Introduction to bone and soft tissue

Question 1

What are the components of the musculoskeletal system?

Answer 1

Bone, muscle and connective tissue comprised of cartilage, ligaments and tendons. Joints are the point at which two separate bones meet.

Question 2

How is the human skeleton organised?

Answer 2

Consists of 206 bones in adults and 270 in children. Made up of the axial skeleton encompassing 80 bones divided into skull, vertebral column and thoracic cage regions. Appendicular bones include pectoral girdle, Upper and Lower Limbs, Pelvic girdle.

Question 3

What are the 4 types of bones? Provide examples

Answer 3

Long bones include fibula, tibia and metacarpals.
Short bones include talus and scaphoid.
Flat bones include ones making up cranial cavity, sternum and scapula.
Irregular bones include all others such as pelvic bone and vertebrae.

Question 4

What are the functions of the skeleton?

Answer 4

1. Support
2. Protection - vital organs protected
3. Movement - works with muscles to allow movement
4. Mineral storage - stores calcium and phosphate
5. Haematopoeisis
6. Stores fat
7. Regulates homeostasis by producing osteocalcin which regulates bone formation and protects against glucose intolerance and diabetes.

MRS SHMP

Question 5

From what and when does intramembranous ossification happen in utero?

Answer 5

Mesenchyme is the tissue from which all connective tissue and bone form. Mostly flat bones formed through this method such as clavicle and cranial bones. Bone formation begins in week 6 in utero.

Question 6

What is the difference between ossification and calcification?

Answer 6

Ossification is the process by which bone forms. This occurs for 4 reasons: initial formation, growth of bone, remoddeling of bone, repair of bone. Calcification is the hardening of the tissue layed down by osteoblasts - this is a process within ossification.

Question 7

What are the 4 stages of intramembranous ossification?

Answer 7

1. Development of ossification centre
2. Calcification
3. Development of trabeculae
4. Formation of periosteum

Question 8

Describe first 2 stages of intramembranous ossification

Answer 8

1. Mesenchymal stem cells aggregate + differentiate into osteoblasts. An osteoblast main cell responsible for bone deposition in body.
2. Form an ossification centre (the osteoblasts) and secrete osteoid.
3. Osteoid is uncalcified bone matrix and as osteoblasts secrete osteoid inwards towards ossification centre, using help of enzyme alkaline phosphatase and calcium, bone matrix calcifies.

Question 9

Describe how calcified bone forms spongy bone in intramembranous ossification

Answer 9

4. Some osteoblasts therefore trapped within central space of bone and these trapped osteoblasts differentiate into osteocytes, housed within lacunae.
5. Ossification centres continue ossification and initially, structures known as spiculae form but as these grow, they eventually touch and join. These grow around blood vessels.
6. Due to them growing around blood vessels, form a random, diorganised arrangement and trabeculae form the spongy bone.

Question 10

Describe how periosteum is formed in intramembranous ossification

Answer 10

9. However, osteoblasts on edge of bone carry out bone remodelling and begin to differentiate into lamellar bone and bind together more strongly in units called osteons. This is compact bone.
10. Lamellar bone is formed around the trabecullae and deposited in layers.
11. Haematopoeisis occurs within the internal spongy bone within red marrow (vascular tissue within trabecular spaces)
12. Osteoblasts remain on bone surface to remodel the bone when needed. This layer known as periosteum which allows for appositional growth.

Question 11

What bones are formed through intramembranous ossification?

Answer 11

Flat bones of skull (fontanels), most of facial bones, mandible and medial part of clavicle. Fontanels undergo intramembranous ossification following birth. Flat bones are those which follow a compact bone/spongy bone/compact bone structure. If bone remodelling done differently, the inner spongy bone layer can be removed to form a medullary cavity for yellow marrow.

Question 12

When does endochondral ossification begin?

Answer 12

Begins in month 2 in utero. Forms all bones below head except clavicle and uses hyaline as blue print.

Question 13

How is the bone collar formed in endochondral ossification?

