Specialised Connective Tissues: Cartilage and Bone PDF

SUPPORTING CONNECTIVE TISSUE CARTILAGE AND BONE DR CHARLOTTE PHELPS SENIOR TEACHING FELLOW SESSION GUIDELINES SESSION OUTCOMES During this session you will learn about: 1. Cartilage as a specialised, supporting connective tissue 2. Types of cartilage and location (hyalin...

SUPPORTING CONNECTIVE TISSUE CARTILAGE AND BONE DR CHARLOTTE PHELPS SENIOR TEACHING FELLOW SESSION GUIDELINES SESSION OUTCOMES During this session you will learn about: 1. Cartilage as a specialised, supporting connective tissue 2. Types of cartilage and location (hyaline, elastic, fibrocartilage) 3. Bone as specialised, supporting connective tissue 4. Different types of bone tissue (woven vs lamellar; spongy vs compact) 5. Bone formation: intramembranous and endochondral ossification 6. Calcium homeostasis RECAP CONNECTIVE TISSUE CLASSIFICATION Connective tissue Connective tissue is classified based on the characteristics of its cellular and extracellular components. Connective tissue Cartilage Bone Blood Main criteria: proper Type of cells Hyaline Compact Arrangement and type of fibers Loose Dense Fibro- Composition of extracellular matrix cartilage Spongy Areolar Regular Elastic Adipose Irregular Reticular Elastic CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue FUNCTIONS Provides flexible support e.g. pinna of ear Absorbs shock (non-compressible) e.g. intervertebral discs Prevents friction in joints e.g. articulating surfaces Template for many bones (e.g. long bones, vertebrae) Assists with repair of fractures CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue FEATURES Three types of cartilage: Hyaline cartilage Elastic cartilage Fibrocartilage Special features: Semi-rigid, flexible structure Avascular No nerve or lymphatic supply Cells (chondrocytes) receive nutrients by diffusion through the extracellular matrix CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE The primary components of cartilage include: Chondrogenic cells Cells Chondroblasts Chondrocytes Cartilage Ground substance Matrix Fibers CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE - CELLS Chondrogenic cells Chondrogenic cells (from mesenchyme) divide and become chondroblasts. Stellate cells. Chondroblasts Chondroblasts (active cells) secrete matrix, allowing appositional growth on the outside of cartilage. Present in outer covering of cartilage ‘chondrogenic layer’ of perichondrium. Chondrocytes Chondrocytes (mature cartilage cells) become trapped in lacuna, surrounded by matrix. Responsible for maintaining the matrix. Further growth by chondrocytes dividing and laying down more matrix (interstitial growth) to form isogenous groups surrounded by territorial matrix. Source: Junqueira's Basic Histology: Text and Atlas (2024). CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE - CELLS Lacuna Chondrocyte* Chondrogenic cells Chondroblasts Chondrocytes Territorial matrix *Lacunae more obvious than chondrocytes (artefact) due to Chondrocytes in shrinkage of the chondrocytes isogenous group during tissue preparation. Source: Gartner, L. (2021). Textbook of Histology. Elsevier.  CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE - PERICHONDRIUM The perichondrium is dense irregular connective that surrounds the cartilage. Two layers: Outer fibrous layer 1. Outer fibrous layer Type I collagen, fibroblasts, blood vessels Inner Perichondrium chondrogenic 2. Inner chondrogenic layer layer Spindle-shaped cells proliferate and differentiate into chondroblasts Chondrocytes flatter near perichondrium and rounded in deeper regions Source: Osmosis. (2024). Elsevier. CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue GROWTH Cartilage grows by two mechanisms: 1. Appositional growth ‘growth from outside’ Apposed to means ‘added to’ Chondroblasts (in chondrogenic/deep layer of perichondrium) lay down new layers of matrix added to pre-existing matrix. Cartilage expands and widens 2. Interstitial growth ‘growth from within’ Chondrocytes within the cartilage (inside lacunae) secrete new matrix Primarily in young/immature Cartilage grows in length cartilage/during childhood and adolescence CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue GROWTH Interstitial growth Cell nests of 2-4 chondrocytes (isogenous Chondrocytes within the existing groups) laying down matrix between them cartilage divide and form small groups of