Cartilage and Bone Kristine Krafts, M.D.

Cartilage and Bone Lecture Objectives Describe the general functions of cartilage and bone. Compare the function and composition of the three types of cartilage. Describe the two methods of cartilage formation. Describe the cells and extracellular matrix of bone. Compare and contrast immature and mature bone. Describe the two methods of bone formation.

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage Bone Function and characteristics of bone Composition of bone Two types of bone Formation of bone

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage

Functions and Characteristics of Cartilage Cartilage is strong and sort of rigid but also flexible. It can withstand force but it can also bend. It’s made of cells (chondroblasts and chondrocytes) and extracellular matrix. Forms the supporting framework of some organs. Lines the surface of articulating bones. Forms the template for growth and development of long bones.

Metabolic facts that will make you feel sorry for cartilage Bone is very metabolically active. Cartilage is not. Bone has a good blood supply. Cartilage has none. Bone usually heals well. Cartilage does not.

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage

Cartilage Cells Chondroblasts arise from mesenchymal cells and differentiate into chondrocytes. Chondrocytes lie in little artifactual lacunae and produce extracellular matrix. Cartilage is a specialized, supporting connective tissue composed of cells and extracellular matrix.

Cartilage Extracellular Matrix (ECM) Cartilage ECM is composed of: Lots of GAGs and proteoglycans Collagen and/or elastic fibers Nice choices! These substances make the ECM firm and resistant to mechanical forces.

Perichondrium Perichondrium covers the surface of hyaline and elastic cartilage (but not fibrocartilage). Dense connective tissue composed of fibroblasts and type I collagen fibers. Contains blood vessels.

Cartilage is avascular. So what? Cartilage itself has no blood vessels or nerves in its extracellular matrix (bone is a different story). Nutrients must diffuse from perichrondrium through extracellular matrix to chondrocytes. This means metabolic activity is low, the width of cartilage is limited, and cartilage heals poorly.

Cartilage Extracellular Matrix (ECM) The two main thing you need to know about are: Glycosaminoglycans Proteoglycans

Glycosaminoglycans (GAGs) GAGs are unbranched polysaccharide chains made up of repeating disaccharide units. One of the sugars is always an amino sugar. They are highly negatively charged (which means they attract cations and absorb water). They are inflexible and strongly hydrophilic. SO: they are like sponges that suck in water but also resist compression.

Two main types of GAGs Hyaluronic acid (simplest, longest chain, not linked to other proteins). Other types of GAGs (like chondroitin sulfate and keratan sulfate) are much smaller and are always attached to proteins (forming proteoglycans).

Proteoglycans Proteoglycans are just GAGs that are attached to a “core” protein. All GAGs except hyaluronic acid attach to a protein in this way. There are many different kinds of proteoglycans, which is cool because they can form pores of different sizes to regulate the molecules that move through the matrix.

Collagen and Proteoglycans in Cartilage ECM

Another diagram of the same thing

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage

The fibers of the extracellular matrix determine the type of cartilage. Hyaline cartilageType II collagen fibers Elastic cartilage Type II collagen fibers and elastic fibers FibrocartilageType I collagen fibers

Three Types of Cartilage Hyaline cartilage Most common. Articular surfaces of joints, large respiratory passages (nose, larynx, trachea, bronchi), rib- sternum junction, and epiphyseal plate. Functions: support soft tissue, line joints, growth of long bones.

Hyaline Cartilage: Histologic Features Chondroblasts Chondrocytes Perichondrium Extracellular matrix

Hyaline articular cartilage

Three Types of Cartilage Hyaline cartilage Elastic cartilage Located in areas that are pliable and flexible Pinna of ear, external auditory canal, auditory (eustachian) tube, epiglottis, larynx

Elastic cartilage: outer fibrous layer

Elastic cartilage: inner chondrogenic layer

Elastic cartilage: matrix

Elastic cartilage: chondrocytes

Three Types of Cartilage Hyaline cartilage Elastic cartilage Fibrocartilage Located in areas subjected to pulling forces Intervertebral discs, pubic symphysis, and attachments of tendons and ligaments

Fibrocartilage

Fibrocartilage: collagen fibers

Fibrocartilage: chondrocytes

Fibrocartilage: matrix

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage

Two Methods of Cartilage Formation Interstitial growth: growth from within by mitotic division of chondrocytes within lacunae. Appositional growth: growth along the edge of cartilage. Mesenchymal cells of the perichondrium divide and differentiate into chondroblasts. Chondroblasts produce matrix. When surrounded by matrix, they are chondrocytes.

