cartilage and bones

• Main function of connective tissue

  1. Support (stroma, muscle tissue covering), framework (bone, tendons, ligaments) of different organs/structures in the body

  2. Protection against invading pathogens (blood, lymphoid system), nutrition (adipose)
• Connective tissue composition


  • ECM embedded with only a few cellular components, both fixed and wandering cells
  • ECM primarily made up of gelatinous ground substance interspersed with different types of connective tissue fibers
  • Ground substance's high-water content attributed to highly polar and negatively charged GAGs of the proteoglycans
• Types of connective tissue

  • Connective Tissue Proper (Collagenous Connective Tissue)
  • Loose Collagenous (Loose Connective Tissue; Areolar Connective Tissue)
  • Dense Collagenous (Dense Connective Tissue)
  • Mucoid Connective Tissue
  • Reticular Connective Tissue
• Loose Collagenous Connective Tissue


  • More prevalent in the body than dense CT
  • High vascularity vs Dense CT
  • Loose, irregular arrangement of sparse CT fibers
  • Abundant ground substance surrounding cells and fibers
  • Function: forms a layer beneath epithelial lining of many organs, filling spaces between muscle and nerve
  • Location: papillary layer of dermis, submucosa of the bowel, muscle fascia, mesentery
• Dense Collagenous Connective Tissue


  • Few blood vessels
  • Scanty ground substance
  • Abundant CT fibers
  • Fewer cells vs loose CT
  • Classification: Dense Irregular CT, Dense Regular CT
  • Dense Irregular CT location: reticular layer of dermis, capsule of LN, liver, spleen, testes, sheath of large nerves
  • Dense Regular CT location: tendon, ligaments, fibrous membranes
  • Dense Regular CT function: strong tissue support, concentrated in areas where resistance to forces from different directions is needed
• Mucoid Connective Tissue


  • Abundant amorphous jelly-like ground substance with few collagen, elastic or reticular fibers
  • Mainly hyaluronic acid in ground substance
  • Few cellular elements, mainly mesenchymal cells and fibroblasts
  • Locations: Umbilical cord (Wharton's Jelly), dental pulp of young teeth
• Reticular Connective Tissue


  • Predominant cell: reticular cells
  • Most abundant fiber: reticular fibers (Collagen Type III)
  • Reticular cells attached to fibers
  • Function: supporting framework of organs, meshwork for passage of leukocytes
• Types of tissues

  • Reticular Connective Tissue
  • Adipose Tissue
  • Bone and Cartilage
• Reticular Connective Tissue


  • Predominant cell: reticular cells
  • Most abundant fiber: reticular fibers (Collagen Type III)
  • Function: supporting framework of organs; meshwork for passage of leukocytes in lymphoid tissues
  • Location: stroma of liver, myeloid tissue, pancreas, lymph nodes, spleen
  • Stain: silver stain
• Bone and Cartilage


  • Special types of Dense Regular Connective Tissue
  • Difference in cell location inside cavities called lacunae
  • Firm or harder matrix vs dense regular CT
  • Bone - calcified = rigid, hard
  • Cartilage - firm, “soft plastic”
• Cartilage


  • Avascular, no lymph vessels, no nerves
  • Firm consistency of ECM allows tissue to bear mechanical distress without permanent distortion, “shock absorber”
  • Cellular component: Chondrocytes housed in lacunae
  • Synthesizes and maintains ALL ECM components
  • Types: Hyaline, Elastic, Fibrocartilage
  • Perichondrium - sheath of dense irregular CT that surrounds cartilage; except at epiphyses & articular cartilages
  • “Chondrogenic potency” - ability to form cartilage
  • Formation: chondrogenesis = embryonic mesenchyme cells differentiation to chondroblasts chondrocytes
  • Growth: Interstitial - mitotic division of chondrocytes, Appositional - chondroblast differentiation from progenitors in perichondrium
  • Repair: Poor ability/inefficient for repair d/t avascularity; new tissue is not cartilage, resembles dense CT instead
• Hyaline Cartilage


