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Bone tissue

Last updated: April 10, 2021

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The musculoskeletal system is comprised of bones and connective tissue structures, such as cartilage, ligaments, and tendons. These structures are brought into motion by skeletal muscles. To withstand resultant forces, bone tissue resists pressure and tension and is minimally elastic. Bone tissue mainly consists of bone cells (osteoblasts, osteocytes, and osteoclasts) and a mineralized extracellular matrix that is primarily made up of collagen fibrils and hydroxyapatite crystals. Ossification, or bone formation, begins with a framework that consists of either mesenchymal connective tissue (intramembranous ossification) or cartilage (endochondral ossification). Woven bone is formed, which is replaced by the more solid and layered lamellar bone. The outer cortical layers can be macroscopically differentiated from the branched center of the trabeculae.

Function of bone

  • Supportive function
  • Protective function
  • Storage (calcium and phosphorus reservoir)
  • Hematopoiesis

Types of bone

References:[1][2]

Overview

The exact composition or organization of individual bone components differs in the various bones types and maturation stages. All human bones are composed of the same basic elements:

Bone cells

Characteristics of bone cells
Definition Function [3] Location
Osteoprogenitor cells
  • Barely differentiated precursor bone cells that originate from the mesenchyme
Osteoblasts [4]
  • Produce osteoid (organic bone matrix, mainly composed of collagen), which is stored in layers on the mineralized matrix
  • Require alkaline environment (↑ pH) for bone alkaline phosphatase (ALP), an enzyme that cleaves inorganic phosphate from various substrates to make it available for mineralization
  • Control osteoid mineralization (hydroxyapatite formation)
  • Regulate bone remodeling
Osteocytes
Osteoclasts

Bone matrix

Composed of organic and inorganic material:

Bone membranes

  • Periosteum: a membrane of connective tissue that covers the outer surface of the bone in two distinct layers
    • Cambium layer: inner vascular and innervated layer (very sensitive to pain) that is lined with bone cells
    • Fibrous layer: outer layer consisting of connective tissue from which Sharpey fibers extend into the bone
  • Endosteum

The periosteum and the endosteum consist of the same type of bone cells.

Bone marrow

Development of bone

Ossification

Overview of the ossification process
Endochondral ossification Intramembranous ossification
Definition
Process
  1. Mesenchymal cells differentiate into osteoblasts at the ossification center.
  2. Osteoblasts deposit osteoids → osteocytes form after osteoid mineralization → formation of a bone segment
  3. The osteoblasts on the outer surface of the bone segment deposit osteoid layers → appositional growth
  4. Several bone segments fuse to primary trabecular bone.
  5. Blood vessels and undifferentiated mesenchymal cells invade the trabecular bone → formation of bone marrow
  6. Simultaneous construction and remodeling of bone (woven bone → lamellar bone)
  7. Mesenchymal layers that do not become ossified → formation of endosteum and periosteum
Examples

The skull undergoes both processes: some bones (e.g., frontal, parietal bones) are derived from the neural crest and undergo membranous ossification, whereas other bones (e.g., sphenoid, occipital bones) are derived from the paraxial mesoderm and undergo endochondral ossification.

Stages of bone maturity

Bones are arranged into woven bone (primary bone) during embryonic development or bone healing. The structure of woven bone is disorganized and transformed into organized tissue of lamellar bone (secondary bone) through continuous remodeling.

Stages of bone maturity
Woven bone Lamellar bone
Definition
Histology
  • Disorganized collagen fibers
  • Less mineralized bone substance with a high water content
  • Rich in cells

Characteristics

  • Mechanically strong
Remodeling
  • Usually successively remodeled to more stable lamellar bones
  • Load-dependent, continuous remodeling of bone according to acting forces

The direction of collagen fibers of the bone extracellular matrix is an important distinguishing characteristic between immature woven bone and mature lamellar bone.

Trabecular bone (spongy or cancellous bone)

  • Definition: thin lattice-shaped (trabeculae) units inside the lamellar bone
  • Characteristics
    • Trabeculae are aligned along large compressive and tensile forces (trajectory direction).
    • The bone marrow is located in the intermediate space of the trabecular bone.

