Chapter 6 Bones and Skeletal Tissue Skeletal Cartilage: Made up of chondrocytes house in lacunae



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Chapter 6

Bones and Skeletal Tissue

Skeletal Cartilage:

  • Made up of chondrocytes house in lacunae (cavities) within the extracellular matrix (ground substance and fibers)

  • Contains large amount of water (giving it resilience)

  • Lacks nerve fibers, is avascular

  • Is surrounded by a fibrous perichondrium that resists expansion


Hyaline Cartilage:

  • Appears glassy, the fibers are collagenic

  • Provide support with flexibility and resilience

  • Are the most abundant skeletal cartilages including articular, costal, respiratory, and nasal cartilages


Elastic Cartilages:

  • Contain abundant elastic fibers in addition to collagen fibers

  • More flexible than hyaline cartilages

  • Support the outer ear and the epiglottis


Fibrocartilages:

  • Contain thick collagen fibers

  • Are compressible

  • Resilient to stretching

  • Form vertebral disks and knee joint cartilages


Growth of Cartilage:

  • Grow from within (interstitial growth) and by the addition of new cartilage tissue at the periphery (appositional growth)

    • Interstitial Growth – lacunae bound chondrocytes divide and secrete new matrix expanding cartilage from within

    • Appositional Growth – cartilage forming cells in the surrounding perichondrium tissue secrete new matrix against the external face of existing cartilage


Classification of Bones:

  • Classified as long, short, irregular and flat on the basis of their shape and their proportion of compact and spongy bone. (Sketch)


Functions of Bones:

  • Give the body its shape

  • Protect

  • Support body organs

  • Provide levers for muscles to pull on

  • Store calcium and other minerals

  • Site of blood cell production

Bone Structure: Gross Anatomy:

  • Bone markings are important anatomical landmarks

    • They reveal sites of muscle attachments

    • Points of articulation

    • Sites of nerves and blood vessels passage


Long Bone:

  • Composed of a diaphysis (shaft) and epiphyses (ends)

  • Medullary cavity of diaphysis contains yellow marrow

  • Epiphyses contain spongy bone

  • Epiphyseal line is a remnant of the epiphyseal plate (growth plate)

  • Periosteum covers the diaphysis

  • Endosteum lines inner bone cavities

  • Hyaline cartilage covers the joint surfaces


Flat Bones:

  • Consist of two thin plates of compact bone enclosing a diploe (spongy bone layer)

  • Short and irregular bones resemble flat bones structurally

  • In adults, hematopoietic tissue is found within the diploe of flat bones and occasionally within the epiphysis of long bones

  • In infants, red bone marrow is also found in the medullary cavity


Microscopic Anatomy of Bones:

  • The osteon is the structural unit of compact bone

    • Consists of a central canal surrounded by concentric lamellae of bone matrix

  • Osteocytes, embedded in the lacunae, are connected to each other and the central canal by canaliculi

  • Spongy bone

    • Has slender trabeculae (strut or thin plate of bone) containing irregular lamellae which enclose red marrow filled cavities


Chemical Composition of Bone:

  • Composed of living cells (osteogenic cells) Osteoblasts, osteocytes, osteoclasts and matrix

    • Matrix includes osteoid, organic substances that are secreted by osteoblasts and give bone tensile strength. Its inorganic components, the calcium salts (hydroxyapatites) make the bone hard


Formation of the Bony Skeleton:

  • Intramembranous ossification forms the clavicles and most skull bones

    • Ground substance on the bone matrix is deposited between collagen fibers within the fibrous membrane to form spongy bone

    • Eventually compact bone plates enclose the diploe

  • Most bones are formed by endochondral ossification of the hyaline cartilage model

    • Osteoblasts beneath the periosteum secrete bone matrix on the cartilage model forming a bone collar

    • Deterioration of the cartilage model internally opens ups cavities, allowing periosteal bud entry

    • Bone matrix is deposited around the cartilage remnants but is later broken down

    • Post Natal Bone Growth

      • Long bone increases in length by interstitial growth of the epiphyseal plate cartilage and its replacement by bone

      • Appositional growth increases the bone diameter/thickness




  • Bone Homeostasis – Remodeling and repair

    • New bone is continually deposited and resorbed in response to hormonal and mechanical stimuli – this is called bone remodeling

    • An osteoid seam appears at areas of new bone deposits

      • with calcium deposited a few days later

    • Osteocytes release lysosomal enzymes and acids on bone surfaces to be resorbed

      • The dissolved products are transcytosed to the opposite side (face) of the osteoclasts for release into the extracellular fluid

    • The hormonal mechanism of bone remodeling serves blood calcium balance

      • When blood calcium levels decline, PTH is released to stimulate osteoclasts to digest bone matrix, releasing ionic calcium

      • When blood calcium levels rise, calcitonin is released, stimulating removal of calcium from the blood

      • Mechanical stress and gravity acting on the skeleton help maintain skeletal strength

      • Bones thicken, develop heavier prominences, or rearrange their trabeculae in sites where stressed

  • Fracture Repair

    • Are treated by open or closed reduction

    • The healing process involves formation of a hematoma, a fibrous cartilaginous callus, a bony callus, and bone remodeling in succession

  • Homeostatic imbalances of Bone

    • Imbalances between bone formation and resorption underlie all skeletal disorders

    • Osteomalacia and rickets occur when bones are inadequately mineralized

      • Bones become soft and deformed

      • Most frequently cause by inadequate Vitamin D

    • Osteoporosis

      • Any condition in which bone breakdown out paces bone formation causing bones to become weak and porous

      • Postmenopausal are particularly susceptible

Paget’s disease is characterized by excessive and abnormal bone remodeling

Developmental aspects of Bones



  • Osteogenesis is predictable and precisely timed

  • Longitudinal bone growth continues until the end of adolescence

    • Skeletal mass increases dramatically during puberty and adolescence when formation exceeds resorption

  • Bone mass is fairly constant in young adulthood


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