Cartilage and Bone

ANATOMY | TRANS 1 Cartilage & Bone OUTLINE I. Cartilage VII. Bone Classification A. Definition A. Histologic classification B. Characteristics B. Macroscopic or Gross C. Functions classification D. Components VIII. Bone Architecture II. Types of Cartilage A. Architecture of long A. Hyaline bones B. Elastic B. Architecture of lamellar C. Fibrocartilage bones III. Cartilage Formation, IX. Histologic Preparations Growth, & Repair A. Ground bone A. Chondrogenesis B. Decalcified bone B. Cartilage growth X. Bone Formation C. Cartilage regeneration A. Intramembranous & repair ossification IV. Bone B. Endochondral A. Definition and ossification components XI. Epiphyseal plate B. Functions XII. Bone Growth, Remodeling, V. Bone surfaces & Repair A. Periosteum A. Bone growth B. Endosteum B. Bone remodeling VI. Bone Cells C. Bone repair A. Osteoblasts XIII. Clinical Application B. Osteocytes XIV. Review Questions C. Osteoclasts XV. References XVI. Appendix ❗️ Must know CT ECM GAGs RER 💬 Lecturer 📖 Book 📋 Previous Trans SUMMARY OF ABBREVIATIONS Connective tissue Extracellular matrix Glycosaminoglycans Rough Endoplasmic Reticulum LEARNING OBJECTIVES At the end of the lecture, the students should be able to characterize the histology of CARTILAGE and BONE by: ✔ Describing component parts ✔ Differentiating the 3 types of CARTILAGE as to composition of the intercellular substance, histologic features and functions ✔ Classifying BONE according to morphology ✔ Explaining the process of formation, growth and repair and regenerative capacity I. CARTILAGE A. DEFINITION ● Tough, durable form of supporting connective tissue, ● characterized by presence of cells embedded in ECM with high concentrations of GAGs and proteoglycans, interacting with collagen and elastic fibers B. CHARACTERISTICS ❗ Avascular (no blood vessels) ● No lymphatic vessels and nerves ● Nutrition is via diffusion from vascularized outer covering (perichondrium) C. FUNCTIONS ● Allows tissues to bear mechanical stress → Ex. Fibrocartilage found in Intervertebral discs ● Facilitates bone movements Cartilage has a smooth surface, specifically in the articulating surface of bones, permitting almost friction free movements of the joints. ▪ Shock absorbing and sliding regions within joints ▪ Ex. Synovial joints ● Forms the framework supporting soft tissues 💬 → Ex. Ear, nose, walls of the respiratory tract ● Guides development and growth of long bones → Ex. Endochondral ossification D. COMPONENTS ● Cartilage is also a connective tissue, ● It is composed of cells and predominantly of ECM Figure 1. Hyaline cartilage. C - chondrocytes, P - perichondrium, M matrix[Mescher, 2018] EXTRACELLULAR MATRIX (ECM) ● Composed of: → Ground substance ▪ High concentration of GAGs and densely proteoglycans which is responsible for; ▪ Semi rigid consistency → Fibers “CartilagE” ▪ Collagen ▪ Elastic packed SUBDIVISIONS OF ECM ● Territorial Matrix → Immediately surrounds chondrocytes → Stains more intensely with hematoxylin (due to higher proteoglycan content) ● Interterritorial Matrix → Lighter staining (lower proteoglycan content) → Between isogenous groups CELLS 1 Chondroblasts → Immature cartilage forming cells → Elliptical in shape → Found in the periphery (inner chondrogenic layer) → Undergo mitosis and secrete ECM around themselves to become chondrocytes 2 Chondrocytes → Round, mature cells surrounded by ECM, lie within lacunae → May occur singly or in clusters (isogenous groups or aggregates) → Synthesize, secrete, and maintain ECM PERICHONDRIUM ● Comes from greek word peri meaning “around” and chondros meaning “cartilage” ● Structure that forms an interface between the cartilage and tissue being supported ● Arise from superficial embryonic ● Essential for growth and maintenance of the