05. Bone Ossification

The main components of connective tissue are:

• Cells (few)
• Extracellular matrix which includes:
  • Fibers (collagen, reticular, elastic)
  • Ground substances (GAGs, Glycoproteins, Proteoglycans)

The types of connective tissue include:

1. Proper
   • Dense
   • Loose
     • Regular
     • Irregular
2. Reticular
3. Mucoid
4. Specialized
   • Adipose tissue
   • Blood
   • Cartilage
   • Bone

The main function of bone tissue in the adult skeleton is to provide solid support for the body.

Bone tissue protects vital organs by encasing them within a hard structure, preventing injury and damage.

Bone tissue harbors cavities that contain bone marrow, which is essential for the production of blood cells.

Bone tissue serves as a reservoir of calcium (Ca 2+), phosphate, and other ions, which are important for various bodily functions.

Bone tissue forms a system of levers that multiply the forces generated during skeletal muscle contraction, transforming them into bodily movements.

1. Osteoblasts: Cells responsible for bone formation by synthesizing the bone matrix.

2. Osteocytes: Mature bone cells that maintain the bone matrix and communicate with other bone cells.

3. Osteoclasts: Multi-nucleated cells that break down bone tissue, playing a crucial role in bone remodeling.

Mature osteoblasts form a single layer of cuboidal cells resembling simple epithelium. They are active when cuboidal and flatten when inactive, lining the surfaces of bones.

Osteoblasts have two primary functions:

1. Synthesize and secrete the organic components of the bone matrix.
2. Control the deposition of inorganic components in the bone.

Inactive osteoblasts appear as flattened bone lining cells located in the endosteum and periosteum.

Osteoid is a layer of new, not yet calcified matrix (unmineralized matrix) that primarily consists of:

• 90% collagen type I
• GAGs (glycosaminoglycans) such as hyaluronic acid, chondroitin sulfate, and keratan sulfate
• Glycoproteins including osteocalcin, osteonectin, and osteopontin
• Membrane-enclosed matrix vesicles containing alkaline phosphatase and other enzymes

Osteoblasts release matrix vesicles that initiate the mineralization process. These vesicles contain enzymes and proteins that facilitate the deposition of minerals, leading to the formation of the osteoid layer and eventually the mineralized bone.

The stages of mineralization include:

1. Osteoblasts release matrix vesicles - Osteoblasts produce vesicles that are crucial for mineralization.

2. Released matrix vesicles and collagen fibers - The vesicles interact with collagen fibers in the bone matrix.

3. Early mineralization around vesicles - Mineralization begins to occur around the matrix vesicles.

4. Matrix becoming confluent between vesicles - The mineralization process advances, leading to a solidified matrix between the vesicles.

Osteoblasts transition to osteocytes by becoming enclosed singly within the lacunae. This process involves changes in their morphology and cellular structure.

Osteocytes are characterized by:

• Flat, almond-shaped appearance
• Less rough endoplasmic reticulum (RER)
• Smaller Golgi complexes
• More condensed nuclear chromatin

The dendritic processes of osteocytes are extensions that:

• Are surrounded by calcifying matrix
• Occupy the canaliculi (250-300nm diameter) that radiate from each lacuna, facilitating communication and nutrient exchange within the bone matrix.

Osteocytes communicate through canaliculi, allowing for:

• Interaction between osteocytes
• Communication with blood vessels
• Connection to both the external and internal bone surfaces

Osteoclasts are very large, motile cells with multiple nuclei that originate from the fusion of bone marrow-derived cells.

M-CSF and RANCL are two polypeptides produced by osteoblasts that play a crucial role in the development of osteoclasts.

Howship lacunae are enzymatically etched depressions or cavities in the matrix where osteoclasts lie, indicating areas of bone resorption.

The ruffled border consists of surface projections that increase the surface area for bone resorption, facilitating the osteoclast's function in breaking down bone tissue.

The genetic disease caused by underactivity of osteoclasts is osteopetrosis. It is characterized by dense, heavy bones, often referred to as 'marble bones'. In osteopetrosis, osteoclasts lack ruffled borders, leading to defective bone resorption.

