Organisation of the skeleton structure and function

Question 1

What is the diaphysis?

Answer 1

the shaft or central part of a long bone.

Question 2

What is the epiphysis?

Answer 2

the end part of a long bone, initially growing separately from the shaft.

Question 3

What are long bones divided into

Answer 3

Divided into epiphyses, metaphysis and diaphysis

Question 4

What happens to the epiphyses after puberty in adults?

Answer 4

The epiphyses becomes fused with the metaphyses

Question 5

What is the bone mainly composed of and what type of collagen is it predominantly

Answer 5

Over 90% of bone is composed of matrix, which is predominantly Type 1 collagen together with other non-collagenous proteins.

Question 6

What is unmineralised bone matrix called?

Answer 6

Unmineralised bone matrix is called osteoid

Question 7

What are the bone cells?

Answer 7

The bone cells are osteoblasts, osteocytes and osteoclasts.

Question 8

What are the 2 types of bone at a microanatomical level?

Answer 8

At a microanatomic level, there are two types of bone; cortical bone and cancellous(also known as trabecular) bone.

Question 9

What are the 2 forms that bone can exist as?

Answer 9

Woven and lamellar

Question 10

What is a woven bone and how does it look like under polarised light?

Answer 10

This is an immature form of bone that is produced when bone is formed rapidly eg in the neonate or in the early stages of fracture repair

In woven bone, the collagen fibres are randomly arranged, and this can be seen under polarized light

Question 11

What is woven bone subsequently remodelled into?

Answer 11

This is subsequently remodelled into lamellar bone, which is stronger.

Question 12

what is lamellar bone composed of?

Answer 12

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

Question 13

What is cortical bone composed of and what is it known as?

Answer 13

Cortical bone is composed of long parallel columns, known as osteons

Question 14

What are the osteons made up of and what do they surround?

Answer 14

made up of concentric rings of bone, the lamellae, surrounding a central Haversian canal

Question 15

What does the haversian canal contain?

Answer 15

contains blood and lymphatic vessels

Question 16

What are osteocytes and what do they have to connect with other osteocytes?

Answer 16

Osteocytes are terminally differentiated osteoblasts, and have a cell body and cell processes that connect with other osteocytes.

Question 17

Where are the cell processes located?

Answer 17

The cell processes are in canaliculi.

Question 18

What is cortical bone covered by and what does it consist of?

Answer 18

Cortical bone, in turn is covered by periosteum. This consists of fibrous connective tissue.

Question 19

What is the medullary cavity composed of?

Answer 19

The medullary cavity or center of bones is composed of cancellous bone, with intervening marrow.

Question 20

What is cancellous bone composed of?

Answer 20

Cancellous bone is composed of a network of bony plates or struts called trabeculae

Question 21

What do cancellous bone connect with?

Answer 21

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

In adults, the spaces between the trabeculae are filled with either haemopoetic bone marrow or adipose tissue.

Question 23

What do osteoblasts synthesize?

Answer 23

These cells synthesise bone matrix ie they make bone

Question 24

Where do osteoblasts lie?

Answer 24

On the surface of the bone

Question 25

What do osteoblasts look like when they are active and inactive?

Answer 25

when they are actively synthesizing bone matrix they appear as cuboidal cells with small nuclei, and when they become inactive, they have the appearance of flat bone-lining cells.

Question 26

Where are osteocytes buried within?

Answer 26

Buried within cortical and cancellous bone are osteocytes.

Question 27

What is the function of osteocytes thought to be?

Answer 27

The function of osteocytes is thought to be mechanosensation.

Question 28

What do osteoclasts do?

Answer 28

These cells resorb bone

Question 29

What are the osteoclasts and where are they found?

Answer 29

They are large cells, often multinucleate, and are found on the surface of bone, often in eroded surfaces or depressions that they have created.

Question 30

What does cartilage consists of and what is it rich in?

Answer 30

Cartilage consists of chondrocytes set in matrix which is rich in proteoglycans

Question 31

What is the most common type of cartilage and where is it found?

Answer 31

Hyaline cartilage is the most common type, and found in the nasal septum, larynx and tracheal rings, most articular surfaces of joints.

Question 32

Where is fibrocartilage found?

Answer 32

Fibrocartilage is found in intervertebral discs and some joints such as pubic symphysis.

Question 33

Where does elastic cartilage occur?

Answer 33

Elastic cartilage occurs in the external ear, external auditory meatus, epiglottis, and parts of the laryngeal cartilage rings

Question 34

What are the 3 types of cartilages?

Answer 34

-Hyaline
-Fibrocartilage
-Elastic

Question 35

What does hyaline cartilage contain?

Answer 35

Contains few fibres

Question 36

What does fibrocartilage contain?

Answer 36

Contains abundant collagen fibres

Question 37

What does elastic cartilage contain?

Answer 37

cartilage contains elastic fibres

Question 38

Steps involved in endochondral ossification

Answer 38

Long bones elongate during development by a process called endochondral ossification, whereby a partially mineralized cartilaginous matrix formed at the growth plate becomes resorbed and replaced by woven bone initially, which is then subsequently remodelled into lamellar bone.

Cartilage formation at the growth plate follows a sequential pattern, involving appositional and interstitial growth. The growth plate enlarges by appositional growth, as mesenchymal cells differentiate into matrix-secreting chondrocytes. These continue to divide within the proliferative zone, thereby further increasing the cartilage mass by interstitial growth. The matrix produced by chondrocytes in the proliferative zone contains poorly organized type II and type IX collagen fibrils, and several non-collagenous proteins including proteoglycans, chondrocalcin, osteonectin and matrix gla protein. The proliferative chondrocytes progressively enlarge to form hypertrophic chondrocytes, which produce a matrix permissive for calcification in the hypertrophic zone by degrading type II collagen and increasing the synthesis of type X collagen and osteopontin. Once calcified, cartilage is partly resorbed by osteoclasts in the zone of vascular invasion and then replaced by woven bone to form the primary spongiosa. This is subsequently remodeled and replaced by lamellar bone forming the secondary spongiosa.

Question 38

What are the steps involved in intramembranous bone formation?

Answer 38

Modelling and remodeling at trabecular and cortical surfaces in the developing and adult skeleton occurs by a process called intramembranous bone formation. This differs from endochondral ossification in that no cartilaginous template is formed before bone deposition. Therefore, intramembranous bone can only form by appositional growth.

The formation of intramembranous bone follows a temporal sequence. Firstly, stromal and bone lining cells differentiate to form mature osteoblasts, which secrete an unmineralised organic matrix, termed osteoid onto the bone surface.

This is composed predominantly of type 1 collagen fibrils, which aggregate to form collagen fibres, and in turn, larger fibre bundles. The higher organization of these collagen fibres varies between different types of bone tissue, reflecting how the matrix was formed. In woven bone, which is formed during periods of rapid formation, such as the developing foetus or in repair after injury, fibres are loosely packed without an ordered spatial arrangement.This contrasts with the more orientated arrangement of collagen fibres in lamellar bone, in which the matrix is formed in successive thin layers. In addition to type 1 collagen, a variety of non-collagenous proteins are also secreted by osteoblasts, and form part of osteoid.

Following osteoid formation, the matrix undergoes a period of maturation creating an environment permissive for mineralisation. Calcium and phosphate is deposited within the organic matrix in the form of hydroxyapatite. In decalcified H&E sections, both calcified bone and osteoid appears pink, because calcium has been removed during histological processing. Focal amounts of calcium that remains renders mineralized bone a dark blue-purple colour. In undecalcified sections, special stains may be used to distinguish unmineralised matrix (osteoid) and fully mineralized bone.