cartilage ● Connective tissue surrounding cartilage → Outer fibrous layer ▪ Dense irregular CT (type I collagen fibers), fibroblasts and blood vessels → Inner chondrogenic layer ▪ Blends with the matrix proper 📖 ▪ Contains chondroblasts Contains mesenchymal stem cells which provide a source for new chondroblasts Figure 2. Components of hyaline cartilage[Lecturer’s PPT] II. TYPES OF CARTILAGE ● 3 types depending on the content of its matrix, and predominant fibers: → Hyaline: non rigid support → Elastic: support with high flexibility → Fibrocartilage: strength under stress Figure 4. Hyaline cartilage ● ● ● ● ● ● ● B. ELASTIC CARTILAGE Similar to hyaline cartilage (type II collagen) ECM: Type II Collagen Fibers Surrounded by perichondrium Abundant elastic fibers Dark bundles unevenly distributed “looks dirty” Fewer isogenous groups than hyaline cartilage Fresh elastic cartilage is yellowish color due to containing an abundant network of elastic fibers in addition to collagen type II EXAMPLES ● Located in areas where both flexibility & support are necessary: → Pinna of ears → External auditory canal → Epiglottis → Cuneiform cartilage of the larynx → Auditory or eustachian tube → Upper respiratory tract Figure 3. Distribution of Cartilage in Adults[Mescher, 2018] A. HYALINE CARTILAGE Most common type of cartilage Forms the temporary skeleton which is gradually replaced by bone ECM: Type II collagen Aggrecan most abundant proteoglycan Chondronectin → Multiadhesive glycoprotein for adherence of chondrocytes to the ECM ● Homogenous and semi transparent in fresh state “glass-like” appearance → Due to the same refractive index of the collagen fibers and ground substance (giving a blended appearance) ● Basophilic/ stains blue because of proteoglycans ● Chondrocytes appear in groups (isogenous aggregate/s) ● ● ● ● ● EXAMPLES ● Fetal skeleton ● Walls of large respiratory passages (nose, larynx, trachea, bronchi) ● Ventral ends of ribs ● Articular surfaces of movable joints and Epiphyseal plates* → Don't have perichondrium ▪ Damage to these areas would mean poor regenerative capability and healing Figure 5. Elastic cartilage. C - chondrocytes, M - matrix, P perichondrium[Mescher, 2018] C. FIBROCARTILAGE ● Combination of hyaline cartilage and dense connective tissue → Presence of fibroblasts ▪ Due to being a combination of hyaline cartilage and dense connective tissue ● Abundant type I collagen than type II, arranged either in regular or irregular configuration → Type I collagen fibers helps in withstanding tensile forces → Reason why one of the functions of cartilage is to bear mechanical stresses ● Less proteoglycan (stains pink) ● Minimal ground substance ● Few isogenous groups → If present, isogenous groups can be seen as aligned aggregates ● The relative scarcity of proteoglycans makes the matrix of fibrocartilage more acidophilic than that of hyaline or elastic cartilage ● No perichondrium → Nutrition comes via diffusion from blood vessels or synovial fluids and not from the perichondrium III. CARTILAGE FORMATION, GROWTH AND REPAIR A. CHONDROGENESIS ● Chondrogenesis: Formation of cartilage Figure 8. Chondrogenesis[Mescher, 2018] 1. Rounding up of mesenchymal cells → Cartilage comes from mesenchyme. During chondrogenesis, there will first be rounding up of mesenchymal cells and condensation 2. Mitosis, chondroblasts → Undergoes mitosis and differentiation into chondroblasts 3. Separated from one another and would reside in the lacunae → These secrete the matrix among themselves and become trapped in the lacuna and become separated from each other and become chondrocytes. 