Bone remodeling

Cellular participants: builders and destroyers

Bone tissue balance is maintained by the coordinated work of two main cell types. The process is ongoing, forming the so-called basic multicellular unit.

• Osteoclasts (osteoclasts) are responsible for the resorption (destruction) of old bone. They form lacunae in the tissue. This process is single-stage: osteoclasts remove the mineral component and collagen simultaneously.
• Osteoblasts (osteoblasts) take over from osteoclasts to fill the resulting voids. Their work takes place in two stages. First, they secrete osteoid (osteoid), an unmineralized matrix. Then, mineralization of this matrix occurs.

During the formation of new bone, some osteoblasts become trapped within their own matrix. These cells transform into osteocytes (osteocytes).

Conditions for proper mineralization

For soft osteoid to turn into strong bone tissue, strict biochemical conditions must be met in the body. Key factors for normal mineralization include:

1. Presence of the active form of vitamin D ($1,25(OH)_2D_3$).
2. Adequate intake of calcium (Ca).
3. Normal levels of phosphorus (P) and the enzyme alkaline phosphatase (alkaline phosphatase).
4. Normal acidity level (pH).

If these conditions are violated, serious mineralization defects develop. In childhood, a lack of building blocks leads to rickets (rickets), while in adults a similar defect causes osteomalacia (osteomalacia).

Remodeling pathologies

Any disruption in the balance between bone destruction and formation leads to diseases:

• Osteoporosis (osteoporosis): occurs when resorption is too active or osteoblasts cannot cope with completely filling the lacunae. The result is a catastrophic loss of bone mass.
• Paget's disease (Paget's disease): a state of abnormally high bone turnover (high turnover state) in which bone renews too quickly and chaotically.
• Osteopetrosis (osteopetrosis): a genetic defect in which osteoclast function is impaired. Normal resorption ceases, leading to abnormal modeling and pronounced tissue sclerosis. Paradoxically, such dense-looking bones become extremely fragile and prone to frequent fractures.

Mechanism of bone growth and development

During embryonic development, the skeleton is laid down as a cartilaginous framework. Further maturation proceeds via two pathways: endochondral (endochondral) and desmal/intramembranous (desmal) ossification.

Growth of long tubular bones is ensured by primary (in the center) and secondary (in the epiphyses) ossification centers. The mechanism of endochondral ossification:

1. Cartilage cells hypertrophy and accumulate glycogen.
2. Cells degenerate, and a provisional zone of calcification (provisional zone of calcification) is formed.
3. Deep layers of perichondral cells turn into osteoblasts, creating primary spongiosa (primary spongiosa).
4. Vessels and new osteoblasts penetrate the matrix, secrete osteoid, and mineralize it.

Bone lengthening occurs in the metaphyses (metaphyses), directly adjacent to the physis (physis)—the cartilaginous growth plate. This zone remains active until skeletal maturity is reached. If the process is temporarily stopped due to severe illness or starvation, the zone of provisional calcification remains as a scar. On an X-ray, this looks like thin white lines—Harris growth arrest lines (Harris growth arrest line).

Macrostructure and types of bone tissue

An anatomically long tubular bone is divided into three parts: epiphysis (epiphysis), the articular end; metaphysis (metaphysis), the transition zone; and diaphysis (diaphysis), the central shaft. Inside the bone lies the marrow space (marrow space) containing hematopoietic bone marrow. It is lined on the inside by the endosteum (endosteum), and on the outside the bone is protected by the periosteum (periosteum). Articular surfaces are covered by highly organized articular cartilage, which appears on MRI as a bright hyperintense rim.

All bone tissue is divided into two types:

• Cortical (compact) bone (cortical bone): forms the outer framework and accounts for 80% of skeletal mass.
• Trabecular (spongy) bone (trabecular bone): a lacy structure at the ends of long bones and in the axial skeleton. It accounts for only 20% of the mass, but provides a colossal surface area for cellular activity. Its trabeculae are located strictly along the load vectors to protect the skeleton from tension by muscle pull and body weight.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Biochemical mechanisms of single-stage resorption by osteoclasts
• Histological structure of the epiphyseal growth plate (physis)
• Differences between endochondral and desmal ossification pathways
• Hormonal regulation of calcium, phosphorus, and vitamin D levels
• Mechanism of formation of Harris growth arrest lines at the cellular level

Other topics in the "Musculoskeletal System" section

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