)
Where is the primary ossification center found?
diaphysis, cartilage is still at ends of bone
When do secondary ossification centers develop?
after birth
Where are secondary ossification centers located?
epiphyses
How to secondary ossification centers develop?
Same as primary
chondrocytes grow, cartilage matrix calcifies, chondrocytes die, blood vessels and osteoprogenitors enter spaces
Where is hyaline cartilage retained and why?
ends of the bone (to become articulating cartilage)
metaphysis (to become epiphyseal or growth plate)
When does epiphyseal cartilage get replaced by bone?
end of puberty
Where is the metaphysis found?
between epiphysis and flared portion of diaphysis
How to long bones increase in length during infancy and youth?
endochondral ossification occurring at epiphyseal plate
What are the zones of an epiphyseal plate?
Zone 1: Resting cartilage–located next to epiphysis
Zone 2: Proliferating cartilage
Zone 3: Hypertrophic cartilage
Zone 4: Calcified cartilage
Zone 5: Ossification–located closest to diaphysis
What’s going on in the zone of resting cartilage?
chondrocytes singularly or small groups
no matrix production, no mitosis
What’s going on in the zone of proliferating cartilage?
chondrocytes in lacunae are undergoing mitosis, actively producing matrix
they look like stacked coins in line w/ long axis of bone
epiphysis is pushed away from diaphysis
What’s going on in the zone of hypertrophic cartilage?
chondrocytes increase in size, accumulate glycogen
matrix compressed/thinned
What’s going on in the zone of calcified cartilage?
cartilage matrix calcifies thru formation of hydroxyapatite crystals
chondrocytes die
**this zone stains basophilic
What’s going on in the zone of ossification?
blood vessels, osteoprogenitor cells and osteoclasts open spaces previously occupied by chondrocytes and produce woven bone (that will be replaced later)
Does the epiphyseal plate remain the same width during the growth of an individual?
YES
cartilage growth = bone tissue replacement
(new cartilage produced in zone of proliferation must equal activity in zone of ossification)
What is a stupid analogy for how endochondral growth looks?
chondrocytes are being ‘chased’ by the bone cells up the length of the bone
What is the epiphyseal line?
epiphyseal plate closure, bone replaces all of the cartilage (bone cells ‘caught up’ with chondrocytes)
How do you widen bones?
appositional growth
osteoblasts secrete matrix along surface of bone
How do you make sure your bones don’t get too thick or thin?
rate of activity of osteoblasts on periosteal layer should equal that of osteoclasts on endosteal layer
How much of your bone do you remodel every year?
5-10%
What determines when remodeling occurs?
negative feedback loop of Ca2+ concentration
What controls where remodeling occurs?
mechanical stresses and gravity
What happens if there is too much Ca2+ in blood?
parafollicular cells in thyroid secrete calcitonin
osteoclasts (therefore bone resorption) are inhibited
osteoblast activity increases
What happens if there is too little Ca2+ in blood?
parathyroid glands secrete parathyroid hormone (PTH)
osteoclasts (and therefore bone resorption) are increased
