Bone Cells and Bone Tissue

• Bone is a specialized connective tissue made of calcified extracellular matrix
• Bone matrix is the hardest connective tissue
• There are 3 types of bone cells: Osteoblasts, Osteocytes, and Osteoclasts

Bone cells

• Osteoblasts differentiate from osteoprogenitor cells and secrete osteoid
• Osteoid allows matrix mineralization to occur
• Osteoblasts build bone through bone deposition
• Osteocytes differentiate from osteoblasts
• Osteocytes are found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes in small canaliculi
• Osteocytes maintain the matrix and detect mechanical stresses
• Osteoclasts resorb (eat) bone, which is also known as bone resorption
• Osteoclasts are large, motile cells with multiple nuclei due to fusion of bone marrow-derived monocytes

Periosteum & Endosteum

• The periosteum is a layer of dense connective tissue on the outer surface of bone
• It is attached to bone by perforating (Sharpey) fibers
• The outer fibrous layer of periosteum contains mostly bundled type I collagen
• The inner layer contains osteoblasts, bone lining cells, and mesenchymal stem cells (osteoprogenitor cells)
• The endosteum is a thin layer that lines all the internal bone surfaces
• Osteoblasts are required for bone growth
• The endosteum also contains osteoprogenitor cells, osteoblasts, and bone lining cells

Types of bone tissue

• Primary bone is woven and immature
• Woven bone is nonlamellar and characterized by random disposition of type I collagen fibers
• Woven bone is temporary and is replaced in adults by lamellar bone
• Woven bone appears first in embryonic development and fracture repair
• Secondary bone is mature bone, and lamellar
• Parallel bundles of collagen in thin layers (lamellae), with regularly spaced cells between; heavily calcified
• There are 2 types of bone arrangements: spongy and compact (cortical)

Compact bone

• Compact bone is solid, like ivory and tightly organized
• There are no spaces present
• Compact Bone is found in the shaft of long bones, outer & inner tables of flat bones of skull, and the outer covering of the vertebrae & ribs.
• Compact bone structure includes: periosteum, endosteum, bone matrix and bone cells

Bone matrix of compact bone

• Formed of lamellae of calcified osteoid tissue that contain osteocytes
• Outer "external" circumferential lamellae are under the periosteum and parallel to it
• Inner "internal" circumferential lamellae are under the endosteum and parallel to the medullary bone cavity
• Concentric bone lamellae are 4-20 cylinders of concentric bone lamellae arranged around the Haversian canal
• Interstitial lamellae are between Haversian systems; where osteocytes are irregularly arranged

Haversian system

• The Haversian system (osteon) is the structural unit of bone
• It is formed of the Haversian canal which runs parallel to the longitudinal axis of the bone
• It is formed of concentric bone lamellae of 4-20 cylinders of concentric bone lamellae
• Osteocytes are found inside their lacunae between the bone lamellae
• Volkman's Canals are transverse or oblique canals
• They connect Haversian canals with periosteal or endosteal blood vessels
• Volkmann's Canals are lined with osteogenic cells and they contain blood vessels
• Perforating fibers of Sharpy: calcified collagenous fibers fixing the periosteum to the bone

Cancellous bone

• Cancellous bone looks like a sponge with many holes or spaces, and is also known as spongy bone
• It is found in the epiphysis of long bones, the central part of flat bones of the skull, bodies of vertebrae & sternum, & in young embryonic bone
• Cancellous bone is irregularly arranged with trabeculae(loose rods) of bone lamellae (branched, anastomosed)
• Bone marrow spaces of irregular shape and size separate the bone trabeculae
• The bone marrow spaces are filled with active red bone marrow
• There is no osteon

Ossification (Osteogenesis)

• Ossification is process of bone formation (growth)
• Methods of bone ossification include intramembranous and intracartilagenous ossification
• Intramembranous ossification: occurs in mesenchymal membranes: flat bones (skull, face, and clavicle)
• Intracartilagenous ossification: occurs in hyaline cartilage model; most bones of the body form by this method

Intramembranous ossification

• Intramembranous ossification transforms a membrane of mesenchymal connective tissue converts into spongy bone
• UMCs form a primary ossification center, which convert into osteoblasts and synthesize bone matrix.

Intracartilagenous ossification

• Intracartilagenous ossification is a type of ossification, when the cartilaginous model is replaced by bone
• Starts at primary ossification centers in diaphyses of fetal long bones
• Secondary ossification centers later develop similarly within the epiphyses
• Stages of endochondral ossification include:
  • Bone collar production by osteoblasts
  • Chondrocytes hypertrophy & die
  • Blood vessels penetrate the bone collar
  • Newly formed osteoblasts move into all available spaces to produce woven bone.

After primary and secondary ossification centers, two regions of cartilage remain

• Articular cartilage: within the joints between long bones, persists through adult life
• Epiphyseal plate or growth plate: specialized epiphyseal cartilage which connects epiphysis to the diaphysis and allows longitudinal bone growth

Intracartilagenous ossification in epiphyseal plate

• Stage of resting cartilage: Chondrocytes are embedded in their matrix.
• Stage of proliferation: Chondrocytes divide and give a large number of chondrocytes
• Stage of maturation and hypertrophy: The chondrocytes grow in size.
• Stage of calcification: The chondrocytes secrete Alkaline Phosphatase; the matrix becomes impermeable causing cell death; leaving empty lacunae separated by thin bars of calcified matrix.
• Stage of Invasion: Some monocytes change into osteoclasts
• Spongy bone formation: Some osteogenic cells change to osteoblast and form trabeculae of spongy bone
• The final stage is to remodel/compress the bone: osteoclast destroy bars of bone, osteoblast arrange around blood vessels