Lecture 12: Bone And Cartilage

Question 1

Describe the three types of cartilage

Answer 1

• There are three major classes of cartilage based on their matrix, especially the predominant types of fibers within the matrix: (Refer to Figure 4-18):
• • Hyaline cartilage
• • Elastic cartilage
• • Fibrocartilage
• Cartilage may form part or all of the skeletal system in vertebrates.
• Most of the skeletal system in all vertebrates begins as cartilage.

Question 2

What are the two types of cartilage Cells?

Answer 2

• Chondroblasts (cartilage-forming cells)

- Chondrocytes (cartilagemaintenance cells)

Question 3

Describe the extracellular matrix of cartilage

Answer 3

• Extracellular matrix:
• Collagen fibers: Mostly type II collagen (type I in fibrocartilage)
• Amorphous ground substance

Question 4

What are the functions/features of cartilage?

Answer 4

• Compressible
• Resists Distortion
• Absorbs shock in joints
• Reduces friction in movable joints
• Necessary for endochondral bone growth
• Involved in bone fracture repair

Question 5

Describe the characteristics of cartilage formation

Answer 5

• Chondroblasts: Lay down cartilage matrix and become chondrocytes
• Chondrocytes: Maintain cartilage matrix
• Lacunae: Pockets within the matrix where the chondroblasts and chondrocytes are found

Reminder: Cartilage is avascular: Slow to heal, Nutrients and oxygen are derived from blood vessels that surround the cartilage and must diffuse through the matrix to the cells.

Question 6

Describe the structure of the perichondrium

Answer 6

• Outer fibrous layer (A): Contains fibroblasts
• Inner chondrogenic layer (B): Gives rise to chondroblasts, which become chondrocytes. The chondrocyte (D) is involved in the production of the collagen and proteoglycans in the matrix. Chondrocytes also secrete chondronectin.
• See slide 7 for diagram picture

Question 7

Describe isogenous groups

Answer 7

• Groups of 2-8 chondrocytes occupying the same lacunae (C).
• Result of mitotic division.
• Cells will become separated as they begin to lay down matrix of their own.

Question 8

Describe the matrix of cartilage structure

Answer 8

• Components of the matrix:
• Collagen type II (type I in fibrocartilage)
• Hyaluronan (hyaluronic acid)
• Chondroitin sulfate, keratan sulfate, heparin sulfate
• Glycosaminoglycans
• Territorial matrix:
• Surrounds each chondrocyte
• High glycosaminoglycan content; low collagen content
• Inter-territorial matrix:
• Surrounds territorial matrix
• Low glycosaminoglycan content; high collagen content

Question 9

Give the characteristics of hyaline cartilage

Answer 9

• Most common type of cartilage:
• Avascular (cells depend on diffusion through matrix for oxygen and nutrients)
• Contains type II collagen fibers
• Surrounded by perichondrium
• Translucent, bluish gray to white
• Solid but flexible
• Chondrocytes are often found in cell groups (resulting from cell division).
• Growth patterns:
• • Appositional
• • Interstitial

Question 10

Where is hyaline cartilage usually found?

Answer 10

• External auditory meatus
• Larynx
• Tracheal cartilages
• Bronchial cartilages
• Fetal long bones
• Articular ends of bones

Question 11

Give the characteristics of elastic cartilage, including where it is usually found

Answer 11

• Specialized by the addition of elastic fibers to the matrix
• Surrounded by perichondrium
• Yellow color because of presence of elastic fibers
• More opaque, flexible, and elastic than hyaline cartilage
• Chondrocytes mostly located singly
• Type II collagen plus elastic fibers
• Found in:
• • Auricle (pinna) of the ear
• • Epiglottis

Question 12

Give the characteristics of fibrocartilage

Answer 12

• Increased collagen in the matrix
• Reduced cellularity compared to hyaline cartilage
• Not surrounded by perichondrium
• Opaque appearance from fibrous texture
• Type I collagen
• Single sparse chondrocytes
• Fibrocartilage location:
• • Intervertebral discs
• • Pubic symphysis
• • Insertion of some tendons and ligaments
• • Closely associated with dense connective tissue or hyaline cartilage

Question 13

Read through slides 13-15, and go over the photomicrograph cards.

Answer 13

Do it.

Question 14

What are the characteristics of bone tissue?

Answer 14

• Bone tissue is based on a canalicular system.
• Bone is highly vascular and is found in close proximity to capillaries.
• Increase in bone length occurs through appositional growth of a hyaline cartilage model.
• Bone tissue is continuously resorbed, reconstructed, and remodeled.
• Bone is formed by osteoblasts which become osteocytes.
• Matrix of bone consists of two major components:
• • An organic component referred to as osteoid
• • An inorganic component referred to as hydroxyapatite which makes up 35 to 65 percent of the matrix

Question 15

What is the difference between appositional growth and interstitial growth

Answer 15

• Interstitial Growth is the developmental “lengthening” of cartilage, which is produced by the chondrocytes continuously producing and secreting new matrix…or something.
• Appositional is growth accomplished by adding new layers on top of existing layers. This also increases the thickness of the bone.