Answer 13

1. Perichondrium is vascularised and begin delivering nutrients to the area through blood vessels to mesenchymal cells, which use this to differentiate into osteoblasts.
2. Newly formed osteoblasts gather at diaphysis wall to form bone collar (bone going around entire circumference) - whole process starts from primary ossification centre

Question 14

How is the central clearing formed from the bone collar in endochondral ossification?

Answer 14

3. Formation of bone collar provides signal to chondrocytes within the central cavity to enlarge, causing matrix to calcify. Chondrocytes are cells which are responsible for building the initial hyaline blueprint.
4. Calcfication means matrix now impermeable to matrix and so causes cell death in the area hence cells in central cavity die, forming a central clearing. Hyaline cartilage is deteriorating.

Question 15

What is the role of the periosteal bud in step 5 of endochondral ossification?

Answer 15

In the middle of the bone, periosteal bud appears which directs new bone growth - periosteal bud invades central cavity and stimulates formation of spongy bone. It conists of arteries, veins, lymphatics, nerves and delivers osteogenic cells.

Question 16

How do osteogenic cells act in the central cavity?

Answer 16

Osteoclasts degarde hyaline cartilage while osteoblasts deposit new spongy bone matrix. Bone is still elongating due to chondrocytes at distal ends.

Question 17

How is medullary cavity formed in endochondral ossification?

Answer 17

Central cavity containing primary ossification centre continues to enlarge as more spongy bone deposited. Osteoclasts also begin breaking down the central portion of newly formed spongy bone to start forming medullary cavity for storage of bone marrow.

Question 18

What is the role of the secondary ossification centre?

Answer 18

Secondary ossification centre appears towards one or both distal ends of bone in epiphyses - these will only appear only after birth. Short bones only have one centre while irregular bones may have several. Once bone almost near full length, chondrocytes only deposit new cartilage in developing epiphyses so ostoblasts begin depositing bone towards start of epiphyseal plates.

Question 19

What are the stages of endochondral ossification?

Answer 19

1. Bone collar formation
2. Cavitation
3. Periosteal Bud Invasion
4. Diaphysis Elongation
5. Epiphyseal Ossification

Question 20

What are 4 key points about endochondral ossification?

Answer 20

1. Development of long bone from a hyaline cartilage model
2. Takes longer than intramembranous ossification
3. Primary ossification center - diaphysis
4. Secondary ossification center – epiphysis

Question 21

What are the 4 types of bone cells and what are their functions?

Answer 21

Osteogenic cells - stem cells
Osteoblasts - immature bone cells which secretes osteoid and catalyses minseralisation of osteoid
Osteoclast - develop from monocytes and macrophages + differ in appearance from other bone cells - multinucleate cell which scretes acid and anzymes to dissolve bone matrix and is derived from bone marrow
Osteocyte - Mature bone cell. Formed when an osteoblast becomes imbedded in its secretions. Sense mechanical strain to direct osteoclast and osteoblast activity.

Question 22

Where are the bone cells found?

Answer 22

Osteogenic cells – Deep layers of periosteum
Osteoblasts – Growing portions of bone, including periosteum and endosteum
Osteocytes – Entrapped in matrix within lacuna
Osteoclasts – Bone surfaces and at sites of old, injured or unneeded bone

Question 23

What is the organic component of the bone matrix?

Answer 23

Type 1 collagen and ground substance, containing proteoglycans, glycoproteins, cytokine and growth factors.

Question 24

What is the inorganic component of the bone matrix?

Answer 24

Calcium hydroxyapatite and osteocalcium phosphate.

Question 25

What are the 2 types of bone?

Answer 25

Immature bone is the first bone that is produced. It is laid down in a ‘woven’ manner and hence, relatively weak. It is mineralized and then replaced by mature bone. Mature bone is mineralized woven bone. Has a lamellar (layer) structure – relatively strong.

Question 26

What are the 2 types of mature bone?

Answer 26

Cortical bone is compact/dense and suitable for weight baring. Cancellous bone is spongy and has a honeycomb structure. Not suitable for weight baring.