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage Bone Function and characteristics of bone

Functions of Bone Support and protection of soft tissues Attachment of muscles for movement Location of hematopoietic bone marrow where blood cells are produced Stores and releases calcium, phosphate and other ions

Characteristics of Bone Bone is highly vascularized and very metabolically active. Bone remodels (turns over) constantly throughout life. Ideally, bone removal occurs at same rate as bone production. Inhibition of bone turnover leads to poor quality, unhealthy bone.

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage Bone Function and characteristics of bone Composition of bone

Composition of Bone Cells Osteoblasts: produce bone matrix Osteocytes: lie in lacunae Osteoclasts: resorb bone Bone matrix Organic matrix: osteoid, composed of type I collagen fibers and ground substance Inorganic (mineralized) matrix: hydroxyapatite

Osteoblasts Found along edge of bone, with osteoid underneath. Active osteoblasts are cuboidal-columnar with abundant RER, Golgi and secretory granules; inactive osteoblasts are flattened.

Osteoblast Functions Make bone! Produce osteoid (organic bone matrix) with type I collagen fibers, proteoglycans and glycoproteins. Deposit inorganic hydroxyapatite crystals: Ca 10 (PO 4 ) 6 (OH) 2 Bone formation promoted by testosterone, estrogen, growth hormone, weight-bearing exercise and muscle use.

Osteocytes Osteocytes are involved in maintenance of bone matrix. Their death results in resorption of matrix. Each osteocyte sits in its own lacuna. Osteocytes have small, dark, flattened nuclei with minimal rough ER and Golgi.

Osteoclasts Multinucleated giant cells ( μm). Lie in Howship’s lacunae. Arise from fusion of blood monocytes. Numerous lysosomes. Actively resorbing osteoclasts adjacent to bone have cytoplasmic membrane branching (“ruffled border”)

How do osteoclasts resorb bone? Clear zone: an area of cytoplasm that seals off an acidic micro-environment containing lysosomal enzymes. This is where bone resorption occurs.

How do osteoclasts resorb bone? The osteoclast transfers collagenase and protons into this microenvironment, breaking down collagen fibers and calcium salts. Then, the osteoclast phagocytizes the remnants of bone resorption.

Bone Matrix Inorganic 50% of dry weight of bone Mostly calcium Organic Type I collagen Ground substance (proteoglycans and glycoproteins)

Serum Calcium and Bone Resorption When serum calcium is low, parathyroid hormone is released, which tells osteoblasts to make osteoclast stimulating factor. Osteoclasts then resorb bone, which increases serum calcium. Cool! When serum calcium is high, calcitonin is released, which inhibits osteoclastic resorption of bone, leading to decreased serum calcium.

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage Bone Function and characteristics of bone Composition of bone Two (four) types of bone

Two Gross Types of Bone Compact (lamellar, cortical) bone On the outside. Very dense; no marrow spaces. Spongy (cancellous) bone On the inside. Sponge-like structure due to bony trabeculae surrounded by marrow tissue. Microscopically, compact and spongy bone are similar!

Compact Bone and Spongy Bone

Compact (cortical) bone

Spongy (cancellous) bone

Two Microscopic Types of Bone Immature (primary, woven) bone The first bone laid down. Formed quickly. Later replaced by mature bone. Mature (secondary, lamellar) bone Arranged in layers (lamellae). Lacunae with osteocytes are found between lamellae.

Two Microscopic Types of Bone Immature (primary, woven) bone The first bone laid down. Formed quickly. Later replaced by mature bone.