cells, isogenous groups, Chondrocytes pushed which produce matrix to become apart as cartilage grows separated from each other by a (interstitial growth) thin partition of matrix. CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE The primary components of cartilage include: Chondrogenic cells Cells Chondroblasts Chondrocytes Cartilage Glycosaminoglycans (GAGs) Ground substance Proteoglycans Matrix Collagen type II (mainly) Fibers Elastic (in elastic cartilage) CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE - MATRIX Two main extracellular matrix components: Ground substance Abundant glycosaminoglycan (GAG) Major GAGs = hyaluronic acid, chondroitin sulphate and keratan sulphate Bound to a core protein (aggrecan) to form a large proteoglycan (-ve charge) Attached to collagen lattice by so-called link protein. Traps water Shock absorption (non-compressible) Fibers Predominantly type II collagen (90-95%) + some IX, X, XI Provides mechanical stability Source: Standring. (2021). Gray's Anatomy. Elsevier. Source: Thomas, P. (2023). Cell Biology. CARTILAGE LO1: Cartilage as a specialised, supporting connective tissue STRUCTURE - Matrix Because of its high content of sulphated GAG, cartilage stains with basic dyes (i.e. haematoxylin), which gives it a blue colour in H&E preparations. Isogenous cell nest Chondrocyte groups Chondrocytes (C) arranged in separated by an clusters of 2-4 cells interterritorial matrix (IM) Represents cell divisions from an original chondrocyte Chondrocytes enclosed by territorial matrix (TM) Darkly stained, indicating high concentration of newly synthesised matrix Source: Junqueira's Basic Histology: Text and Atlas (2024). CARTILAGE LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) TYPES There are three types of cartilage, based on the types of fibers present in the matrix. Hyaline cartilage Elastic cartilage Fibrocartilage Characterised by matrix Characterised by elastic Characterised by abundant containing type II collagen fibers and elastic lamellae in type I collagen fibers, matrix fibers, GAGs, proteoglycans, addition to the matrix material of hyaline cartilage. multi-adhesive material of hyaline cartilage. glycoproteins. CARTILAGE TYPES LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) HYALINE CARTILAGE Features of hyaline (hylos. Gr. = glass) cartilage: Most common cartilage type Avascular Surrounded by perichondrium (except articular cartilage and epiphyseal plate) Chondrocytes surrounded by territorial and interterritorial matrices containing type II collagen interacting with proteoglycan Hyaline cartilage matrix has a ‘glassy’ appearance due to small collagen type II fibers that are not resolved in microscopy light Locations: Template for bone formation (endochondral ossification) Epiphyseal plates during growth of long bones Articular cartilage of synovial joints Supports respiratory passages (nose, larynx, trachea, bronchi) Costal cartilage - joins ribs to sternum CARTILAGE TYPES LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) HYALINE CARTILAGE EXAMPLES Trachea Articular cartilage Cartilage rings Note the lack of perichondrium in articular Perichondrium cartilage Artefacts (folds) Source (L): Netter, H. (2023). Netter Atlas of Human Anatomy: Classic Regional Anatomy Approach. Source (R): Jung, C.K. (2014). Articular Cartilage: Histology and Physiology. In: Shetty, A.A., Kim, SJ., Nakamura, N., Brittberg, M. (eds) Techniques in Cartilage Repair Surgery. Springer, Berlin, Heidelberg. CARTILAGE TYPES LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) ELASTIC CARTILAGE Features of elastic cartilage: Avascular Surrounded by perichondrium Identical to hyaline cartilage but with many elastic fibers to make it more flexible Chondrocytes surrounded by territorial and interterritorial matrices containing type II collagen interacting with proteoglycans and elastic fibers. Locations: External ear (pinna) Epiglottis Auditory tube Eustachian tube Cuneiform cartilage of larynx Source: Standring. (2021). Gray's Anatomy. Elsevier. CARTILAGE TYPES LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) FIBROCARTILAGE Features of fibrocartilage: Generally avascular Lacks a perichondrium Consists of chondrocytes and fibroblasts surrounded by type I collagen and a less rigid ECM. Considered an intermediate tissue between hyaline cartilage and dense connective tissue. Large collagen Locations: