Bone Growth & Joints (Exam II)

Question 1

Intramembranous Ossification

Answer 1

• Bone forms directly in mesenchyme (no cartilaginous precursor)
• Mesenchymal cells differentiate into osteoprogenitor cells & then into osteoblasts
• Osteoblasts lay down bone matrix to form many isolated spicules
• Spicules grow and fuse together to form trabeculae
• Connective tissue surrounding this region condenses to form periosteum
• Trabeculae near periosteum grow & fuse together to form compact bone

Question 2

Endochondral Ossification of long bone

Answer 2

A hyaline cartilage model of the bone forms from mesenchyme in the fetus

Primary center of ossification forms at midshaft

• Chondrocytes at midshaft hypertrophy & die
• Perichondrium at midshaft becomes a periosteum; osteoblasts differentiate from it & lay down a periosteal collar of bone
• Chondrocyte lacunae coalesce, leaving thin irregular spicules
• Matrix of cartilage becomes calcified
• An osteogenic bud grows into the coalesced lacunae
• Bud consists of blood vessels & osteoprogenitor cells that adhere to the exterior of the vessels
• Osteoblasts differentiate from osteoprogenitor cells in the osteogenic bud
• Osteoblasts lay down bone matrix on the spicules of calcified cartilage, forming mixed spicules

Secondary centers of ossification form in each epiphysis

• Not associated with a periosteal bone collar
• An epiphyseal plate (growth plate) of hyaline cartilage remains between the primary ossification center & each secondary ossification center
• Interstitial growth continues in the cartilage of the epiphyseal plate, resulting in increase in length of the bone
• Bone forms mainly on diaphyseal side of growth plate, replacing cartilage
• When rate of ossification exceeds the rate of interstitial growth of the epiphyseal cartilage, the epiphyseal plate becomes completely replaced by bone (closure of the epiphyses)
• After epiphyseal closure, the bone can no longer grow in length
• Remodeling eventually removes the calcified cartilage in the spicules & converts woven bone to lamellar

Question 3

Epiphyseal Plates

Answer 3

There are 5 zones:

1. Zone of reserve cartilage (resting zone)
2. Zone of proliferation
3. Zone of hypertrophy
4. Zone of calcification
5. Zone of ossification

Question 4

Bone Growth

Answer 4

• Bone tissue grows only by appositional growth, i.e., by osteoblasts adding new matrix to the periosteal or endosteal surface of existing bone tissue
• A bone (the organ) grows in width by appositional growth, i.e. osteoblasts add new matrix to the periosteal surface
• An immature long bone (the organ) grows in length by interstitial growth of the cartilage in the epiphyseal plate

Question 5

Bone Modeling & Remodeling

Answer 5

Osteoclasts resorb bone matrix in bone modeling & remodeling:

• Modeling = adding (via osteoblasts) or removing (via osteoclasts) matrix in order to affect the overall shape of the bone
  
  • An example is metaphyseal “waisting” (narrowing the former metaphysis to convert it to shaft as the growing bone elongates)
• Remodeling = a turnover process where an area of bone matrix is removed and then replaced by new bone
  
  • An example is the conversion of woven compact to lamellar compact bone

Question 6

Synovial Joints

Answer 6

• Hyaline cartilage covers the articular surfaces of the bones
• Articular cartilage has no perichondrium & is not covered by the synovial membrane
• The rest of the synovial cavity is covered by the synovial membrane, forming the synovial fold
• Synovial fold is one of the few body surfaces not covered by an epithelium

Question 7

Synovial membrane is composed of 2 types of non-epithelial cells:

Answer 7

• Type A synoviocytes (phagocytic)
• Type B synoviocytes (fibroblastic), which appear to make some components of synovial fluid

Question 8

Zone of reserve cartilage

Answer 8

Area of cartilage displaying some mitotic activity, but mostly resting

Question 9

Zone of proliferation

Answer 9

Chondrocytes are rapidly proliferating, forming rows of isogenous cells parallel to the direction of bone growth.

Question 10

Zone of hypertrophy

Answer 10

Chondrocytes mature, hypertrophy and begin dying

Question 11

Zone of calcification

Answer 11

Chondrocytes die and cartilage matrix becomes calcified

Question 12

Zone of ossification

Answer 12

Osteoprogenitor cells invade this area, differentiate into osteoblasts, deposit bone on the surface of calcified cartilage. Many of these mixed spicules are later resorbed as the marrow cavity elongates. Others are replaced by lamellar bone

Question 13

How does bone shaft increase in length?

