Bones

The bones and cartilages of the human skeleton Epiglottis Larynx Cartilage in external ear Cartilages in nose Trachea Lung Cartilage in intervertebral disc Articular cartilage of a joint Costal cartilage Respiratory tube cartilages in neck and thorax Pubic symphysis Meniscus (padlike cartilage in knee joint) Key: = Bones of axial skeleton = Bones of appendicular skeleton Articular cartilage of a joint = Hyaline cartilages = Elastic cartilages = Fibrocartilages

Hyaline cartilage H [amp ] E ×150 (b) Thin epoxy resin section, toluidine blue ×1200; Cb chondroblasts Cc chondrocytes L lipid droplet M cartilage matrix N nucleus P perichondrium

Fibrocartilage H [amp ] E/Alcian blue ×320

Elastic cartilage Elastic van Gieson ×128

Cortical (compact) bone Ground sections, unstained TS ×80 (b) TS ×600 (c) LS ×150; C bone canaliculi H1 and H2 Haversian systems HC Haversian canals I interstitial system L lacunae V Volkmann's canal

Classification of bones on the basis of shape Flat bone (sternum) (a) Long bone (humerus) (d) Irregular bone (vertebra), left lateral view (b) Short bone (triquetral)

The structure of a long bone (humerus of arm) Spongy bone Spongy bone Compact bone Proximal epiphysis Articular cartilage Articular cartilage Epiphyseal line Periosteum Endosteum Compact bone Medullary cavity (b) Yellow bone marrow Diaphysis Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries Distal epiphysis (a) (c)

Artery with capillaries Structures in the Vein central canal A single osteon Artery with capillaries Structures in the central canal Vein Nerve fiber Lamellae Collagen fibers Twisting force

Microscopic anatomy of compact bone Lamella Osteocyte Osteon (Haversian system) Circumferential lamellae Lamellae Lacuna (b) Canaliculus Central (Haversian) canal Blood vessel continues into medullary cavity containing marrow Osteon Spongy bone Perforating (Sharpe’s) fibers Compact bone Periosteal blood vessel Lacuna Central (Haversian) canal Periosteum Perforating (Volkmann’s) canal Blood vessel (c) (a) Endosteum lining bony canals and covering trabeculae Central canal Interstitial lamellae

Periosteum (a) Inactive: H [amp ] E ×128 (b) Active: H [amp ] E ×200

Ossification Intramembranous: patelas Endochondral: epiphysis

Intramembranous ossification Mesenchymal cell Collagen fiber Ossification center Osteoid Osteoblast 1 An ossification center appears in the fibrous connective tissue membrane. •Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. Osteoblast Osteoid Osteocyte Newly calcified bone matrix 2 Bone matrix (osteoid) is secreted within the fibrous membrane. • Osteoblasts begin to secrete osteoid, which is mineralized within a few days. • Trapped osteoblasts become osteocytes.

Intramembranous ossification (continued) Mesenchyme Condensing to form the periosteum Trabecula of woven bone Blood vessel 3 Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels, which form a random network. The result is a network (instead of lamellae) of trabeculae. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. Fibrous periosteum Osteoblast Plate of compact bone Diploë (spongy bone) cavities contain red marrow 4 Bone collar of compact bone forms and red marrow appears. • Trabeculae just deep to the periosteum thicken, forming a woven bone collar that is later replaced with mature lamellar bone. • Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.

Intramembranous ossification H [amp ] E ×75; B woven bone M primitive mesenchyme Ob osteoblas

Endochondral ossification in a long bone Secondary ossificaton center Articular cartilage Spongy bone Epiphyseal blood vessel Deteriorating cartilage matrix Hyaline cartilage Epiphyseal plate cartilage Spongy bone formation Primary ossification center Medullary cavity Bone collar Blood vessel of periosteal bud 1 Formation of bone collar around hyaline cartilage model. 2 Cavitation of the hyaline carti- lage within the cartilage model. 3 Invasion of internal cavities by the periosteal bud and spongy bone formation. 4 Formation of the medullary cavity as ossification continues; appearance of sec- ondary ossification centers in the epiphy- ses in preparation for stage 5. 5 Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages.

Growth in length of a long bone Resting (quiescent) zone Growth (proliferation) zone Cartilage cells undergo mitosis Hypertrophic zone Older cartilage cells enlarge Calcified cartilage spicule Calcification zone Matrix becomes calcified; cartilage cells die; matrix begins deteriorating Osteoblast depositing bone matrix Ossification (osteogenic) zone New bone formation is occurring Osseous tissue (bone) covering cartilage spicules

Long bone growth and remodeling during youth Articular cartilage Growth Remodeling Bone grows in length because: Growing shaft is remodeled by: 1 Cartilage grows here 1 2 Bone resorbed here Cartilage replaced by bone here Epiphyseal plate 3 2 Cartilage grows here Bone added by appositional growth here 4 Cartilage replaced by bone here 3 Bone resorbed here

Epiphysial growth plate H [amp ] E/Alcian blue ×40

Endochondral ossification - metaphysis H [amp ] E/Alcian blue ×198

Stages in the healing of a bone fracture External callus Hematoma Bony callus of spongy bone Internal callus (fibrous tissue and cartilage) New blood vessels Healed fracture Spongy bone trabeculae 1 Hematoma formation 2 Fibrocartilaginous callus formation 3 Bony callus formation 4 Bone remodeling

Bone remodelling and repair H [amp ] E ×480; B bone D cartilage degeneration E epiphyseal cartilage GP epiphysial growth plate H hypertrophy zone M maturation zone O osteogenic zone Ob osteoblast Oc osteoclast P proliferative zone R reserve cartilage zone SC secondary ossification centre V blood vessels WB woven bone

Bone anatomy and stress Load here (body weight) Head of femur Compression here Tension here Point of no stress