Structure of Bones

Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Because the matrix is mineralized (rather than aqueous), nutrients and waste cannot diffuse through the matrix. Bone has developed a unique structure to allow the functions to occur. The structure of bone allows the bone to be hard, but not too brittle, and gives bone the strength to resist compressive and bending forces. As a result, bone is ideally suited for the functions of support, protection of vital organs, and movement. In addition, bone produces blood cells in the marrow and is the body’s primary storage site for calcium.

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Overview

Definition

Bone, as an organ, is made up of:

  • Osseous tissue: a type of connective tissue hardened by the deposition of minerals (primarily calcium and phosphate)
  • Blood
  • Bone marrow
  • Cartilage and fibrous connective tissue
  • Adipose tissue
  • Nervous tissue

Bone can refer to:

  • The entire organ
  • The osseous tissue within the organ only

Function of bones

  • Protection (e.g., the skull protects the brain, the ribs protect the heart and lungs)
  • Support
  • Movement
  • Blood cell formation
  • Storage:
    • Minerals
    • Phosphate

Classification of bones by location

Axial bones:

  • Skull
  • Vertebral column and sacrum
  • Rib cage: ribs and sternum

Appendicular bones:

  • Clavicle
  • Scapula
  • Arms and hands
  • Pelvic girdle
  • Legs and feet

Classification of bones by shape

  • Long bones: length greater than width:
    • Upper extremity: humerus, radius, ulna, metacarpals, and phalanges
    • Lower extremity: femur, tibia, fibula, metatarsals, and phalanges
  • Short bones: length approximately equal to width:
    • Carpal bones (wrist)
    • Tarsal bones (ankle)
  • Flat bones: enclose and protect soft organs:
    • Most skull bones
    • Ribs
    • Sternum
    • Scapula
  • Irregular bones: bones not fitting into other categories:
    • Vertebrae
    • Some skull bones (e.g., sphenoid, facial bones)

Bone Structure

Types of bone tissue

Compact bone: 

  • A smooth, solid outer layer of osseous tissue
  • Locations:
    • Present in every bone in the body
    • In long bone: forms a cylinder, encases a medullary cavity
  • Function: strength to withstand compressive forces

Spongy (cancellous) bone: 

  • Loosely organized, inner-layer osseous tissue
  • Consists of a lattice of small, thin pieces of osseous tissue called trabeculae or bony spicules:
    • Transfers force on the bone to the outer compact bone
    • Constantly reforming to meet the body’s needs (e.g., exercise increases the trabeculae; prolonged weightlessness in space reduces the trabeculae) 
  • Locations: internal to compact bone:
    • At the ends of long bones 
    • In the middle of short, flat, and irregular bones
Internal structure of a femur head structure of bones

Image of the internal structure of a femur head: Note the compact bone along the outside and spongy/cancellous bone in the center.

Image by Lecturio.

Bone membranes

Two primary membranes surround bone tissue: the periosteum (externally) and the endosteum (internally).

Periosteum:

  • Outer layer surrounding bone on the external surface (except at the joints, which are covered in articular cartilage)
  • Vascularized and innervated
  • Consists of 2 layers:
    • Fibrous layer
    • Osteogenic layer
  • Fibrous layer of periosteum: 
    • Tough outer layer of collagen
    • Sharpey fibers: collagen fibers from the fibrous layer of the periosteum:
      • Continuous with muscle tendons on top of the bone
      • Penetrate deep into the bone matrix to secure the periosteum and overlying muscle to the bone
  • Osteogenic layer of periosteum: 
    • Contains bone-forming cells: 
      • Osteoblasts
      • Osteoclasts
      • Osteogenic cells
    • Critical in bone growth and healing after injury

Endosteum:

  • Lines internal surfaces of bone:
    • Lines the medullary cavity in long bones
    • Covers the trabeculae in spongy bone
  • Contains the same bone-forming cells as the osteogenic layer of the periosteum

Structure of long bones

The 3 primary anatomic regions of long bones:

  • Diaphysis:
    • The shaft
    • Forms the long axis of long bones
    • Consists of a thick layer of compact bone, surrounding a central medullary cavity containing bone marrow
  • Epiphysis:
    • Ends of the bones (at joints)
    • Wider than the diaphysis:
      • Strengthens the joint
      • ↑ Surface area for tendon and ligament attachment
    • Primarily composed of spongy bone
    • Outer layer of compact bone
    • Covered in articular cartilage: 
      • A type of hyaline cartilage
      • Articular cartilage + lubricating fluid → ↓ friction → significantly easier joint movement
      • Acts as a shock absorber
  • Metaphysis:
    • In between the epiphysis and diaphysis
    • Remnant of the epiphyseal plate or line: the hyaline cartilage allowed for bone elongation in childhood
Bone marrow inside the femur

Bone marrow inside the femur

Image: “603 Anatomy of Long Bone” by OpenStax College. License: CC BY 4.0

Structure of short, irregular, and flat bones

  • Outer layers: thin plates of periosteum-covered compact bone 
  • Inner layer: endosteum-covered spongy bone
  • In flat bones, the inner spongy bone is:
    • Known as diploë
    • Sandwiched between 2 layers of compact bone
Structure of a flat bone

Structure of a flat bone

Image: “Cross-section of a flat bone showing the spongy bone (diploë) lined on either side by a layer of compact bone” by OpenStax College. License: CC BY 4.0

Bone markings

Bone markings are areas of bone where tendons, ligaments, and fascia attach, including articulations, projections, and holes.