Answer 13

Shaft increases in length by interstitial growth of cartilage at epiphyseal plate

Question 14

How does bone shaft increase in width?

Answer 14

Shaft continues to increase in width through deposition of bone from the periosteum

Question 15

Metaphysis

Answer 15

Once the diaphysis and epiphysis have ossified, the bone is able to continue to grow in length through interstitial growth of the cartilage at the growth plate

Question 16

Classification of fracture by cause

Answer 16

1. Traumatic fracture – Accidental exceeding of normal range of loading to which bone has adapted.
2. Pathological fractures – caused by normal loading of bone weakened by disease (i.e. osteoporotic fractures or fractures secondary to removal of bone tumors)

Question 17

Classification of fracture by character of break

Answer 17

1. Simple – one break in the bone; fragments remain aligned; skin remains closed
2. Comminuted – bone is broken in multiple places
3. Open – skin is broken, leaving an open wound down to the fracture sit

Question 18

Phase I of Fracture Repair: Inflammation

Answer 18

• At time of fracture, blood vessels rupture in the cortex (i.e. in Haversian canals), in bone marrow and periosteum, and sometimes in adjacent or overlying muscles.
• Blood flows into the fracture region forming a large hematoma around the fracture, which then coagulates within a few hours. Influx of cells (fibroblasts, osteoprogenitor cells, chondroblasts) helps to stabilize the breakage area.
• Injured periosteum begins osteogenic response
• Osteoclasts begin removal of necrotic tissue
• Stage lasts ~ 4 days, overlaps with soft callus stage

Question 19

Phase II of Fracture Repair: Soft Callus Formation

Answer 19

• Fibrous connective tissue and cartilage gradually transform the blood clot into a temporary internal callus that ties the bone fragments together.
• Blood vessels begin to regrow through the area
• External callus forms beneath the periosteum
• Stage lasts 3-4 weeks

Question 20

Phase III of Fracture Repair: Hard Callus Formation (Primary Bony Callus)

Answer 20

• Temporary callus is replaced first by a primary bony callus consisting of woven bone (about 6 weeks to form and months and years later may still be noticeable).
• Osteoclasts continue to remove areas of necrotic bone.
• Begins 3-4 weeks after injury and continues for 2-3 months, until get bony union

Question 21

Phase IV of Fracture Repair: Remodeling (Secondary bony callus and callus reduction)

Answer 21

• Replacement of primary callus by a secondary bony callus made of lamellar bone. Woven bone is removed by osteoclasts, and internal osteon remodeling occurs.
• The final stage of healing involves reduction of callus (by removal of excess bone from exterior of periosteal collar; restoration of medullary cavity), and if necessary, some further remodeling.
• The amount of time it takes for completion healing depends on the bone involved, the severity of the fracture, the apposition of the ends, the stability of the fractured ends, and the nutritional state and age of the individual (union is much slower in adults). Avg = 1+ yr.

Question 22

2 main divisions of joints

Answer 22

• 1. Synarthroses: joints that are closely bound, permitting little or no movement
• 2. Diarthroses: joints that permit free movement (= synovial joints)

Question 23

Synarthroses

Answer 23

• 1. Fibrous Joints: Two bones united by a fibrous connective tissue, thereby binding the bones together in an essentially fixed position
• 2. Cartilaginous Joints: More mobile than fibrous joint. These include:
     * Synchondroses (temporary joints found during growth period - e.g. growth plates). Formed of hyaline cartilage, which is replaced once growth ceases by a bony union called a synostosis.
     * Symphyses (Bone surface covered with hyaline cartilage, between is a layer of fibrocartilage). Permit limited movement, accomplished through deformation of the fibrocartilage pad itself.