Question 16

What are the 3 types of bone tissue

Answer 16

1. Woven Bone
2. Spongy Bone
3. Compact Bone, also called Lamellar Bone

Question 17

Describe Woven bone

Answer 17

• Occurs during bone development and bone repair
• Produced rapidly
• Haphazard collagen foundation
• Less structural integrity

Question 18

Describe spongy bone

Answer 18

• Also called trabecular or cancellous bone
• Has a 3-D lattice of branching, bony spicules intertwined to form trabeculae surrounding the bone marrow spaces in the long bones and flat bones

Question 19

Describe compact bone

Answer 19

• Lacks cavities and forms a dense plate on the outside of long bones or flat bones.
• Consists of concentric (Haversian) lamellae (D) which encircle a central blood vessel (and associated nerves) forming an osteon, or Haversian system (Refer to Figures 4-20 and 4-21).
• Osteocytes are found between the lamellae located in lacunae (C) and are connected to each other and the Haversian canal (F) via canaliculi (E).
• Volkmann’s canals (G) run perpendicular to the Haversian canals and connect the Haversian canals to each other and to the surface of the bone. (Review Photo Flashcards for diagram pertaining to these letters)

Question 20

Describe the 3 types of cells involved in bone formation

Answer 20

• Osteoblast
• Secretes bone matrix
• Secretes collagen and catalyzes mineralization
• Osteocyte
• Maintains bone matrix
• Help control calcium and phosphate levels in matrix
• Osteoclast
• Remodels bone through bone resorption

Question 21

Describe osteoprogenitor cells and osteocytes

Answer 21

• Osteoprogenitor cells:
• Stem cells which in the adult are described as bone lining cells.
• In the adult they are found in the inner portion of the periosteum, in the endosteum, and lining vascular canals of compact bone.
• These cells are derived from mesenchyme of the embryonic somite (sclerotome) and possess mitotic potential.
• Osteoprogenitor cells give rise to osteoblasts and bone lining cells.
• Osteocytes:
• Derived from osteoblasts and are trapped by the matrix they secrete.

Question 22

Describe the characteristics of osteoblasts

Answer 22

• Derived from osteoprogenitor cells
• Give rise to osteocytes
• Characterized by:
• • Alkaline phosphatase (Not found in osteocytes)
• • Vitamin D3 (Regulates expression of osteocalcin)
• Major protein products:
• • Type I collagen
• • Noncollagenousproteins:
• – Osteocalcin (High Binding affinity for hydroxyapatite)
• – Osteonectin
• – Osteopontin
• – Osteoprotegerin
• – RANKL

Question 23

Describe osteoclasts and the ARF cycle

Answer 23

• Osteoclast cells:
• Derived from the monocyte lineage, which, in turn, is derived from the monocyte precursors in bone marrow.
• The ARF (activation-resorption-reversal-formation) cycle occurs at adult remodeling sites and during development.

Question 24

Describe the parathyroid hormone

Answer 24

• Parathyroid hormone (PTH) is the primary regulator of bone turnover
• At low PTH levels, bone formation by osteoblasts is stimulated.
• At high PTH levels, osteoblasts are stimulated to release osteoclast-differentiation factors.
• PTH stimulates the differentiation of monocyte precursors to form osteoclasts. It also stimulates the formation of ruffled borders on osteoclasts.
• Elevated PTH levels result in eroded bone and fibrosis of the resulting spaces (osteitis fibrosa).
• Some could argue that PTH controls calcium levels in the blood? Maybe?

Question 25

How do osteoblasts regulate osteoclasts?

• Review slides 42-43

Answer 25

• Parthyroid hormone binds to receptors on the osteoblast.
• The osteoblast is stimulated to synthesize M-CSF and RANKL.
• • M-CSF = Monocyte colony stimulating factor
• The osteoblast releases M-CSF.
• M-CSF binds to the M-CSF receptor on the monocyte that just happened to drop by.
• The monocyte is now a macrophage and expresses RANK.
• RANK binds to RANKL (this couples together the osteoblast and the macrophage).
• The macrophage becomes a multinucleated immature osteoclast
• The osteoblast secretes osteoprotegerin:
• • Osteoprotegerin binds to RANKL with greater affinity than RANK.
• • This inhibits maturation of osteoclasts.
• • Parathyroid hormone blocks the synthesis of osteoprotegerin.
• A non-functional osteoclast uncouples from the osteoblast and becomes a functional osteoclast.
• Calcitonin acts to reduce bone resorption.

Question 26

What are the two major products secreted by osteoclasts?

Answer 26

Both involved in bone reabsorption

• Cathepsin K: A lysosomal protease involved in bone remodeling and resorption that can catabolize elastin, collagen, and gelatin (organic matrix)
• H+Cl− ions: Dissolve the inorganic matrix