SDL: Development of the skeleton

Question 1

What is the extracellular matrix?

Answer 1

Large network of proteins and other molecules that surround, support, and give structure to cells and tissues in the body

Question 2

How is the skeletal system organised?

Answer 2

Skeletal system is highly specialised connective tissue system

Comprised or paucicellular matrix together with cells that manufacture the matrix

Consists of bones, cartilage, joints + ligaments

Question 3

What is the skeleton divided into?

Answer 3

Axial- vertebrae, ribs, pelvic bones and ribs
Appendicular- upper and lower limb

Question 4

What are the most common types of bones

Answer 4

• Long (humerus, femur) and flat (sternum, ribs, pelvic bones)

Question 5

What is long bone divided into?

Answer 5

• Epiphysis
• Metaphayses
• Diaphysis

Question 6

What is the epiphysis?

Answer 6

End part of long bone, initially growing separately from the shaft

Question 7

What is the metaphyses?

Answer 7

Neck portion of a long bone between epiphysis and diaphysis. Contains growth plate, part of bone that grows during childhood, as it grows it ossifies near the diaphysis and the epiphyses

Question 8

What is the diaphysis?

Answer 8

Shaft, central part of long bone, long tubular structure in all long bones, comprised of medullary cavity, which contains bone marrow and blood vessels.

Question 9

What is bone comprised of?

Answer 9

• Mineralised bone matrix and bone cell
• Over 90% of bone is comprised of matrix, most abdundantly type I collagen with other non-collagenous proteins.
• Unmineralised bone matrix called osteoid

Question 10

What are the functions of bone?

Answer 10

• Provide structural support (e.g. axial skeleton)
• Protect vital organs (e.g. skull, ribs)
• Enable movement (long bones of upper and lower limbs)
• Store calcium and participate in calcium homeostasis

Question 11

What are the types of bone?

Answer 11

• Cortical
• Cancellous

Question 12

Where is cortical bone found?

Answer 12

Lies in outer position of long bones + vertebrae

Question 13

What is cortical bone composed of?

Answer 13

Long parallel columns called osteons

Question 14

What are osteons made of?

Answer 14

Concentric rings of bone, lamellae, surrounding a central Haversian canal containing blood and lymphatic vessels

Question 15

What bone cells are buried within bone and what do they lie within?

Answer 15

Osteocytes, which lie within lacunae

Question 16

What are osteocytes differentiated from?

Answer 16

Osteoblasts, and they have a cell body and cell processes that connect with other osteocytes

Question 17

Where are the cell processes?

Answer 17

In canaliculi

Question 18

What is cortical bone covered by?

Answer 18

Periosteum, this consists of fibrous connective tissue

Question 19

What is the medullary cavity and what is it composed of?

Answer 19

Centre of bones, composed of cancellous bone, with intervening marrow

Question 20

What is cancellous bone comprised of?

Answer 20

Network of bony plates or struts called trabeculae

Question 21

What are trabeculae?

Answer 21

Thin columns and plates of bones that create a spongy structure

These connect with each other and to the endosteum of cortical bone

Question 22

In adults, what is the space between trabeculae filled with?

Answer 22

Haemopoetic bone marrow or adipose tissue (fat)

Question 23

How is cancellous bone different from compact (cortical) bone?

Answer 23

This type of bone is lighter and less dense than cortical bone, and has irregular cavities that contain red bone marrow

Question 24

What are the 3 types of bone cells?

Answer 24

• Osteoblasts
• Osteocytes
• Osteoclasts

Question 25

What is the function of osteoblasts?

Answer 25

• Make osteoid (unmineralised, organic portion of bone matrix, forms prior to maturation of bone tissue)
• Synthesise bone matrix, make bone, lie on surface of bone, when they’re actively synthesising bone matrix they appear cuboidal with small nuclei, when they become inactive they have the appearance of flat bone-lining cells
• Found on cartilage/ fibrous tissue via ossification

Question 26

What is the function of osteocytes?

Answer 26

Derived from osteoblasts

Linked by thin processes of cytoplasm connected by cell junctions

Nuclei spaced far apart

Buried with cortical and cancellous bone.

Mature bone cells (osteocytes)

Function is thought to be mechanosensation (transduction of mechanical force

Question 27

What is the function of osteoclasts?

Answer 27

• Remove/ Reabsorb bone, they’re large, often multinucleate
• Bone removed by dissolving mineral and breaking down matrix, this is recycling of bone governed by hormones, vitamins and available calcium
• Found on surface of bone
• Remodel bone in response to weight etc.

Question 28

What is cartilage made of?

Answer 28

• Chondrocytes
• Cartilage consists of chondrocytes set in matrix, which is rich in proteoglycans (proteins present in connective tissue), hence why cartilage is solid but flexible

Question 29

What are the 3 types of cartilage?

Answer 29

-Hyaline/articular cartilage
-Fibrocartilage
-Elastic cartilage

Question 30

What is the most common type of cartilage?

Answer 30

Hyaline, which is found in napal septum, larynx, tracheal rings, most articular surfaces of joints

Question 31

Describe hyaline cartilage

Answer 31

Contains few fibres, most common type of cartilage, found in:
- napal septum
- larynx
- tracheal rings
- most articular surfaces of joints

Question 32

Describe fibrocartilage

Answer 32

Contains abundant collagen fibres

Found in intervertebral discs and some joints such as pubic symphysis

Question 33

Describe elastic cartilage

Answer 33

• Contains elastic fibres
• Occurs in external ear, external auditory meatus, epiglottis, parts of laryngeal rings

Question 34

What is the name of the process in which long bones elongate?

