Lecture 13: Osteogenesis And Joints

Question 1

List some characteristics about intramembranous bone formation.

Answer 1

• Aggregation of mesenchymal cells:
• • Controlled by following signals: Wnt, Hedghog, FGF, TGF-β
• Mesenchymal cells → osteoblasts
• Osteoblasts secrete osteoid, trapping some of the osteoblasts = blastema.
• Trapped osteoblasts → osteocytes: (through heat)
• Osteocytes form a functional syncytium
• Mineralization occurs via calcium ions.
• Osteoblasts form an epitheliallike covering over the surface of primary bone tissue and can secrete more osteoid on the surface:
• • Osteoid includes collagen I and non-collagen proteins.
• Primary ossification center becomes a trabecula.
• Numerous trabeculae fuse together to form spongy bone.
• Initial bone is woven bone:
• • Collagen fibers are arranged randomly.
• Collagen fibers become aligned:
• • Bone becomes lamellar.
• Lamellae may become symmetrically arranged around a blood vessel forming an osteon (Haversian system).
• Membrane bone usually consists of two layers of compact bone enclosing a layer of spongy bone (diploe).
• See Slides 6-13

Question 2

List the steps of endochondral bone formation

Answer 2

• Cartilage is REPLACED by bone.
• Primary ossification center occurs in future diaphysis of cartilage model.
• Chondrocytes become hypertrophic.
• Chondrocytes secrete vascular endothelial growth factor.
• Blood vessels break through perichondrium, bringing in osteoprogenitor cells.
• Hypertrophic cartilage cells undergo apoptosis, leaving behind thin strands of calcified matrix.
• Osteoblasts use calcified strands as substrates for deposition of osteoid.
• Osteoid is calcified.
• Simultaneously, cells derived from initial perichondrium begin to secrete osteoid appositionally.
• Perichondrium now = periosteum.
• Secondary ossification centers occur in the epiphyses.
• Epiphyses and diaphyses are separated initially by epiphyseal plate (growth plate):
• Reserve zone
• Proliferative zone
• Hypertrophic zone
• Vascular invasion zone

Question 3

THOROUGHLY go through slides 16-37. Seems like a lot, but…

Answer 3

DO IT!

Question 4

What is the definition of a joint and what are the three types of joints called?

Answer 4

• A joint is where two bones come together.

Three Types:

1. Cartilaginous Joint
2. Fibrous Joint
3. Synovial Joint

Question 5

Describe Cartilaginous Joints

Answer 5

• Bones are joined by hyaline or fibrocartilage
  Types:
• Symphysis: Symphyses are joined by fibrocartilage:
  –Examples of symphyses include intervertebral discs and the pubic symphysis
• Synchondrosis: Synchondroses are joined by hyaline cartilage:
  – Examples of synchondroses include epiphyseal plates and the first sternocostal joint.

Question 6

Describe Fibrous Joints

Answer 6

• Bones are joined by collagenous and/or elastic fibrous CT. Types:
• Suture: Examples are the joints between the bones of the calvaria.
• Gomphosis: This is a “peg-in-the-socket joint such as the teeth in the alveoli.
• Syndesmosis: Bones are joined by an interosseous fibrous membrane such as the fibrous membrane between the tibia and fibula.

Question 7

Describe Synovial Joints

Answer 7

• These are the movable joints exemplified by a connective capsule surrounding a fluid-filled joint space.
• Synovial joints are often reinforced by thickenings of the outer part of the capsule referred to as ligaments.
• Ligaments stabilize the capsule and the joint.
• Ligaments control and restrict direction and range of motion.

Question 8

What are the three types of Synovial Joints

Answer 8

(Based on the type of movement of the joint)

• Joints allowing movement in one plane only are monaxial:
• • Hinge joints include the humeroulnar and knee joints.
• • Pivot joints include the atlantoaxial and radioulnar joints.
• Joints allowing movement in two planes are biaxial:
• • Condyloid joints include the metacarpophalangeal and atlantooccipital joints.
• • A saddle (sellaris) joint is exemplified by the first carpometacarpal joint.
• Joints allowing movement in three planes are triaxial:
• • These include the ball-and-socket joints such as the glenohumeral and the femoroacetabular joints.

(See Slide 43)

Question 9

Describe the histology of the synovial joint

Answer 9

• Articular cartilage:
• • Hyaline cartilage
• • Lacks perichondrium
• • Not lined by synovial membrane
• Joint capsule
• • Vascularized dense CT
• • Lined by synovial membrane
• • Attached to edges of articular cartilage
• Synovial membrane:
• • Highly vascularized (fenestrated capillaries)
• • 1-3 layers of synovial cells
• • No basal lamina
• Synovial fluid:
• • Contains mucin (hyaluronic acid-protein complex)
• • Produced by synovial cells
• • Synovial cells: Type A: = Macrophage-like, Type B = Fibroblast-like

See Slides 46-51