Tissue Adaptation Flashcards
Why does bone remodel? Of these reasons, which is considered to be the primary driver of bone remodelling (also, what evidence do we have to support this assumption)? What is the typical duration of a bone remodelling cycle in humans? How often does it occur?
Older bone tissue may be less structurally sound and can have accumulated microdamage. Remodelling allows for the removal of old bone and its replacement with newer, healthier bone.
The primary driver of bone remodelling is adaptation to mechanical stress and load-bearing. This is supported by Wolff’s Law, which states that bone adapts its structure to the mechanical loads placed upon it. For example, weight-bearing exercises lead to increased bone density in areas exposed to those mechanical forces, while disuse or immobility can result in bone loss.
On average, it can take several months to complete a full cycle. Some sources suggest that the entire skeleton may undergo remodelling over a period of about 10 years. However, the remodelling rate can differ for specific bones or regions within bones. Certain areas of the skeleton, like the jaw and finger bones, remodel more quickly, while long bones or the axial skeleton may have slower remodelling rates.
How long are osteoclasts and osteoblasts active for in a bone remodelling cycle?
Osteoclasts are responsible for bone resorption, which is the process of breaking down and removing old or damaged bone tissue. The duration of osteoclast activity can range from a few weeks to several months. During this phase, osteoclasts resorb bone by creating small cavities or tunnels in the bone matrix. The resorption process involves the release of enzymes and acids that dissolve and digest the bone tissue.
Osteoblasts are responsible for bone formation, which is the process of synthesizing and depositing new bone tissue to replace the resorbed bone. The duration of osteoblast activity is typically longer than that of osteoclasts and can last several months. Osteoblasts secrete collagen and other proteins, which become mineralized to form new bone matrix. This process eventually leads to the filling in of the cavities created by osteoclasts.
Describe briefly the three stages of mechanotransduction in bone.
Mechanotransduction is the term used to describe the conversion of a mechanical stimulus into a cellular response.
i. mechanochemical transduction of signal: mechanical detection results in mechanochemical transduction in osteocytes;
ii. cell-to-cell signalling: signal transferred through osteocyte network via canalicular network through intracellular transport of signalling molecules such as prostaglandins, RANKL and sclerostin;
iii. effector cell response: signal received by effector cell (osteoblasts and bone lining cells) leading to production of osteoregulatory factors that affect bone remodelling/ bone function.
Why does lamellar bone tissue have a higher material strength than woven bone tissue?
Lamellar bone is laid down relatively slowly and is characterised by an organised, layered matrix. The collagen and associated mineral is arranged in sheets (lamellae) such that the collagen fibrils are parallel in each lamella. Adjacent lamellae have a perpendicular collagen fibril orientation, where the preferred direction of the fibrils
changes from one lamella to the next.
Woven bone forms very rapidly under conditions of stress or trauma when bone and/or an increase in mechanical strength are required quickly. The collagen fibres are orientated randomly; the apatite crystals therefore have less preferential orientation, which may explain the lower strength of woven bone.
Under what physiological conditions does woven bone tissue form?
Woven bone forms very rapidly under conditions of stress or trauma when bone and/or an increase in mechanical strength are required quickly.
What tissue changes occur in tendons and ligaments when subjected to increased mechanical loads and during immobilisation conditions?
Similar to bone, ligaments and tendons remodel in response to mechanical demands. They become stronger and stiffer with increased load and vice versa. These changes have been shown to be largely biochemical where increases in collagen concentration and GAGs, and higher collagen turnover have been reported with increased exercise. Increases in cross-sectional area of tendons is observed with loading. This is critical in ensuring the tendon has a greater strength to cope with increased force production by the muscle fibres of the muscle organ.
The effects of exercise in all connective tissues (bone, ligaments, tendons) are much less pronounced than the effects of immobilisation. Tissue quality is rapidly lost during periods of immobility, with a slower regeneration with remobilisation, resulting in a tissue with reduced biomechanical properties ie. tissue
quality does not return to 100%. Immobilisation decreases the number of collagen fibrils and increases the proportion of large diameter fibrils to small diameter fibrils. It also decreases the GAG and water content of ligaments.