Question 24

Diarthroses (Synovial Joints)

Answer 24

• 1. Structurally characterized by bony articulations which are in contact but not in continuity with one another
• 2. Areas of contact are characterized by a very low coefficient of friction due to the presence of synovial fluid

Question 25

Parts of a Synovial Joint

Answer 25

* 1. Articular surfaces: Usually covered with hyaline cartilage (articular cartilage) * Contains no nerves or blood vessels. * Nutrition instead derived from: * Vascular net in synovial membrane around its periphery * From synovial fluid * From blood vessels underlying marrow spaces * Collagen fibers in articular cartilage arranged in arched configurations for optimization of mechanical force distribution. * 2. Fibrous joint capsule * Forms a cuff around the articular end of the bone forming the joint * Perforated by articular vessels and nerves * 3. Synovial membrane * Lines the non-articular parts of all synovial joints, synovial bursae, and synovial tendon sheaths * Therefore, you will see synovial membrane lining the fibrous joint capsule, but NOT over the articular cartilage. * Contains two types of cells: * A cells are macrophage-like; remove debris from joint space * B cells resemble fibroblasts and are thought to secrete components of synovial fluid * Underlying connective tissue has rich vascular network * No basal lamina between lining cells of the membrane and underlying connective tissue * 4. Synovial fluid: fluid is viscous (like egg white - hence name, synovia). It has a pale yellow/clear appearance

Question 26

Characteristics of Synovial Joints

Answer 26

* Richly innervated * Rich blood supply with many anastomoses -- needed to prevent blockage of blood flow due to normal movements around joints * Fluid filled sacs or bursae tend to surround joints near the areas where tendons insert into bones * Clinical correlate: inflammation or infection in joint capsule will often spread to surrounding or neighboring bursae (=bursitis).

Question 27

Pediatric Clinical Correlation (Joints)

Answer 27

* Normally, joints more mobile and elastic in children than adults due to thicker hyaline cartilage articular surfaces and more resilient ligaments * Children have far more joints than adults: these are the synchondroses formed by the growth plates of their long bones

Question 28

Pathologies involving growth plate (e.g. slipped epiphysis)

Answer 28

* Have failure through hypertrophic zones of a growth plate * Results in damage to the network of vessels surrounding the articular end of the bone ---\> may lead to avascular necrosis of secondary ossification center. * Damage at growth plate will lead to subsequent length discrepancies between an affected and non-affected limb * Individuals who have slipped one epiphysis are often at risk for another,

Question 29

Sprain

Answer 29

Excessive stretching or tearing of soft tissues surrounding joint

Question 30

Dislocation

Answer 30

Bones participating in joint out of alignment

Question 31

Changes in joint morphology with age/disease

Answer 31

Articular hyaline cartilage surface becomes undulating in configuration, and little shred-like projections appear. Altered viscosity or changes in production of quantity of synovial fluid may lead to dry joints. Pathologies: * Osteoarthritis * Rheumatoid arthritis * Gout

Question 32

What evidence might you see in a section of demineralized bone that would allow you to conclude with certainty that the bone had formed by endochondral ossification?

Answer 32

You could draw that conclusion if you could see any evidence that there had been a pre-existing cartilage model of the bone -- for example an epiphyseal plate or bone spicules with a core of calcified cartilage.

Question 33

How can you distinguish calcified cartilage from bone?

Answer 33

The matrix of calcified cartilage is usually more basophilic than bone matrix. In addition, bone matrix contains osteocytes located within lacunae, but calcified cartilage matrix contains no cells at all. This is because the chondrocytes all hypertrophied and died when the calcification process restricted the diffusion of nutrients into the cartilage. The thin walls separating the enlarged lacunae then broke down, creating irregularly shaped spicules of calcified cartilage matrix. Osteoblasts then laid down bone on these bits of cartilage, creating mixed spicules with a core of calcified cartilage and an exterior layer of woven bone.

Question 34

Briefly outline the morphological steps involved in intramembranous ossification

Answer 34

Some of the morphologically observable events in intramembranous ossification are: Mesenchymal cells aggregate in the area where the bone will form. This is called condensation. The mesenchymal cells differentiate into osteoprogenitor cells and then into osteoblasts. The osteoblasts lay down osteoid, which then mineralizes to form spicules of woven bone. The osteoblasts become surrounded by bone matrix and become osteocytes in lacunae. New osteoblasts from the mesenchyme continue to add bone to the surface of the spicules. The individual spicules eventually grow together (i.e., anastomose with one another) to form trabeculae. Some of the trabeculae reach the periosteum (which has also differentiated from the mesenchyme). There they continue to anastomose to produce compact bone. In the interior of the bone the trabeculae persist as spongy bone.

Question 35

What is a synchondrosis? Give an example

Answer 35

A synchondrosis is a temporary, cartilaginous, non-synovial joint in which hyaline cartilage is interposed between two areas of bone. They are present during periods of bone growth. Once growth ceases, the cartilage is replaced by bone to form a synostosis. Epiphyseal plates are an example of synchondroses.