4. Continuous multiplication within lacunae → There will be continuous matrix secretion and mitosis, eventually resulting in formation of new cartilage EXAMPLES ● Intervertebral discs ● Attachments of certain ligaments ● Pubic symphysis ● The dividing cells are typically called chondroblasts and chondrocytes when proliferation has ceased; both have basophilic cytoplasm rich in RER for collagen synthesis. B. CARTILAGE GROWTH ● Interstitial growth → Mitotic division of pre-existing chondroblasts → Can be found in epiphyseal plates, articular cartilage → In cartilages with no perichondrium ● Appositional growth → Differentiation of new chondroblasts from the perichondrium → more important in postnatal development Figure 6. Fibrocartilage. C - chondrocytes, M - matrix, D - Dense connective tissue[Lecturer’s PPT] Figure 9. Cartilage growth[Lecturer’s PPT] C. CARTILAGE REGENERATION AND REPAIR ● Slow, often incomplete (in adults) ● Generally poor due to avascularity ● Depends on perichondrium → Optimum nutrition depends on perichondrium, if there is no perichondrium, regeneration and repair will result in a much slower and poorer capability ● Figure 7. Isogenous groups of fibrocartilage. C - chondrocytes, D dense connective tissue, M - matrix[Mescher, 2018] ● ● ● IV. BONE A. DEFINITION AND COMPONENTS Bone is a specialized type of connective tissue characterized by its rigidity and hardness Comprised of the following components: Matrix (Inorganic, Organic) Cells (Osteocytes, Osteoblasts, Osteoclasts) B. FUNCTIONS ● Provides structural support and protection → Giving shape & form to the body ● Permit movement → Insertion of muscles ● Serves as a storage for minerals → Ex: Calcium ● Contains bone marrow → Myeloid tissue = bone cell precursors B. ENDOSTEUM ● Internal surface of bones/trabeculae ● Single row of bone lining cells, osteoblasts, osteoclasts & progenitor cells MATRIX ORGANIC PORTION ● Mainly type I collagen fibers → Stain pink with eosin ● Minimal ground substance: → Proteoglycans → Glycoproteins → Osteonectin → Osteocalcin → Phosphatases ▪ Components are important for binding collagen, calcium, calcification ● ● ● ● INORGANIC PORTION 50% of dry weight of bone Calcium hydroxyapatite (most abundant) Calcium phosphate (significant amount) Other ions: → HCO3→ Citrate → Mg2+ → K+ → Na+ V. BONE SURFACES A. PERIOSTEUM ● Double layer of CT surrounding outer bone surface ● Similar to perichondrium → Well vascularized → Absent at articular surfaces LAYERS OF PERIOSTEUM ● Outer fibrous → Dense CT most type I collagen → Small blood vessels, collagen bundles, fibroblasts ● Inner osteogenic → Osteoprogenitor cells → Osteoblasts → Osteoclasts → Bone lining cells (inactive osteoblasts) ● Perforating/Sharpey fibers → Bundles of periosteal collagen fibers that act like a glue to bind periosteum to the bone Figure 11. Endosteum[Mescher, 2018] ● ● ● ● VI. BONE CELLS A. OSTEOBLASTS Origin: Mesenchymal stem cells Exclusively at the surface of the bone matrix Single layer of Cuboidal cells → Inactive form: Bone lining cells ▪ Flattened with heterochromatic nuclei → Active form: Osteoblasts are Cuboidal or columnar in shape with basophilic cytoplasm Functions: → Secrete OSTEOID (Organic bone matrix) Osteoid: located between the osteoblast layer and pre existing bone matrix → Synthesize osteocalcin and alkaline phosphatase in membrane- enclosed matrix vesicles → Formation of hydroxyapatite crystals The first visible step in calcification → Mineralization of osteoid 📖 📖 Figure 12. Photomicrograph of developing bone. M - mesenchyme, Ob - osteoblast, Os - osteoid, B - bony matrix, Oc - osteocyte[Mescher, 2018] Figure 10. Perforating fibers of the periosteum[Mescher, 2018] B. OSTEOCYTES ● Almond shaped cells containing long processes ● Differentiated osteoblasts surrounded by matrix enclosed singly within lacunae ● Communicate with one another via long cytoplasmic processes that contain gap junctions that aid in the transport of nutrients