Answer 34

Endochondral ossification

Question 35

What is endochondral ossification?

Answer 35

When a partially mineralised cartilaginous matrix formed at the growth plate becomes reabsorbed and replaced by woven bone initially, which is then remodelled into lamellar bone

Question 36

What is woven bone?

Answer 36

Bone in which collagen fibres are randomly arranged, and this can be seen under polarised light.

This is immature bone that’s produced when bone is formed rapidly

Woven bone = spongy

Question 37

What is lamellar bone?

Answer 37

Lamellar bone is composed of sheets or lamellae of parallel collagen.

This can be visualised under polarised light

Almost all bones in healthy adults consists of lamellar bone, this bone is stronger than woven

Question 38

Describe cartilage formation at the growth plate

Answer 38

• Growth plate enlarges by appositional growth, as mesenchymal cells differentiate into matrix-secreting chondrocytes
• These continue to divide with proliferative zone, increasing cartilage mass by interstitial growth
• Matrix produce by chondrocytes contains poorly organised type II and type IX collagen fibrils and several non-collagenous proteins including proteoglycans, chondrocalcin, osteonectin and matrix gla protein
• Chondrocytes progressively enlarge, form hypertrophic chondrocytes, which produce a matrix permissive for calcification in the hypertrophic zone by degrading type II collagen and increasing synthesis of type X collagen and osteopontin
• Once calcified, cartilage partly reabsorbed by osteoclasts in zone of vascular invasion (invading capillary loops from the metaphysic break through the last transverse septum of mineralised cartilage to enter the hypertrophic chondrocyte lacuna), then replaced by woven bone to form primary spongiosa
• Primary spongiosa remodelled and replaced by lamellar bone forming secondary spongiosa

Question 39

What pattern does cartilage formation at the growth plate follow?

Answer 39

Sequential patter

Question 40

What is intramembranous bone formation?

Answer 40

• The process of modelling and remodelling at trabecular and cortical surfaces in the developing and adult skeleton

Question 41

How does intramembranous bone formation differ from endochondrol ossification?

Answer 41

• There’s no cartilaginous template formed before bone deposition, therefore intramembranous bone can only form by appositional growth

Question 42

What sequence does formation of intramembranous bone follow?

Answer 42

Temporal

Question 43

Describe the formation of intramembranous bone

Answer 43

• Stromal + bone lining cells differentiate, form mature osteoblasts, secrete unmineralised organic matrix (osteoid) onto bone surface
• Composed of mainly type I collagen fibrils, aggregate to form collagen fibres, and in turn larger fibre bundles. Higher organisation varies between types of bone tissue, reflect how matrix was formed
• In woven bone, (formed during periods of rapid formation, e.g, developing foetus, repair after injury etc.) fibres loosely packed with ordered spatial arrangement, contrasting more orientated arrangement in lamellar bone, in which matrix formed in successive thin layers
• Variety of other non-collagenous proteins also secreted by osteoblasts + form part of osteoid
• Post osteoid formation, matrix undergoes maturation, creates environment permissive for mineralisation
• Calcium + phosphate deposited in organic matrix in form of hydroxyapatite

Question 44

What is H&E?

Answer 44

Staining of tissue sections with haematoxylin, which stains cell nuclei blue

Question 45

What happens to the calcified bone and osteoid in decalcified H&E sections?

Answer 45

Calcified bone and osteoid appear pink

Question 46

What happens to the calcified bone and osteoid in Eosin?

Answer 46

Calcified bone and osteoid appear pink

Question 47

Why does the calcified bone and osteoid in decalcified H&E and Eosin stain pink?

Answer 47

Because calcium has been removed during histological processing

Question 48

What do small amounts of calcium that remain change the colour of mineralised bone to?

Answer 48

A dark blue-purple colour

Question 49

What is bone remodelling?

Answer 49

Specialised form of intramembranous bone formation that takes place throughout life in adult skeleton

Question 50

What is the purpose of bone remodelling?

Answer 50

Remodel bone so it remains structurally adapted to mechanical needs of body, in addition to calcium homeostasis

Question 51

Outline bone remodelling

Answer 51

• Follows strict temporal + spatial sequence
• Bone’s removed in small packets by osteoclasts
• Osteoid laid down by osteoblasts in same location, filling resorption cavity and replacing the bone

Question 52

Describe osteoporosis

Answer 52

• Total amount of bone reduced, but bone fully and normally mineralised
• Results in thinner trabecular which are poorly connected with each other
• Causes structural and mechanical integrity of bone to be diminished, so bone fractures on minimal trauma

Question 53

Describe osteomalacia

Answer 53

Total amount of bone preserved but amount of mineralised bone is reduced and there’s increased amount of osteoid (unmineralised bone matrix)

Question 54

What is closely packed, ‘compact’ bone?

Answer 54

Cortical bone

Question 55

What is calcified, spongy looking bone?

Answer 55

Cancellous bone, in some bones spaces in spongy bones contain bone marrow

Question 56

What does cortical bone look like microscopically?

Answer 56

Orderly arrangement with concentric cylinders of bone laid down around central blood vessels containing osteocytes. More blood vessels connect the central vessels horizontally to form a complex mesh of small arteries and veins.

These cylindrical structures are Haversian systems or ‘osteons’

Question 57

How are osteons packed to form cortical bone?

Answer 57

Closely together

Question 58

How are osteons arranged in cancellous bone?

Answer 58

As irregular plates