Connective Tissue II

Cartilage

• Cartilage is avascular and is a specialized connective tissue.
• Extracellular matrix is firm and less pliable than connective tissue proper.
• Primary cellular component: chondrocytes.
• Functions: supports soft tissues, provides shock absorption and sliding movement within joints, and is essential for development and growth of long bones.

Perichondrium

• A sheath of dense connective tissue surrounding cartilage in most locations.
• Forms an interface between cartilage and the tissue it supports.
• Contains blood vessels, nerves, and lymphatic vessels.
• Exceptions: fibrocartilage and articular cartilage (a type of hyaline cartilage) lack a perichondrium.

Histogenesis

• A. Embryonic Mesenchyme: embryonic connective tissue that gives rise to all other connective tissue types.
• B. Mitotic proliferation of mesenchymal cells.
• C. Differentiation of mesenchymal cells into chondroblasts, which lay down the matrix.
• D. Multiplication of cells gives rise to chondrocytes, which are surrounded by a territorial (capsular) matrix.
• Note: The superficial mesenchyme develops into the perichondrium.

Hyaline Cartilage

• Cells: peripheral chondrocytes are elliptical; more centrally located chondrocytes become rounder and often occur in isogenous groups (clusters derived from a single chondrocyte after mitotic division).

• Territorial matrix (capsular matrix): zone surrounding chondrocytes rich in GAGs, poor in collagen.

• Extracellular matrix: mainly Type II collagen fibrils (not well seen by light microscopy).

• Ground substance contains chondroitin sulfate and keratan sulfate covalently linked to core proteins forming proteoglycans and hyaluronic acid.

• Water binds to negatively charged GAGs, increasing resilience and providing a shock absorber effect.

• Chondrogenic layer: contains chondrocytes within lacunae, capsule, chondrocyte aggregates, chondroblasts, and fibrous perichondrium (typical histology from development).

Hyaline Cartilage Locations

• Articular surfaces of bones.
• Respiratory tract: nose, larynx, trachea, and bronchi.
• Costal cartilage (sternal ends of ribs).
• Epiphyseal plates (growth plates) of bones.

Elastic Cartilage

• Hyaline-like in all respects except the ECM contains an abundant network of elastic fibers in addition to Type II collagen.
• More pliable and distensible than hyaline.
• Locations: auricle (pinna) of the ear, walls of external acoustic meatus (canals), auditory (Eustachian) tubes, and the epiglottis.

Hyaline vs Elastic Cartilage (Key differences)

• Hyaline: Type II collagen; basophilic matrix; chondrocytes often in isogenous groups; perichondrium present.
• Elastic: Type II collagen plus abundant elastic fibers; perichondrium present (like hyaline) but matrix contains elastic fibers providing greater elasticity.
• Both have lacunae housing chondrocytes; territorial matrix around cells; perichondrium typically present except at articular surfaces.

Fibrocartilage

• Chondrocytes can appear singly or in isogenous groups in long rows; separated by coarse Type I collagen fibers.
• Extracellular matrix contains numerous collagen fibers (Type I) in irregular bundles between chondrocyte groups or in parallel arrangements between columns of chondrocytes.
• There is no perichondrium.
• Capsules (perichondrial coverings) are seldom seen.

Fibrocartilage Locations

• Associated with dense connective tissue
• Notable sites: intervertebral discs, pubic symphysis, and menisci of the knees.

Table 7.1: Cartilage Types, Characteristics, and Locations

• Hyaline
  • Identifying characteristics: Type II collagen; basophilic matrix; chondrocytes usually arranged in groups (isogenous groups)
  • Perichondrium: usually present
  • Location: trachea, bronchi; ventral ends of ribs; nose; larynx; some laryngeal cartilages; epiphyseal plates
• Elastic
  • Identifying characteristics: Type II collagen; elastic fibers
  • Perichondrium: present
  • Location: pinna of ear (pinna); auditory canal; epiglottis; some laryngeal cartilages
• Fibrocartilage
  • Identifying characteristics: Type I collagen; acidophilic matrix; chondrocytes in parallel rows between bundles of collagen; often associated with dense collagenous connective tissue and/or hyaline cartilage
  • Perichondrium: absent
  • Location: intervertebral discs; articular discs; pubic symphysis; insertion of tendons; meniscus of knee

