Types of Cartilage

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Cartilage Components:

Cartilage is characterized by an ECM enriched with GAGs and proteoglycans. These molecules interact with collagen and elastic fibers.

Cartilage Function:

• Support.

• Shock absorber and aids sliding of surfaces at joints.

• Essential for long bone growth.

Types of Cartilage:

There are three types of cartilage.

Characteristics of Cartilage:

• Avascular - receives nourishment via diffusion from nearby capillaries and from synovial fluid from joint cavities.

• Exhibit low metabolic activity.

• No nerves or lymphatics.

• Perichondrium in most locations, except articular cartilage, epiphyses and fibrocartilage.

• Gel-like consistency due to electrostatic bonds between collagen and GAGs and the binding of water to GAGs.

• Whitish-blue if fresh.

• Type II collagen.

• No fibers.

• Chondrocytes in lacunae.

• Isogenous nests.

• Chondroitin-4 and -6 sulfates.

• Chondronectin (glycoprotein) in matrix promotes adherence of chondrocytes to matrix collagen.

• Capsular or territorial matrix rich in GAGs.

• Interterritorial matrix.

Embryology of Cartilage:

Mesenchyme differentiates into chondroblasts and perichondrium. Perichondrium occurs as outer, fibrous perichondrium and inner, chondrogenic perichondrium.

Interstitial Growth of Cartilage:

Occurs only during the early phases of cartilage formation, at epiphyseal plates of long bone and in articular cartilage. Growth from within.

Appositional Growth of Cartilage:

Appositional growth - Inner, chondrogenic layer deposits cartilage matrix on previously existing cartilage. Growth from the outside. The pregnant woman - football field model.

Articular Cartilage:

Has free edge with no perichondrium. Proteoglycans in free surface make surfaces slippery. Subchondral bone with capillaries supply nutrients, as well as synovial fluid.

Degenerative Arthritis:

Ground substance erodes and fibrils form rough surface.

Nucleus Fibrosus and Pulposus:

AF = annulus fibrosus. NP = nucleus pulposus - a remnant of the notochord intervertebral disc.

Functions of Bone:

Provide mechanical support (ribs) Permit locomotion (log bones) Provide protection (skull) Serve as metabolic reservoir of mineral salts.

Types of Bone:

Dense, cortical or compact bone; Trabecular, spongy or cancellous.

Woven vs Lamellar:

Based on pattern of collagen Woven - produced first and is haphazard and weak Lamellar - is remodeled and strong.

Mesenchyme:

Mesenchyme - an embryonic connective tissue arising almost completely from mesoderm, except some from neural crest ectoderm in the head. Mesenchyme may exist in adults as stem cells that can dedifferentiate and differentiate into many tissues.

Osteoprogenitor Cells:

Derived from mesenchyme and become osteoblasts.

Osetoblasts:

Osteoblasts produce osteoid (unmineralized bone matrix).

Osteocalcin:

Osteocalcin - a glycoprotein embedded in the osteoid matrix that strongly binds calcium.

Osteoid:

Osteoid - produced mainly by osteoblasts and to a small degree by osteocytes. Unmineralized matrix is collagen I and GAGs.

Mineralized Bone Matrix:

Mineralized bone matrix - osteoid that has had minerals, primarily hydroxyapatite, deposited in it, converting it to bone.

Hydroxyapatite:

Hydroxyapatite - a mineral salt of calcium and phosphate.

Bone Cell Terminology:

Osteocytes:

Osteocytes maintain a thin layer of osteoid.

Osteoclasts:

Osteoclasts - resorb salts, primarily hydroxyapatite, from bone.

Ossification:

Ossification - the entire process of bone formation.

Calcification:

Calcification - the process of deposition of calcium salts, mainly hydroxyapatite in collagen and ground substance of bone, cementum, dentin and enamel.

Bone Remodeling:

Primary Haversian System of Osteon:

A system of concentric bone lamellae around a vascular channel in immature bone.

Secondary Haversian system or definitive osteon:

Concentric bone lamellae around a Haversian canal of compact bone.

Cement line:

Distinct refractile borders of bone matrix forming the outer border of secondary osteons. They have a different orientation of collagen hence reflect light differently. They generally lack canaliculi.

Interstitial Lamellae:

Irregularly shaped fragments of dense bone generally lacking Haversian canals.

Periosteum:

Periosteum - specialized dense connective tissue encasing bone. Outer, fibrous periosteum - primarily a connective tissue investment of fibroblasts, collagen and elastic.

Sharpey's fibers:

Connect fibrous periosteum to bone. Inner, osteogenic periosteum - in presence of oxygen differentiates into osteoblasts.

Inner circumferential lamella or endosteal lamella:

Around medullary cavity and beneath the endosteum.

Outer circumferential lamella or periosteal lamella:

Extend around the shaft just under the periosteum.

Haversian Canals:

Vascular channel in center of osteon.

Volkmann's canals

Oblique or transverse vessels connecting Haversian canals and to the periosteum and endosteum.

Bone Marrow:

Endosteum

• Thin layer composed of many osteoprogenitor cells lining the marrow cavity of the diaphysis and all cavities of cancellous bone.

Bone marrow

• Richly vascularized connective tissue specialized in the production of blood elements.

Red bone marrow

• Hemopoietically active.

Yellow bone marrow

• Low hemopoietic activity with abundant adipose.

Osteoclasts:

• 5-50 nuclei.

• Huge.

• Derived from monocytes.

• Resorption bays or Howship's lacunae.

Mineralization of Osteoid:

Osteocalcin (glycoprotein) binds extracellular calcium, increasing the local concentration. Hydroxyapatite lies alongside collagen and a layer of water, the hydration shell, forms around the crystal. This facilitates exchange of ions between crystal and body fluids. Alkaline phosphatase (abundant in osteoblasts) increases calcium and phosphate concentration. Osteoblasts produce matrix vesicles with calcium and phosphate and bud them off to the matrix and form the initial nidus for the initial precipitation of hydroxyapatite. Other cells also produce matrix vesicles including ameloblasts and odontoblasts, as well as chondrocytes, hence the frequent mineralization of cartilage.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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