The condition resulting from overactivity of osteoclasts is osteoporosis. It is characterized by calcium loss from bones and reduced bone mineral density (BMD).

In normal bone, the structure shows a dense network of bone tissue, while in bone affected by osteoporosis, the structure is much sparser and more porous, with larger spaces between the bone tissue.

The three main regions of a long bone are:

1. Epiphyses: The ends of the long bone, which are typically wider than the shaft and contain spongy bone and red marrow.

2. Metaphyses: The regions between the epiphyses and the diaphysis, where growth occurs in children and adolescents.

3. Diaphyses: The shaft of the long bone, which is primarily composed of compact bone and contains the medullary cavity filled with yellow marrow.

• Compact or cortical bone: 80%
• Spongy or cancellous bone: 20%

• Woven bone (Immature): Found in embryos and during fracture repair.
• Lamellar bone (Mature): Found in adults.

• Compact bone: Found in the diaphyses (shaft) of long bones.
• Spongy bone: Found in the epiphyses (ends) of long bones.

Woven bone, also known as primary bone, is characterized by:

• Nonlamellar structure
• Random disposition of type I collagen fibers
• Lower mineral content compared to lamellar bone
• Higher proportion of osteocytes

Lamellar bone is characterized by multiple layers or lamellae of calcified matrix. The Type I collagen fibers are aligned, and the lamellae can be organized either parallel to each other or concentrically around a central canal (osteon).

Spongy bone, also known as cancellous bone, is made of trabeculae that are filled with bone marrow. It is located inside bones, providing support and housing the bone marrow.

Lamellar bone consists of concentric layers known as lamellae, which contain lacunae housing osteocytes, canaliculi for nutrient exchange, and osteoblasts and osteoclasts involved in bone remodeling.

The structural organization of compact bone is called the Haversian system or osteon.

The components of the Haversian system in compact bone include:

1. Haversian system/osteon
2. External circumferential lamellae
3. Inner circumferential lamellae
4. Interstitial lamellae

The main components of an osteon include:

1. Haversian canal - contains blood vessels.
2. Concentric lamellae - layers of bone matrix surrounding the Haversian canal.
3. Lacunae - small spaces that house osteocytes.
4. Canaliculi - tiny channels that connect lacunae and allow communication between osteocytes.
5. Volkmann canal - channels that connect Haversian canals and contain blood vessels.
6. Interstitial lamellae - remnants of older osteons found between current osteons.
7. Cementing line - boundary between adjacent lamellae.

The Haversian canal serves as a central channel that contains blood vessels and nerves, providing essential nutrients and oxygen to the bone tissue and facilitating communication within the bone structure.

Canaliculi are small channels that connect lacunae, allowing osteocytes to communicate and exchange nutrients and waste products. This network is crucial for maintaining the health and function of bone cells within the mineralized matrix.

Interstitial lamellae are remnants of older osteons found between current osteons, while concentric lamellae are the layers of bone matrix that surround the Haversian canal in a functional osteon. Interstitial lamellae do not form a complete ring like concentric lamellae.

Interstitial lamellae are remnants of old osteons that fill the spaces between newer osteons in bone tissue. They represent the areas where older osteons have been partially resorbed and replaced by new bone formation.

The stages of osteon formation include:

1. First-generation osteons: These are the initial osteons formed in the bone.
2. Second-generation osteons: These osteons are formed as the bone continues to remodel and adapt.
3. Third-generation osteons: These are the most recent osteons, representing the latest phase of bone remodeling and are typically more densely packed.

Each generation of osteons is characterized by concentric circles of lamellae, with older generations being less dense than newer ones.

The periosteum consists of two layers:

1. Fibrous layer: This outer layer provides structural support and attachment for tendons and ligaments.
2. Cellular layer: This inner layer contains osteogenic cells that are involved in bone growth and repair.