  • Most abundant type of cartilage in the body
  • Features: consists of cells (chondrocytes) inside cavities (lacunae) that are dispersed in intercellular substance (cartilage matrix)
  • Location: skeleton of the fetus in-utero, articular surface of movable joints, sternal end of ribs, epiphyseal plate of long bones, large respiratory passages (trachea, external nose, larynx, bronchi)
• Elastic Cartilage


  • More flexible than hyaline cartilage
  • Features: same with hyaline cartilage, except that it contains elastic fibers aside from Collagen type II fibrils
  • Function: maintains shape while allowing flexibility
  • Location: auricle, external acoustic meatus, auditory tube, epiglottis, some parts of the larynx
• Fibrocartilage


  • Can withstand greater stress than hyaline and elastic cartilage
  • Transitional stage between hyaline cartilage and dense CT; Collagen type I
  • Features: cells occur isolated or in aligned isogenous aggregates
  • Location: intervertebral discs, pubic symphysis, surface layers of tendons and ligaments that press on bone (knee, jaw)
  • Stain: picrosirius & hematoxylin
• Bone


  • Consists of calcified matrix (presence of calcium in the matrix)
  • Cells: Osteoprogenitor cells, Osteoblasts, Osteocytes, Osteoclasts
  • Function: provide rigid framework for the body; protect vital organs – skull, rib cage; storehouse for calcium and phosphorus; houses the bone marrow; levers for muscles
• Bone Matrix


  • Inorganic minerals: Calcium, phosphorus
• Cells of bone tissue

  • Osteoprogenitor cells
  • Osteoblasts
  • Osteocytes
  • Osteoclasts
• Function of bone


  • Provide rigid framework for the body; protect vital organs such as the skull and rib cage; storehouse for calcium and phosphorus; house the bone marrow; act as levers for muscles
• Components of bone matrix

  • Inorganic minerals
  • Organic compounds
• Inorganic minerals in bone matrix

  • Calcium, phosphorus crystals of calcium hydroxyapatite
  • Bicarbonate, citrate, magnesium, potassium, sodium ions
• Formation of bone


  • Osteogenesis/ossification = embryonic mesenchyme cells differentiate to osteoprogenitor cells - osteoblasts - osteocytes
  • Osteoclasts are from a different lineage, same with monocytes (CFU-GM)
• Intramembranous Ossification


  1. Mesenchymal tissue to bone

  2. Flat bones of the skull
• Endochondral Ossification


  1. Hyaline cartilage to bone

  2. Limbs, pelvis, vertebral column
• Cells of bone tissue

  • Osteoprogenitor cells: fusiform cells; line periosteum and endosteum
  • Osteoblasts: synthesizes organic constituents of bone GS, and precursors of collagen fibers in the bone tissue; have receptors for parathyroid hormone
  • Osteocytes: osteoblasts that get trapped in the developing matrix; little secretory capability, enough to maintain the bone matrix that surrounds them
  • Osteoclasts: multinucleated cells; ruffled border where bone resorption happens
• Periosteum and Endosteum


  • All surfaces of bone are covered by tissue with “osteogenic potency”; Periosteum covers external surface, Endosteum covers internal surface
• Structure of bone


  • Calcified intercellular substance arranged in thin layers called lamellae; cavities called lacunae dispersed in the bone lamellae; small canals called canaliculi connected to other canaliculi; lacunae house osteocytes, canaliculi house the cytoplasmic processes
• Forms of bone tissue

  • Compact Bone (cortical; dense)
  • Spongy Bone (cancellous)
• Compact Bone


  • Haversian Systems: Osteons consist of up to 4-20 bone lamellae arranged concentrically around a tiny channel called Haversian canal; Interstitial Lamellae are remnants of Haversian systems; Circumferential Lamellae are most developed in long bones
• Spongy Bone


  • Spicules; trabeculae form cavities occupied by bone marrow; osteocytes receive nutrition directly or indirectly
• Compact Bone vs Spongy Bone

  Comparison between the two types of bone tissue