Cortical bone (compact bone)

  • Definition: homogeneous and dense cortical layer of lamellar bone
  • Main components
    • Osteon
      • Definition: concentric bony lamellae with a central Haversian canal
      • Direction: several concentric lamellae whose collagen fibers change direction from one lamella to the other
      • Boundary: cement lines
    • Interstitial lamellae
      • Remnants of partially destroyed osteons
      • Separate osteons from one another
    • Circumferential lamellae: provide an outer and inner boundary for the cortical bone through at least one lamellar layer

Lamellar bones vascularization and canals

  • Bone canals and associated vessels
    • Haversian canal: a canal in the center of the osteon that contains Haversian vessels, which supply the bone with blood
    • Volkmann canal (perforating channels): canals radiating from the periosteum that are at a right angle to Haversian vessels and are supplied by Volkmann vessels

The course of lamellar bone vessels is strictly defined by the Haversian and Volkmann canal structures. In contrast, woven bone vessels are disorganized.

Development of long bones

Epiphyseal plate

The part of the bone where longitudinal growth takes place. Layers include (from epiphysis to diaphysis):

  1. Zone of resting cartilage: deposits undifferentiated precursor chondrocytes that provide the proliferation zone with new chondrocytes
  2. Proliferation zone: mitosis of chondrocytes
  3. Zone of hypertrophy: enlarges cartilage through chondrocyte hypertrophy, which leads to collagen (type X) production by hypertrophic chondrocytes and mineralization of the longitudinal septa
  4. Zone of calcification
  5. Zone of ossification: colonization of the mineralized longitudinal septae by osteoblasts osteoid formation → mineralization

References:[5]

The human skeleton is in a continuous dynamic state of remodeling. Not only does this apply to the replacement of immature woven bone by lamellar bone, but also for adaptation of adult bones to their individual load.

Bone remodeling [6]

Blasts Build, Clasts Crumble.

Bone remodeling in cortical bone

Bone remodeling in trabecular bone

Regulation of bone remodeling [7]

Estrogen has a positive effect on bone balance because it inhibits osteoclast formation and activation and increases OPG formation.

Bone healing [8]

Fractures occur when bones are strained beyond their maximum load. Fractures can heal in two different ways.

During childhood or adolescence, a fracture near a joint may damage the unclosed epiphyseal plate, which can result in growth disorders (e.g., asymmetry, inhibition, acceleration) during healing.

References:[9][10]

  1. Sadler TW, Langman J. Langman's Medical Embryology. Lippincott Williams & Wilkins ; 2009
  2. Musculoskeletal System - Skull Development. https://embryology.med.unsw.edu.au/embryology/index.php/Musculoskeletal_System_-_Skull_Development. Updated: May 9, 2018. Accessed: June 28, 2018.
  3. Blair HC, Larrouture QC, Li Y, et al. Osteoblast differentiation and bone matrix formation in vivo and in vitro. Tissue Engineering Part B: Reviews. 2017; 23 (3): p.268-280. doi: 10.1089/ten.teb.2016.0454 . | Open in Read by QxMD
  4. Kwan Tat S, Padrines M, Théoleyre S, Heymann D, Fortun Y. IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev. 2004; 15 (1): p.49-60.
  5. Patton KT, Thibodeau GA. Anthony's Textbook of Anatomy & Physiology. Mosby ; 2014
  6. Rucci N. Molecular biology of bone remodelling.. Clin Cases Miner Bone Metab. 2008; 5 (1): p.49-56.
  7. Ross MH, Pawlina W. Histology. Lippincott Williams & Wilkins ; 2006
  8. Sela JJ, Bab IA. Principles of Bone Regeneration. Springer Science & Business Media ; 2012
  9. Kasper DL, Fauci AS, Hauser SL, Longo DL, Lameson JL, Loscalzo J. Harrison's Principles of Internal Medicine. McGraw-Hill Education ; 2015
  10. Hall JE. Guyton and Hall Textbook of Medical Physiology. Elsevier ; 2016