Question 36

What is a gomphosis? Give an example

Answer 36

A gomphosis is a type of fibrous joint. The only example of a gomphosis is the articulation between the root of a tooth and the bone of the maxilla or mandible. In a gomphosis the outer layer of the root of the tooth (the cementum) is connected to the bone by many bundles of collagen fibers called Sharpey's fibers, which collectively form the periodontal ligament.

Question 37

Synovial membranes line all the surfaces of a synovial cavity that are not covered by articular cartilage. What is unusual about the types of cells that make up a synovial membrane?

Answer 37

A synovial membrane is composed of one or two layers of synovial cells. These are a mixture of macrophage-like type A cells and fibroblast-like type B cells. The unusual feature is that the synovial membrane is not an epithelium, yet it lines a free surface in the body (the synovial cavity). This is one of the very few locations where a free internal or external surface of the body is not covered by an epithelium.

Question 38

What is a cutting cone?

Answer 38

A cutting cone is involved in the remodeling of compact bone. It is a group of osteoclasts that cut a cylindrical channel through the bone. They form on the internal surfaces of the compact bone that surround an existing blood vessel (e.g., a Haversian canal) or a newly forming vessel, and remove the bone from around the vessel. Osteoblasts then lay down new bone, gradually filling in the area excavated by the cutting cone with concentric lamellae. They fill the area from its outer edge inward, working gradually toward the blood vessel until a new osteon is formed with a Haversian canal at its center.

Question 39

A bone that is undergoing surface remodeling usually includes sites along its inner and outer surfaces that are known as “reversals”. What is a reversal and how would you identify one morphologically?

Answer 39

During bone modeling, tissue often needs to be removed from some parts of the surface by osteoclasts and added to neighboring areas by osteoblasts. The boundary between an area of bone removal and bone addition is called a reversal, because the processes occurring in the two regions are the opposite of one another. A reversal can be identified morphologically by finding a point along the bone where the type of cells found on the surface changes abruptly from osteoclasts to osteoblasts. The cement line, which demarcates the edge of a secondary osteon that is formed by intracortical remodeling, is really a reversal line that marks the outermost point where osteoclasts have removed bone at a cuting cone, and where osteoblasts are first seen at the closing cone.

Question 40

Bone is one of the target organs for growth hormone. Which cells do you think growth hormone stimulates in order to produce an increase in the height of an individual?

Answer 40

It stimulates the chondrocytes of the epiphyseal plates, causing them to proliferate. When they divide, these chondrocytes form the rows of cells that are characteristic of the zone of proliferation. Notice that the rows are oriented parallel to the axis of bone elongation. When the cells in any given row lay down cartilage matrix and gradually separate from one another, most of the new matrix thus contributes to increase in bone length rather than bone width.

Question 41

What is responsible for creating interstitial lamellae in cortical bone?

Answer 41

Remodeling exists rexisting bone and lays down new bone in the same location which is responsible for creating interstitial lamellae of cortical bone

Question 42

What type of process is occuring here? What do the arrows indicate?

Answer 42

Endochrondal ossification. Perichodonrium (left & right arrow), periosteum (bottom), periosteal bone collar.(center)

Question 43

What type of process is occuring here? Describe it.

Answer 43

Intramembranous ossification.

Question 44

What type of process is occuring here?

Answer 44

Intramembranous ossification.

Question 45

What structure is this annotated image associated with?

Answer 45

Joint

Question 46

What part of this image is continous with the periosteum? Where is there no articular cartilage found?

Answer 46

Joint cavity except where articular cartilage is. Joint capsule continous with periosteum

Question 47

Where is the epiphyseal plate? Articular cartilage? Synovial membrane?

Answer 47

Epiphyseal plate (1) Articular cartilage (3) Synovial membrane (bottom arrow)

Question 48

What is the bottom arrow pointing to? What is it composed of?

Answer 48

Osteogenic bud composed of osteoprogenitor cells, hematopoeitic cells, and blood vessels

Question 49

What type of ossification occured here? How do you know?

Answer 49

Spongy bone endochrondal ossification. ## Footnote Cartilage pore (spicules) show endochondral ossification Basophilic tissue in center shows endochondral ossification

Question 50

What are the zones in this image?

Answer 50

Hypertrophy Calcified Ossification

Question 51

What are the zones of this image?

Answer 51

Epiphysys Resting Proliferating Hypertrophy Metaphysis