Bone tissue histology

Question 1

Which are the mode of formation of bone tissue?

Answer 1

Connective (intramembranous ossification) & Cartilage model (endochondral ossification).

Both forms result in spongy bone & compact bone.

Question 2

What’s the connective model ? Which bones develop like this ?

Answer 2

Intramembranous ossification that occurs in area os mesenchyme. Osteoblats differentiate directly.

Bones that develop this way are:

• flat bone (crâne)
• part of the mandible and clavicles

Question 3

What’s endochondral ossification? Whichs bones are concerned by it?

Answer 3

Occurs in pre-existing cartilage models. Mesenchymal cells differenciates into chondrocytes.

It concerns:

• most long bones,
• vertebral column,
• ribs
• pelvis

Question 4

Describe Intramembranous ossification (connective model)

Answer 4

1) Formation of primary bone: Mesenchymal cells aggregate in a richly vascularised connective tissue and differentiate in :
- Osteoprogenitors
- Osteoblasts : secrete osteoide, calcified in a few days. Form a network of trabecular bone (woven bone). Vascular mesenchyme becomes periosteum.

2) Secondary bone formaton: Lamellar bone replaces woven bone
- Ostéoclasts migrates to the trabecular surface and resorb primary bone matrix
- Deposition of concentric lamellae autour des vaisseaux par les ostéoblastes forme des ostéons dans l’os haversien.
- The space between trabeculaeau contains primary bone marrow
- Trabeculae thicken and form external and internal bone plates (compact bone).
- Diploe is made of spongy bone. Its vascular tissue becomes red bone marrow.

Question 5

Which are the 4 steps happenning in the primary ossification centers for endochondral ossification ? Where does it take place?

Answer 5

Happens in the diaphysis.

1) Begins with Hyaline cartilage templates
- developed from mesenchyme by condensation
- with differentiated chondoroblasts and chondrocytes

2) Primary ossification center in the diaphysis.
- Perinchondrium becomes periosteum

3) under periosteum, chondrocytes a) hyperhtrophy b) die.
a) secrete collagen type X, angiogenic factors (VEGF) and enzyme alkaline phosphatase. and Secrete MMP which degrade surrounding matrix. Cartilage is errodes. A primitive marrow cavity appears.

4) Blood vessels invade the diaphysis, bringing mesenchymal and osteoprogenitor cells. Osteoblasts lay down woven (primary) bone. Calcified cartilage template resorbed by osteoclasts. Osteoblasts lay down primary woven bone.

Diaphysis is remodelled to form a medullar cavity.

Question 6

In endochondral ossification, where are the secondary ossification centers located ?

Answer 6

Happens in the epiphysis, periphery of the bone possessing hyaline articular cartilage.

Question 7

Where is the epiphtyseal growth plate located?
What is its structure?
What is its role?

Answer 7

At the junction between epiphysis and diaphysis, so between the primary and secondary ossification centres.

Cartilagae tissue containing chondrocytes, arranged in columns.

Responsible for bone elongation, provedes a temporary scaffolding on which new bone is laid down.

Question 8

What replaces growth plate in the adult?

Answer 8

Spongy lamellar bone

Question 9

How many zones can we distinguish in growth plates? Which ones?

Answer 9

5 zones:

• Reserve
• Proliferative
• Maturation and hypertrophy
• Provisional calcification

Question 10

What is the reserve zone of the growth plate (aka resting cartilage)?

Answer 10

Small clusters of flattened / rounded quiescent chondrocytes. Randomly arranged.

Question 11

What is the proliferative zone?

Answer 11

Belongs to growth plate.

• Chondrocytes undergo rapid mitosis (infuence of growth hormone).
• Daugther cells are stacked in columns.

Question 12

What is the maturation and hypertrophy zone?

Answer 12

Mitosis ceases
Lacunae enlarged
Cellular accumuulation of : 
-lipid
-glycogen
-alklaline phosphatase

Question 13

What is the zone of provisional calcification?

Answer 13

This partitions or spicules of basophilic calcified cartilage.
Dead chondrocites are resorbed
Lacunae eroded

Richly ascularized primary marrow extends into the newly opened spaces
Osteoblasts differentiate from mesenchymal cells

Question 14

What is the ossification zone?