  • Articular markings:
    • Condyle: rounded surface at an articular area
    • Epicondyle: eminence superior to a condyle
    • Facet: flat surface where bones articulate
  • Projections:
    • Crest: ridge of a bone
    • Process: prominence feature
    • Protuberance: projection of bone
    • Spine: a sharp process
    • Tubercle: smaller rounded process
    • Tuberosity: rough surface
  • Holes:
    • Canal: channel through bone
    • Foramen: passage through a bone 
    • Fossa: hollow or depressed area
    • Groove or sulcus: elongated depression or furrow
    • Sinus: irregularly shaped cavernous area

Bone Cells and Matrix

The 2 primary components of bone are cells and matrix.

Bone cells

Bone contains a relatively small number of cells compared to the amount of matrix. In addition to other functions, the cells synthesize and break down the bone. Four major types of osseous cells exist:

  • Osteogenic cells: 
    • Stem cells arising from embryonic fibroblasts
    • Can differentiate into osteoblasts → stimulated by stress (e.g., from exercise) and fractures
    • Found in the endosteum and osteogenic periosteum
  • Osteoblasts:
    • Synthesize the collagen matrix (organic portion of bone)
    • Deposit calcium salts on the matrix (mineralization)
    • Unable to divide → all new osteoblasts must come from osteogenic cells
    • Found in endosteum and osteogenic periosteum
  • Osteocytes:
    • Osteoblasts trapped in the bone the osteoblasts created
    • Located in spaces within the mineralized matrix known as lacunae
    • Play no significant role in bone synthesis or resorption
    • Primary role: sense strain and communicate the message to surface osteoblasts
  • Osteoclasts:
    • Dissolve/resorb bone
    • Purpose of bone resorption:
      • Removal of old, injured, or unnecessary bone
      • Release of stored calcium (to maintain tightly regulated calcium levels)
    • Develop from the fusion of monocytes in the bone marrow → results in large, multinucleated cells with a “ruffled border” (deep folds in the plasma membrane to increase the surface area)

Bone matrix

Bone has a mineralized matrix (as opposed to an aqueous matrix in most other tissues, through which nutrients can easily diffuse). Bone has both organic and inorganic components:

  • Organic component (⅓ of the matrix by weight): 
    • Collagen fibers
    • Proteoglycans
    • Glycoproteins
  • Inorganic component (⅔ of the matrix by weight):
    • Hydroxyapatite crystals: calcium phosphate salts (85% of inorganic component by weight)
    • Calcium carbonate (10% of inorganic component by weight)
    • Other ions: magnesium, sodium, potassium, fluoride, sulfate, and hydroxide (5% of inorganic component by weight)
  • The combination of organic (protein) and inorganic (mineral) components allow the bones to be strong and solid, but not too brittle.

Microscopic Structure

Microscopic structure of compact bone

In long bones, the majority of cells and matrix are arranged in functional units known as osteons.

Osteons:

An osteon (also known as a haversian system) is a cylinder of cells and matrix running longitudinally within compact bone. An osteon is made up of a longitudinal central canal, which is surrounded by concentric rings of osteocytes, and bone matrix known as lamellae.

  • Osteon central canal (haversian canal):
    • Longitudinal canal at the core of each osteon
    • Contains:
      • Blood vessels
      • Lymphatic vessels
      • Nerves
    • Lined with endosteum 
    • Provides nutrients and removes waste from osteocytes immediately adjacent to the central canal
  • Perforating canals:
    • Contain neurovasculature
    • Run perpendicular to the central canals, connecting the canals to:
      • External nerves and vessels
      • One another
  • Lamellae: 
    • Concentric rings of calcified matrix
    • Osteocytes are located between lamellae in spaces known as lacunae.
    • Canaliculi (tiny canals in the matrix) allow thin, finger-like projections of the osteocytes to connect to one another via gap junctions:
      • Allows for nutrient delivery and waste removal from osteocytes in outer rings, without being immediately adjacent to vasculature
      • Allows for communication of strain signals
  • Collagen:
    • Fibers “corkscrew” down the matrix in a given lamella
    • Different helical arrangements are present within adjacent lamellae:
      • Right-handed coils vs. left-handed coils
      • Varying tightness of coils
    • Creates a “crisscrossing” network of collagen → significantly ↑ strength to resist bending and compression

Circumferential lamellae: 

  • Lamellae running parallel to the bone surface around the entire circumference of the bone
  • Not part of an osteon functional unit
  • Locations:
    • Immediately within the osteogenic layer of periosteum
    • Lining the inner medullary cavity

Interstitial lamellae: 

  • Irregular regions of bone tissue between osteons
  • Remnants of old osteons partially broken down during bone remodeling
Microscopic structure of compact bone

Microscopic structure of compact bone

Image: “Cross-sectional view of compact bone showing the basic structural unit, the osteon” by OpenStax College. License: CC BY 4.0

Microscopic structure of spongy bone

In spongy bone, rather than forming concentric rings within osteons, lamellae form concentric rings, which create the trabeculae.