The endosteum is a thin layer that lines the inner surface of the bone. It contains various types of cells, including osteoblasts and osteoclasts, which are essential for bone remodeling and maintenance.

The main structural components of compact bone include:

• Osteons (Haversian systems): The basic structural unit of compact bone.
• Lamellae: Concentric rings of bone matrix.
• Lacunae: Small spaces that house osteocytes.
• Canaliculi: Tiny channels that connect lacunae and allow for nutrient exchange.
• Central canal: Contains blood vessels and nerves.
• Perforating canals: Connect central canals and allow for blood flow between them.

Trabeculae in spongy bone are:

• Structural components: They are thin, rod-like structures that form a network within spongy bone.
• Function: They provide structural support and help distribute stress across the bone.
• Bone marrow: The spaces between trabeculae are filled with red bone marrow, which is involved in blood cell production.

The development of an osteon involves several key stages:

1. Tunneling: Osteoclasts tunnel into old bone, creating a resorption cavity.
2. Formation of Resorption Cavity: The cavity is formed as osteoclasts continue to resorb bone.
3. Closing Cone: Osteoblasts begin to fill in the cavity with new bone material, leading to the formation of the closing cone.
4. Osteon Formation: The final structure, the osteon, is established with a central canal surrounded by concentric lamellae of bone matrix.

The first stage of bone fracture repair is the formation of a fracture hematoma. During this stage, a large, red hematoma forms at the site of the fracture, which is crucial for initiating the healing process.

In the bone fracture repair process:

1. Osteoclasts clear the damaged area by resorbing dead bone tissue.
2. Osteoblasts begin rebuilding the bone by laying down new bone matrix.

Osteons are significant in the bone fracture repair process as they form new structures to bridge the fracture site and replace the temporary bone matrix (woven bone) with more organized, solid bone tissue.

The progression of callus formation during bone fracture repair includes:

1. Fibrocartilaginous (soft) callus formation, where a soft bridge forms at the fracture site.
2. Hard (bony) callus formation, where the fibrocartilaginous callus is replaced by a more solid, bone-like structure.
3. Remodeling, where the callus is remodeled, and the bone returns to its original shape.

Intramembranous ossification involves osteoblasts differentiating directly from mesenchymal cells to produce osteoid, primarily forming flat bones. In contrast, endochondral ossification occurs when osteoblasts invade a pre-existing hyaline cartilage matrix, resorb it, and deposit osteoid to form new bone, typically in long bones.

The first step is the development of the ossification center, where osteoblasts secrete organic extracellular matrix within a network of blood capillaries and collagen fibers.

During the calcification stage, calcium and other mineral salts are deposited in the extracellular matrix, and osteocytes become visible within lacunae with canaliculi extending from them.

Trabeculae are formed through mesenchymal condensation, where blood vessels and osteoblasts come together to create spongy bone trabeculae.

The final step is the development of the periosteum, which includes the formation of compact bone tissue and spongy bone tissue.

Mesenchymal cells differentiate into chondrocytes, which are essential for forming the cartilage model during endochondral ossification.

The surrounding matrix begins to calcify, leading to the deterioration of the cartilage matrix as the process progresses.

Chondrocytes become hypertrophic and eventually die, which is a critical step in the transition from cartilage to bone.

The primary ossification center is where osteoblasts replace the dying cartilage, marking the transition from cartilage to bone in the diaphysis.

Osteoclasts break down some of the new bone to form the medullary cavity.

Cartilage is replaced by bone in the epiphyses at the secondary ossification centers.

After endochondral ossification, the articular cartilage and epiphyseal plates remain.

The epiphyseal plates are crucial for bone growth during development, as they allow for the lengthening of bones before they ossify into epiphyseal lines.

When growth stops, the epiphyseal growth plate closes, meaning that the cartilage is completely replaced by bone.

The distal femur typically fuses around age 16–18.

The closed epiphyseal line indicates the cessation of growth, as the growth plate has fully transitioned from cartilage to bone.

The proximal humerus usually fuses around age 20–22.

The iliac crest may fuse around ages 21–25.