what are the two main fomrs that bones exist in
spongy (cancellous) bone and compact bone
explain the structure of spongy bone and key features
Spongy (cancellous) bone
Structure:
- made of trabeculae (thin struts/plates of bone)
- this forms an open network
- found mainly in the epiphyses (bone ends)
Key Features:
- spaces between trabeculae contain red bone marrow
- no blood vessels in the matrix
- nutrients diffuse through canaliculi
It’s also lightweight, handles multidirectional stress, and site of blood cell production
An example is the femoral head → the spongy bone distributes body weight
explain compact bone structure and key features
Its structural unit is osteons
Each osteon contains:
- concentric lamellae → rings of calcified matrix
- central canal → blood vessels + nerves
- osteocytes inside lacunae
- canaliculi → tiny channels connecting the osteocytes (bone cells)
Compact bone is highly organized which allows for strength and nutrient delivery.
what is the long bone organization from outside –> inside
C. long bone organization (integrated layers)
From outside → inside:
1. Periosteum
This is the outer membrane (like the skin of the bone)
Contains osteoblasts which are for growth and repair
- Compact bone
Circumferential lamellae which wraps around the bone
Osteons for structural strength
Interstitial lamellae which fill gaps between osteons
Perforating (transverse canals) connect osteons perpindicular to surface - Spongy bone
Innermost
Key concept → bone is not solid, its hierarchically organized
what are the three types of lamallae?
Circumferential lamellae which wraps around the bone
Interstitial lamellae which fills gaps between osteons
Concentric lamellae which are rings of calcified matrix
what is the cartilage tissue like and why does it rebuilt itself slowly compared to bone?
Cartilage tissue
Structure:
Cells → chondrocytes
Matrix → gel like
No blood vessels, avascular
– the perichondrium is like the skin of the cartilage
Why cartilage rebuilds itself slowly compared to bone
This is because:
1. No direct blood supply (avascular)
2. Nutrient diffusion is slow
3. Low metabolic rate
Appositional growth also takes place in cartilage and is longer and if the injury is bad it cant be healed from it
what are the two bone growth mechanisms/direction?
Appositional growth → width
Endochondral ossification → length
what is the process of apoositonal growth ?
Appositional growth (growth in diameter)
This occurs at the bone surface.
Process:
1. Osteogenic cells are located in the inner lining of the periosteum which differentiate into osteoblasts
2. Osteoblasts deposit new lamellae on outside
3. Bone diameter increases
4.Osteoclasts remove bone inside
5. Medullary cavity enlarges
6. Bone grows outwards but stays lightweight by hollowing inside using osteoclasts
what is the remodeling comcept? why does an athlete have a thicker cortical bone due to loading?
Remodeling concept
Deeper lamellae replaced by osteons
Continuous bone turnover
This takes place when deeper lamellae are recycled then replaced by osteons
The osteoclasts remove matrix at the inner surface to enlarge the medullary cavity
Example: an athlete has a thicker cortical bone due to loading
what is endochondrial ossificaton?
Endochondral Ossification
This is how most bones form and grow in length
Core principle: bone replaces a cartilage model
The Initial skeleton of embryo are formed by hyaline cartilage
This cartilage is then gradually replaced by bone through endochondral ossification
Uses cartilage as a small model, and bone grows in diameter and length (where diameter involves appositional bone deposition)
what is step 1 in endochondrial ossificaiton
Step 1 → cartilage model enlarges
Chondrocytes near the center of the shaft enlarge
These enlarged chondrocytes grow and die/disintegrate, cavities form. This creates space for bone formation
what is step 2 in endochondrial ossificaiton
Step 2 → Blood vessels grow around the edge of the cartilage model
Blood vessels grow and surround the cartilage
the perichondrium becomes osteoblasts. The osteoblasts then create an outer bone layer (superficial layer of bone along the shaft)
what is step 3 in endochondrial ossificaiton
Step 3 → blood vessels penetrate cartilage and enter central region to make the primary ossification center
Blood vessels enter shaft (diaphysis)
Fibroblasts turn to osteoblasts
Spongy bone begins to form at the primary ossification center
Bone formation spreads along the shaft toward both ends
what is step 4 of endochondrial ossification
Step 4 → remodeling + medullary cavity where growth continues
The shaft thickens and the medullary cavity is formed
Osseous tissue of the shaft thickens
Cartilage near the epiphysis is replaced by shafts of the bone
Bone grows in length and diameter
what is step 5 of endochondrial ossificaition
Step 5 → secondary ossification centers where the capillaries and osteoblasts migrate into the epiphysis
This is what creates the secondary ossification centers at the epiphysis
what is step 6 of endochondrial ossificaiton
Step 6 → epiphysis fill with spongy bone
Two cartilage areas remain
The articular cartilage which is the joint surface remaining exposed to joint cavity
The epiphyseal cartilage/plate which separates the epiphysis from the diaphysis
what is step 7 of endochondrial ossification
Step 7 → bone length growth at epiphyseal plate
Two opposite processes occur:
Epiphyseal side → cartilage is produced (chondrocytes are producing)
Diaphyseal side → cartilage replaced by bone (osteoblasts replacing cartilage with bone)
This pushes the epiphysis away so that bone lengthens
what is epiphyseal closure
At puberty:
Hormones accelerate bone growth
Osteoblasts replace cartilage faster since they produce bone faster than chondrocytes produce cartilage
Epiphyseal cartilage/growth plate narrows until it disappears (epiphyseal closure)
Eventually disappears, leaves an epiphyseal line in adults
This results in the epiphyseal line remaining and height growth stops