Skeletal muscles can increase their cross-sectional area by up to 60% in response to increased exercise/training. How do skeletal muscles increase their size and performance in response to exercise?
Skeletal muscles are permanent cells in the adult, meaning that they cannot undergo mitosis. Therefore any increase in muscle size in the adult is accounted for by hypertrophy ie. an increase in existing muscle fibre diameters. This is accompanied by an increase in density of blood vessels.
Compare and contrast bone remodelling and bone modelling
Bone modelling is the process of shaping and forming bones during growth and development, primarily occurring in childhood and adolescence. It involves the addition of new bone tissue and is driven by osteoblasts. Modelling takes place at specific growth plates and adapts bones to changes in size and proportions.
Bone remodelling is an ongoing process that occurs throughout a person’s life. It serves to maintain bone health, repair micro-damage, and adapt bones to mechanical stresses. It involves a dynamic balance between osteoblasts (bone builders) and osteoclasts (bone resorbers) and is influenced by factors such as hormones and mechanical loading. Remodelling happens throughout the entire skeleton, replacing old or damaged bone tissue with new bone or removing excessive bone tissue.
Define the minimum effective strain and explain why it is different in each individual. Does the MES change during one’s lifetime?
In summary, MES is not a fixed value and can vary from person to person based on genetics, age, gender, physical activity levels, nutrition, and overall health. Furthermore, MES may change during one’s lifetime, often decreasing with age due to natural changes in bone density. It can also be influenced by lifestyle factors and underlying health conditions.
In terms of mechanotransduction, why does bone require dynamic loads to initiate a bone adaptive response and not static loads?
In summary, dynamic loads are more effective for initiating a bone adaptive response because they induce fluid flow, create electrical charges, provide oscillatory strains, and stimulate a more robust cellular response. These qualities make dynamic loading essential for maintaining bone health and optimizing bone strength and density. Static loads, while not devoid of benefits, are generally less effective in stimulating bone adaptation.
Why is early weight-bearing and rehabilitation important following a period of immobilisation?
Early weight-bearing and rehabilitation are crucial after a period of immobilization due to surgery, injury, or other reasons. They help:
Rebuild muscle strength and function.
Restore joint mobility and prevent stiffness.
Maintain bone health and circulation.
Correct muscle imbalances.
Improve balance and proprioception.
Focus on functional recovery for daily activities.
Enhance psychological well-being.
Prevent complications and reduce recovery time.
In what way does the effect of exercise on bone change with age?
In summary, exercise has a positive impact on bone health throughout life. In childhood and adolescence, it helps build bone density and optimize peak bone mass. During adulthood, it maintains bone density and prevents age-related bone loss. In middle age and beyond, exercise remains essential for reducing the rate of bone loss, improving balance, and reducing the risk of fractures. However, the type and intensity of exercise may need to be adjusted based on age and individual health conditions, and consultation with a healthcare professional is advisable for personalized recommendations.
Why are osteocytes considered to have the most suitable location for detection and propagation of mechanical signals?
Osteocytes are embedded within the mineralized bone matrix in small spaces called lacunae, interconnected by even smaller channels known as canaliculi. This location places them in close proximity to the bone tissue and the fluid-filled canaliculi. Can communicate with surrounding bone matrix and fluid
Why is high impact loading (such as running) able to produce a greater bone adaptive response than low impact loading (such as walking)?
High-impact activities subject bones to greater mechanical stress. The forces experienced during running are substantially higher than those during walking due to the rapid changes in ground reaction forces with each stride. This increased stress stimulates more significant bone adaptation.
Elite athletes have a higher BMD than non-athletes. Is this evidence that exercise increases bone mineral density?
While the higher BMD observed in elite athletes is indicative of the benefits of exercise for bone health, it is not the sole piece of evidence. The relationship is influenced by multiple factors, including exercise type, intensity, nutrition, genetics, and hormonal factors.