bundles arranged at Intervertebral disks (annulus fibrosus) right angles to each other Mandible Sternoclavicular joints Rows of Pubic symphysis chondrocytes in Articular discs of some joints (e.g. menisci of knee) hyaline cartilage matrix CARTILAGE TYPES LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) CLINICAL CORRELATIONS For your interest Hyaline cartilage Hyaline cartilage does not repair itself well. Damage to articular cartilage = osteoarthritis. May become calcified either normally (during endochondral ossification or ageing) or pathologically Elastic cartilage ‘Cauliflower ear’, common in rugby players or wrestlers. Blunt trauma to ear causes perichondrium to separate, blood vessels to rupture, haematoma forms, disrupts oxygen and nutrients from entering cartilage, chondrocytes die, cartilage replaced with scar tissue, calcifies. Fibrocartilage Slipped disc (i.e. tear/break in annulus fibrosus). Common in lumbar spine. Leads to severe pain in lower back and extremities due to displaced disc compressing lower spinal nerves. CARTILAGE TYPES LO2: Types of cartilage and location (hyaline, elastic, fibrocartilage) REVIEW Hyaline cartilage Elastic cartilage Fibrocartilage Structure Location Perichondrium Chondrocytes (in lacunae) ECM Fibers BONE BONE LO3: Bone as specialised, supporting connective tissue FUNCTIONS Provides structural framework to support and protect tissues (e.g. cranium, ribs) Acts as a series of levers to effect movement caused by muscle contraction Forms joints Serves as the body’s main reservoir of calcium (99%) Houses haematopoietic tissue (blood cell formation) BONE LO3: Bone as specialised, supporting connective tissue FEATURES Types of bone textures: Compact bone - outer layer of dense, smooth bone Spongy bone - inner layer, trabecular / honeycomb Special features: Hard, supportive connective tissue with calcified matrix Contain various types of tissue - bone (osseous) tissue, nervous tissue, cartilage, connective tissue Long bones have a hollow cavity filled with marrow External and internal surfaces of bone lined by membranes (except joint surfaces) BONE LO3: Bone as specialised, supporting connective tissue STRUCTURE BONE LO3: Bone as specialised, supporting connective tissue STRUCTURE Osteoprogenitor Osteoblast Mesenchymal origin Cells Osteocyte Osteoclast Bone Collagen Organic Proteoglycans Matrix Glycoproteins Calcium Inorganic Phosphate BONE LO3: Bone as specialised, supporting connective tissue STRUCTURE For Histology, bone must be either: Decalcified then stained (maintains Decalcified bone, H&E stained collagen, but loses inorganic matrix) Ground and polished (maintains inorganic components but loses collagen and cells) Ground section of compact bone BONE LO3: Bone as specialised, supporting connective tissue MATRIX Bone matrix has both inorganic and organic components: Inorganic - 65% bone mass Gives bone its hardness Mostly hydroxyapatite crystals (Ca10(PO4)6(OH)2) ⚬ Mostly calcium phosphates The combination of mineral and collagen = strength and Newly formed bone (osteoid) is not yet calcified flexibility Mature bone can only grow by appositional growth Organic - 35% bone mass Gives bone flexibility Mostly (90%) collagen Type I fibers Also proteoglycans, glycosaminoglycans (GAGs), glycoproteins BONE LO3: Bone as specialised, supporting connective tissue CELLS The cells of bone are osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts. Osteoblasts Produce new bone matrix (osteogenesis/ossification) Synthesise and release proteins and organic components Osteocytes Mature bone cells Occur in pockets called lacunae, sandwiched between layers of matrix called lamellae Recycle calcium salts and help repair damaged bone Osteoclasts Remove bone matrix Important in calcium and phosphate regulation BONE LO3: Bone as specialised, supporting connective tissue CELLS - PERIOSTEUM The periosteum is a thin sheath of dense fibrous connective tissue on the surface of bone. Outer fibrous layer Osteogenic layer Osteoprogenitor Osteoblasts BONE LO3: Bone as specialised, supporting connective tissue CELLS - ENDOSTEUM The endosteum covers the spongy walls and extends into all cavities of the bone (including Haversian and Volkmann’s canals). Endosteum lined by osteoprogenitor cells / osteoblasts Osteoblasts secrete matrix and become surrounded by it, becoming osteocytes BONE LO3: Bone as specialised, supporting