Bone

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Anatomy > Skeletal system > Bone

Overview of Bone

Bone is a highly specialized form of connective tissue that forms the structural framework of the vertebrate body. It provides mechanical support, facilitates movement through its articulation with muscles, protects vital organs, acts as a reservoir for calcium and phosphorus, and houses the bone marrow responsible for hematopoiesis.

Bone exists in two primary forms:

• Compact bone – Dense and rigid outer layer forming the cortex of bones.
• Cancellous bone (or spongy bone) – Porous inner matrix primarily found at the epiphyses and inside flat bones.

The outer surface of bone is covered by the periosteum, a fibrous membrane that plays a key role in bone nourishment and repair.

Structure and Composition of Bone

Gross anatomy of bone

Macroscopic view of bone structure

Bones exhibit a hard, pinkish-white exterior and a deep red vascularized interior in the living state. Long bones like the femur or humerus contain a medullary cavity filled with yellow marrow, while short and flat bones contain red marrow housed in trabecular spaces.

Bones vary in density:

• Compact bone forms the outer shell
• Cancellous bone forms a meshwork internally, lighter but strong

Periosteum

The periosteum is a double-layered membrane:

• Outer fibrous layer – Dense irregular connective tissue
• Inner cellular layer – Contains osteoprogenitor cells and osteoblasts

It is richly vascularized and innervated, and critical in bone growth and repair.

Cells in bone marrow

Bone marrow

Bone marrow exists in two types:

• Red marrow – Hematopoietically active; found in vertebrae, sternum, ribs, and proximal ends of long bones.
• Yellow marrow – Composed mainly of adipocytes; found in the medullary cavities of long bones.

Microscopic Anatomy of Bone

Cross-section of compact bone (Sharpey)

Haversian system

The fundamental unit of compact bone is the osteon or Haversian system. It consists of:

• A central Haversian canal containing blood vessels, nerves, and lymphatics
• Concentric rings of lamellae
• Lacunae containing osteocytes, connected via tiny canals called canaliculi

Other lamellae include:

• Interstitial lamellae – Between osteons
• Circumferential lamellae – Encircle the bone's entire perimeter

Endochondral ossification – fetal femur

Bone cells

There are four major types:

• Osteoblasts – Bone-forming cells
• Osteocytes – Mature bone cells embedded in matrix
• Osteoclasts – Bone-resorbing, multinucleated giant cells
• Bone lining cells – Quiescent cells that cover inactive bone surfaces

Bone matrix

Bone matrix is composed of:

• Organic matrix – Mainly type I collagen (~30%)
• Inorganic minerals – Mostly hydroxyapatite crystals (~70%)

These elements impart both tensile strength and rigidity to bone.

Strength Comparison of Bone and Other Materials

Material	Tensile Strength (psi)	Compression Strength (psi)	Shear Strength (psi)
Medium Steel	65,000	60,000	40,000
Granite	1,500	15,000	2,000
White Oak	12,500	7,000	4,000
Compact Bone	13,000–18,000	18,000–24,000	7,000–11,800

Development and Ossification

Ossification types

Bone is formed by two processes:

• Intramembranous ossification – Direct from mesenchyme (e.g. skull, clavicle)
• Endochondral ossification – From a cartilage model (e.g. long bones)

Intramembranous ossification – fetal parietal bone

Bone growth and epiphyses

Long bones grow at the epiphyseal plate between the diaphysis and epiphysis. Growth ceases when the plate ossifies.

Three types of epiphyses:

• Pressure epiphyses – Bear weight (e.g. femoral head)
• Traction epiphyses – Muscle attachment points (e.g. trochanters)
• Atavistic epiphyses – Evolutionary remnants (e.g. vertebral arches)

Bone remodeling

Bone constantly undergoes remodeling through the coordinated action of:

• Osteoclasts – Resorb bone
• Osteoblasts – Lay down new bone

Remodeling helps maintain calcium homeostasis and repair microdamage.

Additional images

• 
  Cells in bone marrow
• 
  SEM of deproteinated trabecular bone
• 
  Structure detail of an animal bone

See also

• Skeletal system
• Compact bone
• Spongy bone
• Osteogenesis
• Bone remodeling
• Periosteum

External links

Bones of the human skeleton

Gray's Anatomy

• Gray's Anatomy Contents
• Gray's Anatomy Subject Index
• About Classic Gray's Anatomy
• Glossary of anatomy terms

Anatomy atlases (external)

[1] - Anatomy Atlases

Adapted from the Classic Grays Anatomy of the Human Body 1918 edition (public domain)