Question 27

Describe structure of compact bone

Answer 27

Is made up of repeated structural units known as osteons. These consist of concentric lamellae around a central Haversian canal. The Haversian canal consists of an artery, vein, nerve and lymphatic supply. Lamellae contain small spaces known as lacunae which contain osteocytes. Tiny Canaliculi radiate from lacunae filled with extracellular fluid. Transverse perforating canals known as Volkmans canal travel to the Haversian canal from beyond the periosteum.

Question 28

Describe the structural components of long bones

Answer 28

Periosteum – Connective tissue covering
Outer Cortex – compact bone
Cancellous bone
Medullary cavity - contains yellow bone marrow
Nutrient Artery
Articular cartilage: on surface of bone at a joint only

Question 29

Describe structure at ends of long bones

Answer 29

Articular cartilage at joint, epiphysis, physis, metaphysis and diaphysis in the centre.

Question 30

What are the types of bone growth?

Answer 30

Interstitial growth is the lengthening of bone while appositional growth is the increase in diameter or thickness. Chondrocytes are responsible for interstitial growth while osteoblasts are responsible for appositional growth.

Question 31

How does long bone lengthen?

Answer 31

Physeal plate contains the hyaline cartilage. In the reserve zone, matrix is produced while it undergoes mitosis in the proliferative zone. Epiphyseal side is where hyaline cartilage is active and dividing to form hyaline cartilage matrix. Diaphyseal side is where cartilage calcifies and dies and then replaced by bone.

Question 32

What is appopsitional growth and how does it occur?

Answer 32

It is the deposition of bone beneath the periosteum to increase thickness.

1. Ridges in periosteum create groove for periosteal blood vessel
2. Periosteal ridges fuse, forming an endosteum-lined tunnel
3. Osteoblasts in endosteum build new concentric lamellae inward toward center of tunnel, forming a new osteon
4. Bone grows outwards as osteoblasts in periosteum build new circumferential lamellae. Osteon formation repeats as new periosteal ridges fold over blood vessel.

Question 33

Describe calcium homeostasis

Answer 33

When thyroid gland releases calcitonin, osteoclast activity is inhibited and calcium reabsorption in the kidneys decreases lowering calcium levels. When levels too low, parathyroid releases PTH which causes osteoclasts to release calcium from bones, calcium is reabsorbed from urine by the kidneys and greater vit D synthesis increases calcium reabsorption from the kidneys.

Question 34

How are joints classified?

Answer 34

Fibrous, Synovial and Cartilaginous. Fibrous joints include sutures, syndesmosis and interosseous membranes. Synovial joints can be of many different types. Cartilaginous joints are synchondroses and symphyses.

Question 35

What are the types of synovial joints?

Answer 35

Plane (between tarsal bones), Hinge (elbow), Condyloid (between radius and carpal bones of wrist), Pivot (between C1 and C2 vertebrae), Saddle (between trapezium carpal bone and metacarpal 1) and Ball and socket (hip joint)

PHC PSB

Question 36

What are the features of synovial joints?

Answer 36

Is the most common type of joint. Most mobile. 
Joint capsule:
Articular capsule (Outer)  – keeps bones together structurally
Synovial membrane (Inner)  - contains synovial fluid
Synovial fluid – reduce friction during movement

Question 37

What is the role of ligaments?

Answer 37

Prevent excessive movement that could damage joint. When there are more ligaments and tighter ligaments, results in greater stability BUT less mobility. Less ligaments and laxer ligaments mean greater mobility BUT less stability.

Question 38

What are 2 reasons why ligaments may fail?

Answer 38

1. Disproportionate, inappropriate or repeated stress to ligaments results in injury
2. Excessive ligament laxity leads to hypermobility and hence greater risk of injury

Question 39

Why is the shoulder joint unstable?

Answer 39

There is a mismatch in terms of joint articulation as glenoid fossa is shallow. Joint capsule is also weak and it lacks strong ligaments. Movement is dependent on the rotator cuff muscles but joint is extremely mobile.

Question 40

Why is the hip joint stable?

Answer 40

Joint articulation is stable as there is a complete fit and deep socket along with a strong joint capsule. There is a strong network of ligaments and joint is supported by muscles. While joint is less mobile, it is stable.