Building and maintaining the skeleton

Question 1

What is the role of the bone?

Answer 1

Bone is important in providing structural support and rigidity to the limbs, protection, movement and in regulating homeostasis. It is one of the hardest tissues in the body and is metabolically active and highly vascularised. It is the site for hoemopotasis in the bone marrow.

Question 2

What is the origin of the skeleton?

Answer 2

Head derives from the neural tube of the ectoderm and body derives from the paraxial mesoderm.

Question 3

What are mesenchymal cells?

Answer 3

Stem cells found in the bone marrow which is a progenitor for many cell types such as RBC, connective tissue and skeletal tissue like bone. It is derived from the mesoderm

Question 4

What is the origin of the vertebrae?

Answer 4

Paired somites from the paraxial mesoderm

Question 5

What is the role of the hox genes?

Answer 5

Guide the morphology (shape) of the skeleton during development. As bone develops, it starts off as either cartilagenous precursor or membrane bound. It is deposited all over the embryo but becomes organised to the edges by Hox genes for appositional growth where new bone is on the outer surface and older bone is at the centre.

Question 6

How does morphogenesis occur?

Answer 6

Limb buds begin to develop around five weeks and have an apical hardened ectoderm edge called the Apical Ectodermal Ridge originating from the embryonic tube. This is important for bone morphogenesis and is located in the progress zone and releaes fibroblast growth factors to cause bone morphogensis. The zone of polarising regulates morphogenesis by inhibiting it or further stimulating it.

Question 7

What is the progress zone?

Answer 7

Layer of mesodermal cells below the apical ectodermal ridge of the limb bud

Question 8

What is the zone of polarising activity?

Answer 8

Region which sends signals to instruct the limb bud to form along the anterior-posterior axisc. and regulates bone morphogenesis.

Question 9

What is the role of the apical ectodermal ridge?

Answer 9

Maintains outgrowth of the limb bud to keep mesenchymal cells in the progess zone in an undifferentiated state.

Question 10

What are the types of ossification?

Answer 10

Entochondral ossification from a hyaline cartilage precursor or Intramembranous ossification directly from mesenchymal connective tissue as osteoblasts

Question 11

Where are the ossification centres?

Answer 11

The primary areas of bone formation in the epiphysis and diaphysis which are seperated by an epiphyseal plate.

Question 12

What are chondrocytes?

Answer 12

They are cells which begin as chondroblasts. It secretes/synthesises the extracellular matrix such as collagen and become entombed within it to differentiate into a chondrocyte. The empty space they lie in is the lacunae. These chondrocytes become stacked and form piles to force the ossification centres away from each other to increase bone growth. Responsible for entochondral ossification.

Question 13

How does intramembranous ossification occur?
bone.

Answer 13

Directly from mesenchymal cells which differentiate into osteoblast cells. These secrete an extracellular proteoglycan matrix called osteoids to allow it to bind to calcium for calcficiation and be deposited as lamallae or layers of the matrix. This release of osteoids causes osteoblasts to become entrapped and form the ossification centre and become osteocytes. Osteoids continue to be secreted from osteoblasts which allows blood vesses to enter and form the medullary cavity. On the outer surface, osteoid secreted from osteoblasts forms the outer periosteum layer. In the inner surface, it forms cortical compact

Question 14

How does endochondral ossification occur?

Answer 14

Formation of bone from hyaline cartilage. The mesenchymal cells differentiates into chondrocytes. These release extracellular matrix to form the hyaline cartilage model. The outer layer of cartilage is the perichondrium and the inner layer is formed of chondroblasts. At the centre, chondroblasts secrete collagen X and fibronectin for calcification of hyaline cartilage. This leads to cell apoptosis as nutrients cannot enter. Blood vessels enter through these dead areas and forms the medullary cavity. Blood contains osteogenic cells which causes the perichondrium -> periosteum. The osteoblasts cause the formation of compact bone in the diaphyseal region of the periosteum called the periosteal collar. In the epiphysis, cells continue to proliferate and this is the site where the bones continue to elongate in childhood.

Question 15

What are osteoids?

Answer 15

Unmineralised organic tissue which undergoes calcification to be deposited as lamellae or layers in the bone matrix by ostepblasts.

Question 16

What is the composition of bone?

Answer 16

Bone consists of a double membranous layer of periosteum. Cortical compact bone is the most superificial layer which contains osteons and the trabeculae is at the centre of bone and contains the medullary cavity where blood vessels enter. The epiphysis is at the end of bones; the diaphysis is the shaft and the metaphysis is the space between the two.

Question 17

What is the periosteum?

Answer 17

Bones are coated with a thick, hard membrane called a periosteum which has an inner layer and outer layer. The outer layer has a collagenous matrix with fibroblast cells and is highly vascularised via the Volksmann’s cannals for the primary blood supply to the muscles and nerves. The inner layer is the cambium layer. The periosteoum is important as a limiting layer because it prevents bone tissue invading neighbouring tissue.

Question 18

What are the zones in the epiphysis?