Logic Tree for Cartilage (diagnostic approach)

• Start by assessing the extracellular matrix: Is it homogeneous (no visible fibers)? If yes, proceed; if no, evaluate fiber arrangement.
• If fibers are present, determine whether lacunae are randomly arranged.
• If lacunae are randomly arranged and fibers are present, you may classify as Hyaline cartilage or Elastic cartilage depending on fiber type.
• If lacunae are in isogenous groups and fibers are absent or minimal, classify as Hyaline cartilage.
• If fibrous bundles predominate and lacunae are in parallel rows between bundles, classify as Fibrocartilage.
• If elastic fibers are abundant within matrix, classify as Elastic cartilage.

Bone

• Bone is highly vascularized, specialized connective tissue with cells embedded in a calcified extracellular matrix.
• It is a dynamic tissue that changes shape in response to mechanical forces: applied tension promotes bone formation; applied pressure promotes bone resorption.
• Cell types:
  • Osteoblasts: synthesize and secrete osteoid (uncalcified bone matrix).
  • Osteocytes: mature bone cells housed within lacunae.
  • Osteoclasts: large, multinucleated cells that function in osteolysis (bone resorption).

Bone Matrix

• Organic portion: Type I collagen fibers; ground substance contains chondroitin sulfate and keratan sulfate.
• Inorganic portion: Calcium and phosphate precipitates form hydroxyapatite crystals; also significant amounts of amorphous (non-crystallized) calcium phosphate; other ions present include bicarbonate, citrate, magnesium, potassium, and sodium.

Types of Bone

• Compact bone: dense areas without cavities.
• Spongy (cancellous) bone: bone with numerous interconnecting cavities; trabeculae are the bony partitions between cavities.

Compact Bone and Osteon Anatomy

• Haversian system (osteon): the structural unit of compact bone.
• Central (Haversian) canal runs longitudinally through the osteon and contains a blood vessel.
• Concentric lamellae surround the central canal.
• Interstitial lamellae are remnants between osteons.
• Outer circumferential lamellae lie beneath the periosteum; inner circumferential lamellae line the inner surface of the compact bone near the endosteum.
• Periosteum covers the outer surface of compact bone.
• Volkman’s (perforating) canal connects blood vessels from the periosteum to the Haversian system.
• Endosteum lines the internal cavities and covers trabeculae in spongy bone.
• Osteocytes reside in lacunae; extend processes through canaliculi to communicate with other osteocytes and receive nutrients.

Microscopic anatomy of compact bone (summary)

• Osteon: circular unit comprising central canal and surrounding lamellae.
• Central (Haversian) canal: contains blood vessels and nerves.
• Perforating (Volkmann’s) canals: run perpendicular to osteons, connecting blood supply.
• Lamellae: concentric rings of mineralized matrix around the central canal.
• Lacunae: small spaces housing osteocytes.
• Canaliculi: tiny channels connecting lacunae, enabling cell-to-cell communication and nutrient/waste exchange.
• Interstitial lamellae: remnants of old osteons between new ones.
• Cement line: boundary between adjacent osteons.

Endosteum and Periosteum (structural membranes in bone)

• Periosteum: membrane that covers the outer surface of compact bone; contains osteoprogenitor cells and nerves.
• Endosteum: delicate membrane that lines the internal canals and covers the trabeculae of spongy bone; contains osteoprogenitor cells.

Spongy (Cancellous) Bone

• Contains osteocytes within lacunae, arranged along trabeculae.
• Spaces house red bone marrow.
• Osteoclasts and osteoblasts align along trabeculae to form new bone.
• Trabeculae are the bony columns that create the lattice structure of spongy bone.
• Lacunae and canaliculi are present within trabeculae as in compact bone.

Aging and Osteocytes (hypothetical scenario)

• Question: As people age, some central (Haversian) canals may become blocked. What effect would this have on the surrounding osteocytes?
• Answer: Blocking central canals would impede the diffusion of nutrients and removal of wastes from osteocytes within the osteons, potentially leading to osteocyte stress or death due to restricted diffusion through canaliculi and reduced vascular supply.