Bone Structure & Histology

Question 1

Three types of collagen?

Answer 1

Elastin, hyaline and fibrocartilage

Question 2

Features of hyaline cartilage

Answer 2

Articular cartilage, most common type
Avascular
Slippery, smooth and resistent to compression
ECM = Type 11 collage, 70% water, GAGs, hyaluronic acid, , proteins (i.e. chondroitin sulphate)
Chondrocytes maintain cartilage

Question 3

What is the structure of the bone cortex?

Answer 3

Strong, outer part of bone

Made up of compact bone covered externally by periosteum and internally by endosteum

Question 4

What is the structure of compact bone?

Answer 4

Made up of bony columns running longitudinally (along stress lines) = Haversian Systems
Lined externally by periosteum and internally by endosteum

Question 5

What are Haversian Systems (Osteons)?

Answer 5

Bony columns only in compact bone 
Concentric lamellae (layers) of bone with a central canal (Haversian canal) that contains the neuromuscular bundle 

Neurovascular bundles and periosteum and endosteum are interconnect via Volkman’s canals

Question 6

What are the features of periosteum and endosteum?

Answer 6

Periosteum = fibrous & vascular supply
Contains fibroblasts, fibrous tissue, vessels, some osteoprogenitor cells

Endosteum = cellular
Contains many osteoprogenitr cells

Question 7

What structures allow strong grip of tendons at bone?

Answer 7

Sharpey’s fibres

Collagen of the tendons become continuous with the compact bone and fibres penetrate the bone for strong attachment

Question 8

What is the structure of the medullary cavity?

Answer 8

Hollow inside of bone

Contains interconnected struts of trabecular bone (cancellous) with fat (marrow) and vessels between

Question 9

What are the features of trabecular bone?

Answer 9

‘Spongey bone’
Don’t contain aversion systems
Attach to the internal layer of the compact bone and project as struts that are interconnected
Have a thin lining of endosteum
Less structured and ordered than compact bone

Question 10

What is the difference between red and yellow marrow?

Answer 10

Red marrow is haematopoietic and is found in the long bones early on but is replaced by yellow marrow in most bones with age (except flat bones, such as pelvis, skull, spinal cord)

Yellow marrow is largely fat with no haematopoietic function but can be reactivated to red marrow later in life if required

Question 11

What is the arrangement of blood supply to the bone?

Answer 11

Vessels enter the marrow and run ‘north and south’ to supply medullary cavity. There are vessels that perforate the bone to supply the outer compact bone
Periosteum has its own supply, as does the diaphysis and epiphysis

Question 12

What are the features & function of osteoprogenitor cells?

Answer 12

Mesenchymal origin
Give rise to osteoblasts
Found in the periosteum and endosteum (mainly endosteum)
Flattened cells that are difficult to see
Activated by damage to the bone

Question 13

What are the features & function of osteoblasts?

Answer 13

Arise from osteoprogenitor cells
Create new bone - lay down osteoid
Osteoid = growth factors, cytokines, ECM (collage Type 1 + proteins)
Add new layers to the bone surface (outside)
They can become quiescent but become large & plump when active
Similar to plasma cells - can see ribosomes & paranuclear hoff

Question 14

What are the features & functions of osteocytes?

Answer 14

Osteocytes arise from osteoblasts that become entombed by bone while laying down new osteoid
The space that surrounds them = lacunae
They are important in maintaining the health of the bone matrix and for mineralisation of the bone (breakdown local bone to liberate Ca for hydroxyapetite)
Receive nutrition from small cytoplasmic extensions to the endosteum = canaliculae

Question 15

What are the features & functions of osteoclasts?

Answer 15

Arise from a monocyte lineage
Destroy bone in normal remodelling, growth and repair
Large cells with multiple nuclei
Secrete H+, Cl-, proteases to destroy ECM
Maturation and thus activity is influenced by osteoblasts, PTH and oestrogen

Question 16

What process leads to osteoclast maturation and activity?

What influences this interaction?

Answer 16

Osteoblasts contain RANK receptor which interacts with the RANKL (ligand) on immature osteoblasts
When binding occurs osteoblasts mature and increase in activity

OPG produced by osteoblasts circulates and can bind to RANKL to prevent it binding to RANK
Oestrogen reduces OPG and increases RANK/RANKL - decreases maturation and activity

PTH increases maturation & activity by increasing RANKL

Question 17

What two processes form bone in development?
Which is more common?
Which bones result from which process?

Answer 17

Endochondral ossification and membraneous bone formation

Endochondral more common

Endochondral = long bones, appendicular skeleton, weight bearing bones
Membranous = skull, clavical and flat bones of face & jae

Question 18

What is the process of endochondral bone formation?

Answer 18

1. Start with cartilage model
2. Perichondrium becomes periosteum, forming a bone collar around the diaphysis
3. Primary centre of ossification in centre of diaphysis cavity
4. Invasion of shaft by osteoprogenitor cells, mesynchmal cells and blood vessels
5. Cartilage replaced by bone at the at the growth plate forming a perforated trabecular matrix
6. Elongation of the diaphysis occurs as growth plates move away from each other
7. Secondary ossification centres develop in each of the epiphysis of the bone - same process of new bone formation

Question 19

What are the features & layers of the growth plate?

Answer 19

Growth plate (epiphyseal plate) is the interface between the diaphysis and epiphysis 
= cartilage + bone (metaphysis)

Contains layers of chondrocytes at different stages & a layer of new bone - new bone added from the diaphysis side of the growth plate

At the epiphysis side (distal) to the proximal side:

1. Resting zone - resting chondrocytes
2. Proliferative zone - proliferating chrondrocytes to maintain the growth plate
3. Maturation zone - mature chondrocytes
4. Hypertrophic zone - dying chrondrocytes (‘smothered’ by new bone layer)
5. Ossfication zone - dying chrondrocytes replaced with osteoid

Question 20

What is membranous bone formation?

Answer 20

Myesenchymal cells differentiate into osteoprogenitor cells, which develop into osteoblasts
Osteoblasts deposit osteoid directly into the embryological tissue to form new bone

Question 21

What type of bone is first layed down during bone production in development?
What other situations is it also produced?

Answer 21

Woven bone

Produced by endochondral ossification and also during bone repair and in some pathological processes

Question 22

What are some features of woven bone?

Answer 22

It is temporary - is remodelled to lamellar bone

It is weaker, has more cells and collagen and no haversian systems than lamaellar bone

Question 23

How is woven bone remodelled?

Answer 23

Woven bone is remodelled by osteoblast and osteoclast activity
In compact bone remodelling produces Haversian systems (osteons)

1. Osteoclasts tunnel cavities in the bone longitudinally along axis of stress, bringing endosteum and vessels with it
2. Osteoblasts from endosteum lay down osteoid in concentric layers with collagen of each layer running in opposite directions
3. Last concentric layer leaves a space forming the aversion canal where neuromuscular structures run