Answer 18

The bone epyphysis has many zones for bone development from entochondral ossification.
the zone of reserve cartilage which has immature chondroblasts
zone of proliferation where chondroblasts divide and form stacks
zone of maturation where hypertrophy occurs and larger stacks of chondroblasts form to force away the ossification centres
zone of cartilagenous degradation where the cartilage degrades and ossifies to bone
osteogenic zone- epiphyseal plate of mature bone which is an ossification centre

Question 19

What are the characteristics of cartilage?

Answer 19

Cartilage is avascular and smaller than bone so it can rely only on diffusion to porvide nutrients. Cartilage has interstitial growth all aorund itself but as it becomes mature it develops into appositional growth where new bone is directed to the edges by Hox genes. Children have more cartilage than adult bone.

Question 20

What is epiphyseal growth plate?

Answer 20

Region separating the epiphysis from the metaphysis which is the site of bone growth from cartilage precursor chrondroblasts.

Question 21

What is the lamallae?

Answer 21

Layer of bone matrix consisitng of rings of collagen bundles arranged in alternating circular layers which gives bone its rigidity and flexibility.

Question 22

What is circumferential lamallae?

Answer 22

Lamallae surrounding bone along stress lines

Question 23

What regulates the activity of osteoblasts and osteoclasts?

Answer 23

Mechanical stress stimulates osteoblasts to secrete osteoids and differentiate into osteocytes to form new bone in appositional growth and osteoclasts to resorb old/damaged bone.

Question 24

What is concentric lamallae?

Answer 24

Rings of collagen fibres which contain regions of old osteocytes.

Question 25

What are haversion canals/osteons?

Answer 25

Cylandrical tubes which contain capillaries and nerve fibres to supply cortical bone with oxygen and nutrients. Osteons include the surrounding collagen rings called lamallae which contains osteocytes between them called concentric lamallae.

Question 26

What are volkmann’s canals?

Answer 26

Channels which transmit blood vessels from periosteum to communicate and supply to haversian canal/osteon to supply nutrients to bone. It is perpendicular to the Haversian canals/osteons

Question 27

What are the types of osteons?

Answer 27

Primary osteons which is new bone formed from osteocytes and secondary osteons which are old osteocytes found within the lamallae.

Question 28

What are osteoblasts?

Answer 28

Precursors to osteocytes which secrete extracellular matrix called osteoids and become entombed within it to differentiate into osteocytes which lay new bone in random orientation which are later arranged in appositional growth. They contain canaliculi to itneract with other cells and lie within the lacunae.

Question 29

What are osteoclasts?

Answer 29

Responsible for bone remodelling and bone resorption of old/damaged bone and contains a ruffled border to dissolve bone and its minerals.

Question 30

What is the trabeculae?

Answer 30

Networks of lamallae that make up spongy/trabecullae bone formed by osteoclast resoprtion to distribute mechanical load away from joints and towards the compactbone, They have no osteons, but have osteocytes in lacunae and the space is filled with marrow for RBC production and fat which can leak out into the bloodstream if damaged and lead to fat emboli.

Question 31

What is the difference between osteons/ haversian systems and haversion canals?

Answer 31

Haversian system include both the canal and the rings of lamallae. Canals consist only of the blood supply and the capillaries

Question 32

How can joints be classified?

Answer 32

Joints can be classified by the types of tissues. Fibrous joints are connected by fibrous tissue. Cartilagenous joints are connected by cartilage. Synovial joints have a synovial fluid filled capsule where they join.

Question 33

What are the synarthroses joints?

Answer 33

The synarthroses joints are immovable bone between fibrous tissue and includes:
-sutures in the skull
-intranasal betwene maxilla and frontal bone.
-synchondroses-hyaline cartilage joint between diaphysis and epiphysis of a single bone
-gomphoses which are the articulation of the teeth with the maxilla or mandible

Question 34

What are the amphiarthrosis joints?

Answer 34

The syndesmosis: joint which has the tibiafibulararticulation and the radialulnar articulation. There is an interosseus membrane arranged in oblique oritentation of cartilage.
-the symphyses- fibrocartilage joint with some hyaline, eg pubic symphysis and vertebral joints
-cartilage that joints the ribs

Question 35

What are the diarthrosis joints?

Answer 35

Synovial joint which has articular cartilage lining the bone articulation. The synovial layer has a tangentianal layer where cartilage is arranged longitudinally (parallel) to surface, transitional layer with no orientation, radial layer where cartilage is in stacks that move upwards and outwards to become calcified cartilage (with calcium) and then the layer of mature ossification.

Question 36

What is the function of articular cartilage?

Answer 36

Responsible for the distribution of weight to other bones and a smooth frictionless surface for articulation.

Question 37

What is the histology of the articular cartilage?

Answer 37

Contains a tangentianal layer where the collagen fibres are arranged parallel to the surface, a transitional layer with random orientation and raidial layer where these are formed in stacks to force the regions apart.

Question 38

How does bone repair?

Answer 38

Platelets clot and form a haematoma to limit bleeding and subperiosteal cells and endosteal cells proliferate to ossify this and form a hardened callus which is woven and consolidated to lamellar bone where it is remodelled to form part of the marrow.