Osteogenesis Flashcards
Two types of bones
Endochondral
Membrane (Dermal)
Endochondral Bones
Most of the bones in your body
- long bones
- bones of thorax
- vertebrae
- pelvis
Membrane Bones
AKA Dermal bones
- flat bones in skull
Endochondral ossification
Process that forms enochondral bones
- start with a cartilage template resembling (not exact) a small version of the adult bone
- initial bone deposit is woven bone (non-lemellar) aka non-mature bone
- remodeling leads to formation of compact and spongy bone as needed
Primary center of ossification
One of two centers very first place bone formation takes place
Endochondral ossification: Steps
Three aspects:
- Formation of cartilate template upone which bone tissue apposed
- Initation events (2)
- Bone collar formation (de novo)**: perichondrial ossification
- Calcification of cartilage to form surface for apposition of bone tissue
- continued apposition, reabsorbtion and remodeling to attain mature size and structure
In more detail:
- Begin with cartilage template
- In the center of bone, some chontrocytes begin to enlarge (hypertrophy), and eventually die.
- at same time, the periosteal bone collar is formed (bone forming around shaft) – this is de novo bone synthesis (not replacing cartilage; it is new bone deposition)
- Intervening hyaline cartilage will calcify, making region porous so blood vessels can enter
- Entering blood vessels bring in osteogenic cells, eg. osteoblast, (osteoclasts)
- Osteoblasts begin laying down new bone on top of calcified cartilage –> forms Primary center of ossification
- continued bone deposition will enlarge the girth of the bone
- in two epiphyseal ends, same type of bone formation takes place as it did in the primary center of ossification
Epiphyseal Plate
On either long end of the growing bone
Between the primary and secondary centers of ossification
- Hyaline cartilage persists in this region
- it is because of the persistence of this region that long bones are allowed to grow in length
The formation of the secondary center of ossification leads to the appearance of the definitive epiphysial growth plate
- Top of photo: not much proliferation (resting)
- Cells start proliferating up
- Chondrocytes get bigger in hypertrophic zone
- Cartilage matrix darker at bottom: undergoing calcification, calcified cartilage is the template upon which osteoblasts will lay down new bone
Stays the same thickness because at one end you have chondrocytes dying off and on the other end, there is new cell proliferation on other side. (Erode cartilage on one end, adding new hyaline cartilage at other end.)
At closure, the growth plate cartilage stops proliferating and completes osteogenesis
Bone remodeling
Occurs
- developmentally
- throughout life
- fracture repair
Mixed spicule
calcified cartilage core (CC) with overlaying bone (B)
Can see osteoids (light pink layer) under the osteoblasts (not yet mineralized)
Indicative of endochondral bone formation
Compact bone remodeling
- Excavation by osteoclasts
- Once you have exposed bone, osteoblast line up and start depositing new bone
- Bone synthesis and entrapment of osteocytes continues
- Leaves a haversian canal
All bones start off as
woven bone
And it starts as spongy bone
Osteons in development
Can tell new bone is being made because of osteoid (small pink line around outside)
Spongy bone remodelling
- removal of woven bone by osteoclasts
- new lameller bone deposited by osteoblasts
- will see layer upon layer of new bone with embedded osteocytes
- will NOT see haversian canal
Intramembranous ossification
Takes place within the “membranes” of the mesenchyme
- NO cartilage template
- Initiation event: condensed mesenchyme leads to de novo bone formation
- initial bone deposited is woven bone
- remodeling leads to formation of compact and spongy bone as needed
Epiphysis
Diaphysis
Intramembranous ossification
Lamellar bone
Mature bone
highly organized with the collagen fibers being in layers
Alternating layers are generally perpendicular to each other, which accounts for the alternating light and dark appearance in polarized light
Periosteum
the inorganic matrix of bone tissue consisting chiefly of calcium phosphate in a crystalline form
Develops from the perichondrium as one of the first steps of endochondral bone formation
Responsible for growth of GIRTH of the bone
Endosteum
a layer of connective tissue that lines the bone surface facing the medullary cavity
contains cells that have osteogenic capability
Intramembranous Ossification
Initiation (left) and continued appositional growth (right)
B: bone tissue w/ osteocytes
M: mesenchyme
O: osteoblasts
P: periosteum
V: Blood vessels
Section of bone organ forming by endochondral ossification taken from zone of ossification. In this region the mixed spicules of endochondral bone tissue is seen. Appositional growth occurs against a calcified cartilage matrix (C). As resorption and remodeling take place, the cartilage matrix is lost and bone forms on existing bone tissue (B); as a result, mature spongy bone does not have a calcified cartilage core. H&E stain
H&E cross-section of a developing finger bone (phalanx)
a. osteiod
b. calcified cartilage
c. periosteum
d. endosteum
e. osteocyte
f. probable osteoclasts
g. marrow cavity
Remodeling spongy bone
A: regional bone removal (osteoclasts)
B: regional bone deposition (osteoblasts)
last arrow is Osteoid
Paget’s Disease
Accelerated remodeling of bone leads to deposition and accumulation of woven bone. Not both woven and lamellar bone appear giving rise to a characteristic mosaic appearance.