Developing bone: super low-power view

Developing bone: periosteum (outer fibrous layer and inner osteogenic layers)

Developing bone: woven immature bone

Developing bone: osteoblasts

Developing bone: osteoclast

Developing bone: developing osteons

Developing bone: marrow cavity

Two Microscopic Types of Bone Immature (primary, woven) bone The first bone laid down. Formed quickly. Later replaced by mature bone. Mature (secondary, lamellar) bone Bone arranged in osteons (Haversian systems): layers (lamellae) around little canals. Lacunae with osteocytes are found between lamellae.

Haversian system (osteon)

Haversian canal (longitudinal section)

Lamellae around Haversian canal

Osteoblasts lining Haversian canal

Vessels in Haversian canal

Cement line (boundary of osteon)

Nice mature osteon

Volkmann (perforating) canals connect osteons

Mature boneImmature bone GrossCompact or spongyAlways spongy Arrangement of collagen fibers Collagen fibers in each lamella run parallel Collagen fibers are randomly arranged Relative number of osteocytes per unit area Relatively few osteocytesMany osteocytes Staining of bone matrix Staining is even and uniform Staining is blotchy and irregular Immature vs. Mature Bone

Periosteum and Endosteum Periosteum lines the outer surface of compact bone. Two layers: outer dense connective tissue layer and inner layer of osteoprogenitor cells. Sharpey’s fibers are collagen fibers which tightly attach the periosteum to the bone matrix. Contains blood vessels for nutrition of bone. Contains osteoblasts for bone growth, repair and remodeling.

Periosteum outer layer inner layer perforating fibers external lamellae osteon central canal interstitial lamellae trabeculae of spongy bone

Periosteum and Endosteum Periosteum lines the outer surface of compact bone. Endosteum lines the inner surface of compact bone. Also lines the surface of bony trabeculae of spongy bone, and the Haversian canals. Contains osteoblasts for bone growth, repair and remodeling.

Endosteum

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage Bone Function and characteristics of bone Composition of bone Two types of bone Formation of bone

Bone Development (Osteogenesis) Intramembranous ossification Occurs in flat bones (most of skull, including maxilla and mandible). Endochondral ossification Occurs in long bones and irregular bones. Bone can be formed in two ways:

Eight Steps in Intramembranous Ossification 1.Capillaries grow into mesenchyme & release oxygen. 2.Surrounding mesenchymal cells round up, differentiate into osteoblasts and form osteoid. 3.Osteoid mineralizes. Osteoblasts become osteocytes. 4.Bone spicules form and enlarge to form trabeculae of immature bone. 5.A single plate (table) of bone is formed. 6.Immature bone is replaced by mature bone. 7.Remodeling continues, and 2 tables of bone are formed. 8.The two tables are separated by diploe: Tables = compact bone Diploe = spongy bone with marrow

Intramembranous Ossification

Six Steps in Endochondral Ossification 1.Fetal hyaline cartilage develops. 2.Cartilage calcifies, and a periosteal bone collar forms around the diaphysis (long part of the bone). 3.A primary ossification center forms in the diaphysis. 4.Secondary ossification centers form in the epiphyses (ends of the bone). 5.Bone replaces cartilage (except the articular cartilage and epiphyseal plates). 6.Epiphyseal plates ossify.

Endochondral Ossification

Epiphysis: the end of a long bone. Consists primarily of spongy bone, with a layer of compact bone on the outside. Diaphysis: the shaft of a long bone. Consists of compact bone on the outside and a marrow cavity inside.

Endochondral bone formation: super low-power view

Endochondral bone formation: zones of maturation

Endochondral bone formation: resting zone

Endochondral bone formation: proliferative zone

Endochondral bone formation: zone of hypertrophy

Endochondral bone formation: zone of calcification

Endochondral bone formation: zone of ossification

Epiphyseal plate in endochondral ossification

Synovial joint (diarthrosis)

Synovial Membrane

Joint cavity Synovial membrane Articular cartilage

Cartilage and Bone Lecture Outline Cartilage Function and characteristics of cartilage Composition of cartilage Three types of cartilage Formation of cartilage Bone Function and characteristics of bone Composition of bone Two types of bone Formation of bone