Answer 14

At the metaphysis.

Osteoblasts gather on exposed plates of calcified cartilage and secrete osteoid.

Question 15

Which are the main steps of bone repair? which tissues are involved?

Answer 15

Made from a viable periosteum.

3 phases:

• Inflammatory phase
• Reparative phase (callus ossification)
• Remodelling phase

Question 16

What are the steps in the inflammatory phase of bone repair?

Answer 16

• Inflammation and edema
• Bleeding from the marrow cavity and surrounding tissue
• formation of a blood clot (hematoma) in the injured site
• Death of osteocytes and necrose of bone fragments
• Infiltration of fibroblasts and new capillaries
• Migration of different cells to form granulation tissue:
• -leukocytes
• • monocytes
• • macrophages

Question 17

Which are the steps of callus ossification?

Answer 17

• Migration of mesnechymal cells from both the periosteum and endosteum
• Osteoclasts resorb dead bone
• Chondrocytes form a cartilaginous callus around and between fragments to unite them
• Osteoblasts form trabuceale of new woven bone that replaces the calus
• New bone meet

Question 18

What is happening during the remmodelling phase?

Answer 18

• Woven bone remodels into lamellar bone overtime
• New osteons of compact bone are constructed
• Medullar cavity is re-established

Question 19

Which cells make tendons?

Answer 19

Tenocytes or Tendinocytes

Question 20

What are the constitutives elements of tendons?

Answer 20

ECM
Callegen type I
Cells (Tenocytes / Tendinocytes)

Question 21

What is epitenon? What is it made of ?

Answer 21

Loose conjonctive tissue surrounding collagen fiber bundles. It is itself surrounded by the paratenon. The final bundle makes up the tendon. Epitenon contains :

• Vascular & Limphatic structures
• Nervs

Question 22

What’s an enthesis? Which type of enthesis exist?

Answer 22

Transition zones between tissues and sites of mechanical stress concentration. Attach to bones:

• ligaments
• tendons
• capsule

Fibrous (or direct) enthesis and Fibrocartilaginous enthesis.

Question 23

What is fibrous (direct) enthesis for?

Answer 23

Connecting dense conjonctive tissue that directly attaches to the bone. Stress is dissipated among a wider area.

Attachement sites of ligaments, tendons, and capsules to metaphyseal or diaphyseal bone.

Present in the appendicular skeleton.

Limited pathologies.

Question 24

How do we call entheseal diseases?

Answer 24

Enthesopathies

Question 25

What is fibrocartilaginous entheses?

Answer 25

4 zones during the transition from tendon / ligament to bone.

On the epiphyses or apophyses of long or small bones. Fibrocartilage is only present on part of the attachement site.

Narrow attachement site, main source of pathologies.

Question 26

What are the 4 zones of fibrocartilaginous entheses?

Answer 26

1) End of the tendon
2) Uncalcified fibrocartilage
3) Calcified fibrocartilage
4) Bone

Question 27

What is the end of the tendon made of in fibrocartilageneous enthesis?

Answer 27

• Longtudinally oriented tenocytes
• // collagen & elastic fibers
• conective tissues merges with peritenon
• balance the different moduli of elasticity

Question 28

What is the uncalcified fibrocartilage made of in the fc enthesis?

Answer 28

• Chondrocytes in pair and rounded
• aggrecans (that attract water)
• stress is dissipated from the tidemark and into the tendo itself

Question 29

What is calcified fibrocartilage made of?

Answer 29

Abrupte passage from uncalcified to calcified at the calcification front or tidemart .

Basophilic blue line
Mechanical boundary between soft and hard tissue

Irregular passage from calcified zone to subchondral bone.
Collagen Sharpey fibers penetratrantes the trabecular bone.

No periosteum at the attachement site.

Question 30

What’s the use for calcified fibrocartilage (CF)?

Answer 30

• Gradual transmission of forces
• Barrier against diffusion
• Reduce risk of infection

Question 31

What is the subchondral bone plate made of?

Answer 31

Trabecular bone mostly. This contributes to stress dissipation.