Bone

Question 1

Describe and label the structure of bone

Answer 1

Exterior of compact bone, interior of spongy trabecullar/cancellous bone

Question 2

What are the main 2 types of bone?

Answer 2

Lamellar and woven bone

Question 3

Describe the two types of lamellar bone

Answer 3

Compact/cortical bone

• has central blood vessel associated with CT
• osteocytes lie within lacunae and are linked by caniculi arranged in bony layers
• have osteone/Haversian system

Spongy/cancellous/trabecullar bone

• occurs within bones of all shapes
• spicules are aggregated into bony trabecullae with blood vessels in between
• spaces between trabecullae consist of CT (endosteum)

Question 4

Describe woven bone

Answer 4

• temporary form of bone associated with development, fracture repair and bone tumours
• randomly arranged collagen fibres
• in most cases is replaced by lamellar bone

Question 5

What are the functions of bone?

Answer 5

• rigid and hard support
• protects vital organs
• contains bone marrow
• reservoir for Ca, PO4 and other ions
• transforms forces of skeletal muscle contraction into bodily movements

Question 6

What are the 3 main cell types and what are their functions?

Answer 6

1. Osteoblasts - external and internal surfaces, synthesise osteoid (ECM), develop into osteocytes
2. Osteocytes - located in lacuna between lamellae of calcified osteiod, communicate via canaliculi, maintain bony matrix
3. Osteoclasts - external and internal surfaces; large, motile, multinucleated cells; on contact with bone develop a ruffled border sink in and digest bone to form lacuni

Question 7

What are the components of the bony ECM?

Answer 7

• osteoid - organic - collagen type 1 fibres, GAGs, Ca binding glycoproteins

Question 8

Describe the structure of compact bone

Answer 8

• Periosteum & endosteum - vascular CT on inner and outer surfaces; osteoblasts; forms outer and inner circumferential lamellae
• Volkmann’s canals & Haversian - branching network which penetrates diaphysis of long bone; lined with CT which is continuous with endosteum & periosteum; blood vessels & osteo-progenitor cells penetrate the bony shaft & form Haversian systems
• Osteones/Haversian system - central canals lined with osteoprogenitor cells; osteoblasts form concentric lamellae; collagen fibres have a specific orientation which gives strength; constantly being remodelled

Question 9

How do Haversian systems form?

Answer 9

1. Forms under the periosteum - longitudinal ridges form along the diaphysis & periosteal cells differentiate into osteoblasts; periosteal capillary is found in the groove; new bone extends adjacent ridges towards each other
2. Ridges fuse, groove becomes bony tunnel enclosing the blood vessel
3. Additional bone lamellae are deposited around the tunnel, then converted into Haversian canal containing blood vessel
4. Haversian vessel continues to recieve blood through Volkmann’s canal which extend obliquely across the diaphysis
5. When bone reaches full size, outer and inner circumferential lamellae provide boundaries of the compact bone consisting of Haversian systems

Question 10

What are the 2 ways in which the bony skeleton develops?

Answer 10

1. Intra-membranous ossification
2. Endochondral ossification

Question 11

Describe intra-membranous ossification

Answer 11

• forms directly from osteogenic CT
• there is no pre-existing hyaline cartilage model
• flat bones of the skull, mandible and maxilla are formed by it
• occurs in areas of appositional growth and repair

Question 12

Describe endochondral ossification

Answer 12

• replaces hyaline cartilagenous model in embryo with bone
• responsible for growth of long bones in embryo and juvenile

*woven bone is produced first but is replaced by lamellar bone*

Question 13

Describe the first stage of endochondral ossification

Answer 13

1. hyaline cartilage acts as a template
2. osteoprogenitor cells of perichondrium form the periosteal collar
3. chondrocytes proliferate then undergo hypertrophy which initiates the formation of the primary ossification centre
4. hypertrophic chondrocytes secrete VECGF to induce sprouting of blood vessels from perichondrium
5. blood vessels form the periosteal bud brach in opposite directions
6. calcification and apoptosis of hypertrophic chondrocytes occurs

Question 14

Describe the second stage of endochondral ossification

Answer 14

1. secondary centres of ossification and the epiphyseal growth plate forms
2. blood vessels and mesenchyme infiltrate the epiphysis and establish a secondary centre of ossification

Question 15

Describe the 3rd stage of endochondral ossification

Answer 15

1. termination of growth and closure of epiphyseal growth plate
2. blood vessels from the diaphysis and epiphysis intercommunicate
3. all epiphyseal cartilage is is replaced by bone, except for articular surface
4. epiphyseal plate is replaced by an epiphyseal line - gradually occurs from puberty to maturity until long bone can no longer grow in length

Question 16

Explain the process of compact bone remodelling

Answer 16

1. ACTIVATION - osteoclast precursors recruited to Haversian canal & differentiate into osteoclasts which line the bone lamellae facing the canal. They start bone resorption of the inner lamellae and consecutive lamellae towards the outer
2. RESORPTION - additional osteoclast precursors are recruited as lamellar resorption progresses beyond the boundary of original osteon. Osteoclasts revert back to osteoblasts once they stop removing bone
3. REVERSAL - osteoblasts reverse resorption process by organising a layer inside the resorption cavity and start to secrete osteoid. Cement line indicates boundary of newly organised lamella. New bone lamellae continues to be deposited towards the centre of the osteon
4. FORMATION - osteoblasts continue laying down bone and eventually become trapped within the mineralised bone matrix as osteocytes. New osteon/Haversian system formed