connective tissue CELLS - OSTEOPROGENITOR Osteoprogenitor cells are derived from embryonic mesenchymal cells and maintain ability to undergo mitosis to differentiate into osteoblasts. Found in periosteum and endosteum. Active Inactive Osteoprogenitor cells increase in size and capable of Osteoprogenitor (OP) cells - flattened cells differentiating into osteoblasts (OB) Periosteum Osteoprogenitor cells most active during periods of intense bone growth Source: O’Dowd, G. (2023). Wheater's Functional Histology BONE LO3: Bone as specialised, supporting connective tissue CELLS - OSTEOBLASTS Cuboidal osteoblasts Osteoblasts are responsible for formation of bone. When fully active: Cuboidal or polygonal cells Basophilic cytoplasm (abundance of RER) Newly secreted organic matrix (osteoid - Osteoblasts secrete matrix and become uncalcified) surrounded by it, thereby becoming osteocytes. Source: Lowe, J. (2025). Stevens and Lowe’s Human Histology. BONE LO3: Bone as specialised, supporting connective tissue CELLS - OSTEOCYTES Osteocytes are mature bone cells derived from osteoblasts that become trapped within formed bone (lacunae). Their function is to maintain the structural Lacunae integrity of mineralised matrix Cells display slender processes that extend into thin channels (canaliculi) in the mineralised matrix Via these channels, osteocytes communicate with neighbours This also allows osteocytes to receive nutrients and remove wastes BONE LO3: Bone as specialised, supporting connective tissue CELLS - OSTEOCLASTS Osteoclast Osteoclasts are bone resorbing cells (phagocytic) i.e. erode mineralised bone Giant, multi-nucleated cells (can have up to 50 nuclei!) ⚬ Derived from bone marrow precursor which also produces monocytes and macrophages Occupy shallow depressions on bone surface (Howship’s lacunae) that identify regions of bone resorption. Ruffled border is responsible for resorption of bone. BONE LO3: Bone as specialised, supporting connective tissue CELLS - OSTEOCLASTS Seal off area of bone, secrete acid and lysosomal enzymes Dissolve inorganic matrix; digest organic matrix (under influence of parathyroid hormone) Body’s main mechanism in calcium homeostasis Important in the constant turnover and refashioning of bone Source: Gartner, L. (2021). Textbook of Histology. Elsevier. BONE LO3: Bone as specialised, supporting connective tissue CELL TYPES - REVIEW BONE LO4: Different types of bone tissue (woven/lamellar; spongy/compact) TYPES Woven/immature vs. mature/lamellar Bone as a specialised connective tissue is either: 1. Woven/immature when first laid down (embryo, fracture) Collagen in matrix is randomly oriented Osteocytes more plentiful and randomly arranged Less mineral in the matrix 2. Or remodelled to form mature/lamellar bone Osteoblasts lay down bone in layers (lamellae) Source: Fayez, S. et al. (2009). Bone Structure, Development and Bone Biology: Bone Pathology. Bone Pathology. BONE LO4: Different types of bone tissue (woven/lamellar; spongy/compact) TYPES - MATURE/LAMELLAR Mature/lamellar bone is either: Spongy (cancellous, trabecular) bone on the inside (light) Network of trabeculae (spicules) that form interconnecting spaces containing bone marrow Compact (cortical, dense) bone on the outside (dense, strength) Composed of cylindrical units, known as osteon (Haversian systems), usually aligned with the long axis of the bone, composed of concentric rings of bone (lamellae) surrounding a central canal BONE LO4: Different types of bone tissue (woven/lamellar; spongy/compact) TYPES - MATURE/LAMELLAR Haversian systems (osteon) Haversian canals (central) Adaptation to bring blood vessels and marrow cells deep into compact bone Osteocytes in lacunae encircling canal Canaliculi communicate with the Haversian canal, which contains blood vessels Collagen fibers in adjacent lamellae of osteon spiral in different directions providing tensile strength. BONE LO4: Different types of bone tissue (woven/lamellar; spongy/compact) TYPES - MATURE/LAMELLAR Osteoblasts lay down bone in layers (lamellae) Outer circumferential lamellae: Under periosteum Inner circumferential lamellae: Under endosteum Haversian lamellae: Around blood vessels Interstitial lamellae: Remnants of remodelled Haversian systems BONE LO4: Different types of bone tissue (woven/lamellar; spongy/compact) TYPES - MATURE/LAMELLAR Spongy bone Honeycomb of needle-shaped pieces called ‘trabeculae’ Lamellar but without Haversian systems Canaliculi communicate directly with the marrow cavity (nutrients diffuse from capillaries in the Trabeculae surrounding endosteum) Bone marrow Not randomly aligned - trabeculae aligned along lines of stress Open spaces between trabeculae are filled with red and Osteocytes yellow bone marrow BONE LO5: Bone formation: endochondral & intramembranous FORMATION Embryonic formation of the bony skeleton Up to week 8, the embryonic skeleton is made of: Fibrous membranes and hyaline cartilage Process of bone formation depends on origin of bone tissue. Intramembranous ossification Bone develops from fibrous membrane (mesenchymal) Flat bones i.e. skull, mandible, maxilla, clavicle Endochondral ossification Bone develops by hyaline cartilage model All other bones i.e. long, short, irregular bones BONE LO5: Bone formation: endochondral & intramembranous FORMATION - INTRAMEMBRANOUS Intramembranous ossification Ossification occurs in spicules: mesenchymal cells differentiate into osteoblasts, make osteoid, which undergoes calcification to produce woven bone. Osteoclasts and osteoblasts remodel the bone, eventually producing lamellar bone Source: Gartner, L. (2021). Textbook of Histology. Elsevier. BONE LO5: Bone formation: endochondral & intramembranous FORMATION - INTRAMEMBRANOUS Selected centrally Osteoblasts begin to located mesenchymal secrete osteoid, which is cells cluster and mineralised within days. differentiate into Trapped osteoblasts osteoblasts, forming become osteocytes. ossification centre Accumulating osteoid is Trabeculae deep to laid down between periosteum thickens, embryonic blood vessels forming woven bone to form trabeculae. collar (later replaced by Vascularised lamellar bone). Spongy mesenchyme condenses bone persist, vascular externally becoming tissue becomes red periosteum marrow. Source: Robson, L. et al. (2025). Medical Sciences. BONE LO5: Bone formation: endochondral & intramembranous FORMATION - INTRAMEMBRANOUS BONE LO5: Bone formation: endochondral & intramembranous FORMATION - ENDOCHONRAL Endochondral ossification In the centre of the diaphysis, the cartilage is being replaced by bone in a primary centre of ossification (in the foetus) Cartilage begins to undergo hypertrophy and calcification, allowing the penetration of blood vessels which bring them the osteoblast and bone marrow precursors Later on a secondary ossification centre forms in the epiphyses Cartilage is replaced by bone in the epiphysis and diaphysis, except in the epiphyseal plate region Bone continues to grow, until maturity (~18 years old). Source: Marieb, E. N., & Hoehn, K. (2023). Human Anatomy and Physiology. BONE LO5: Bone formation: endochondral & intramembranous FORMATION - ENDOCHONRAL Endochondral ossification - epiphyseal growth plate 1. Zone of resting cartilage 2. Zone of proliferation 3. Zone of maturation and hypertrophy 4. Zone of calcification and degeneration 5. Zone of ossification BONE LO5: Bone formation: endochondral & intramembranous GROWTH Bone growth after birth Growth in length results from the epiphyseal plate (growing cartilage model followed by ossification) Growth in width is by appositional growth from the periosteum Resorption of bone is as important as deposition of bone for growth to occur BONE LO6: Calcium homeostasis CALCIUM HOMEOSTASIS Blood Ca2+ Bone contains 99% of body calcium (Ca2+) Calcitonin released from thyroid gland = increase osteoblast Ca2+ is essential for a range of functions: activity = new bone synthesis = Neurotransmitter release decrease blood Ca2+ Muscle contraction Blood clotting Glandular secretions Blood Ca2+ Parathyroid hormone released Blood calcium levels are regulated by hormonal from parathyroid gland = pathways activating bone to take up or release Ca2+ increase osteoclast activity = bone degradation = increase blood Ca2+ BONE LO6: Calcium homeostasis CALCIUM HOMEOSTASIS Which cells? Which cells? TERMINOLOGY CHECKLIST Cartilage Bone Chondroblast Osteoprogenitor cell Chondrocyte Osteoblast Lacuna (L. a ditch or pond; pl. lacunae) Osteocyte Perichondrium Osteoclast Appositional growth Periosteum, endosteum Interstitial growth Women (immature) vs. lamellar (mature Isogenous group; territorial matrix bone - compact or spongy) Canaliculus (p. canaliculi) Inner and outer circumferential lamellae Interstitial lamellae Osteoid (unmineralised bone matrix)