Similarities between spongy and compact bone:

  • Lamellae form concentric rings
  • Osteocytes live within lacuna between lamellae
  • Osteocytes are connected to each other via canaliculi

Differences between spongy and compact bone:

  • Trabeculae are arranged along lines of force (osteons are parallel to one another in compact bone).
  • Trabeculae form a lattice-like network, creating space within the bone filled with bone marrow.
  • No central canals → not needed because no osteocytes are far from the blood supply (surrounding marrow)
Microscopic structure of spongy bone

Microscopic structure of spongy bone

Image: “Spongy bone is composed of trabeculae containing the osteocytes. Red marrow fills the spaces in some bones.” by OpenStax College. License: CC BY 4.0

Related videos

Hematopoietic Tissues in Bones

Definitions

Bone marrow: a general term for hematopoietic soft tissue occupying the spaces within bone.

  • Red marrow: myeloid tissue (can produce blood cells)
  • Yellow marrow: fatty marrow (no longer produces blood cells)

Locations of bone marrow

  • The medullary cavities of long bones
  • Diploë of flat bones
  • Trabecular cavities of spongy bone

Marrow changes during the lifespan

  • In infants and children: Nearly all bone cavities contain red marrow.
  • In young to middle-aged adults: 
    • Most red marrow has become yellow marrow
    • Red marrow exists in:
      • Vertebrae
      • Ribs
      • Sternum
      • Parts of the pelvic girdle
      • Proximal head of the humerus and femur
    • Yellow marrow can revert to red marrow in severe or chronic anemia.

Clinical Relevance

  • Osteoporosis: a decrease in bone mass and density leading to an increased number of fractures. Osteoporosis is most commonly caused by a loss of protective estrogen and/or testosterone later in life, immobilization, underlying medical disorders, or long-term use of certain medications. Osteoporosis most often presents clinically with frequent fractures and loss of vertebral height. Diagnosis is established by measuring bone mineral density. Management includes lifestyle modifications, maintaining adequate levels of calcium and vitamin D, and the use of bisphosphonates.
  • Osteomalacia and rickets: disorders of decreased bone mineralization. Rickets affects the cartilage of the epiphyseal growth plates in children. Osteomalacia affects the sites of bone turnover in children and adults. Both disorders are most commonly caused by vitamin D deficiency. Rickets commonly presents with skeletal deformities and growth abnormalities. Osteomalacia can present with bone pain, difficulty with ambulation, and pathologic fractures. Treatment includes vitamin D, calcium, and phosphorus supplementation.
  • Paget disease of bone: a focal disorder of bone metabolism commonly affecting the skull, spine, pelvis, and long bones of the lower extremity. The main clinical manifestations of Paget disease are bone pain and the consequences of bone deformities (e.g., fractures, osteoarthritis, nerve impingement). Management of Paget disease includes bisphosphonates, calcitonin, and surgery to manage fractures, deformities, or complications. 
  • Hyperparathyroidism: a condition associated with elevated blood levels of parathyroid hormone (PTH). Hyperparathyroidism may be due to an inherent disease within the parathyroid gland or abnormalities of calcium metabolism. Individuals classically present with “stones (nephrolithiasis), bones (↓ bone mineral density), abdominal groans (nonspecific abdominal pain), and psychiatric overtones (neuropsychiatric symptoms).” Diagnosis is based on laboratory assessment of serum PTH, calcium and phosphate levels, and urinary calcium. Management is typically surgical and the treatment of any underlying conditions.
  • Bone fractures: a partial or complete interruption in the continuity of a bone (periosteum and/or cortex) resulting from mechanical stress (typically injuries or metabolic disorders of the bone). Clinical presentation varies depending on the cause and location of the injury. Presentation generally includes deformity, pain, edema, and inflammation. Diagnosis is made clinically and confirmed with imaging. Management may be splinting or surgery.

References

  1. Saladin, K.S., Miller, L. (2004). Anatomy and physiology. (3rd Ed. Pp. 218–224).
  2. Manolagas, S.C. (2020). Normal skeletal development and regulation of bone formation and resorption. UpToDate. Retrieved Aug 4, 2021, from https://www.uptodate.com/contents/normal-skeletal-development-and-regulation-of-bone-formation-and-resorption 
  3. OpenStax College, Anatomy and Physiology. OpenStax CNX. Retrieved Aug 5, 2021, from http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@11.1

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