Bone Structure & Growth Quiz

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Questions and Answers

Which function of bones is primarily involved in storing calcium and phosphate?

Bone marrow storage

Movement

Ca2+ and PO43- storage (correct)

Support and protection

What type of collagen is primarily found in the bone matrix?

Type 2 collagen

Type 1 collagen (correct)

Type 4 collagen

Type 3 collagen

What is the primary role of proteoglycans in bone structure?

Providing tensile strength

Facilitating movement

Providing compressive strength (correct)

Storing minerals

Which hormones play key roles in calcium and phosphate regulation?

PTH and vitamin D Signup and view all the answers

What happens during defective bone remodelling?

Development of osteoporosis Signup and view all the answers

What is the primary role of Vitamin D in relation to calcium and phosphate regulation?

To promote the synthesis of calcium channels and phosphate absorption Signup and view all the answers

What is osteoporosis characterized by?

Low bone mass and microarchitectural deterioration of bone Signup and view all the answers

What condition occurs in children due to Vitamin D deficiency?

Rickets Signup and view all the answers

Which of the following best describes osteomalacia in adults?

Bone weakness due to unmineralised osteoid Signup and view all the answers

What primarily triggers fractures in post-menopausal women over the age of 50?

Low bone mass due to osteoporosis Signup and view all the answers

What is the effect of vitamin D on kidney tubules?

Promotes calcium and phosphate reabsorption Signup and view all the answers

What happens to bone growth after the fusing of the epiphyseal plate?

Bone growth stops Signup and view all the answers

What is the primary consequence of Vitamin D deficiency on bone mineralization?

Alteration of calcium availability and decreased mineralization Signup and view all the answers

What effect does PTH have on phosphate reabsorption in the kidneys?

Inhibits phosphate reabsorption Signup and view all the answers

Which enzyme is stimulated by PTH for the synthesis of the active form of vitamin D?

1α-hydroxylase Signup and view all the answers

Where does the key step of 1α-hydroxylation for active vitamin D production occur?

In the kidney Signup and view all the answers

What is the chemical formula for bone mineral hydroxyapatite?

Ca10(PO4)6(OH)2 Signup and view all the answers

What is the active form of vitamin D also called?

Calcitriol Signup and view all the answers

In which part of the intestines does Ca2+ absorption primarily occur?

Duodenum Signup and view all the answers

What role does active vitamin D play in the gut?

Promotes Ca2+ absorption Signup and view all the answers

What is the initial process for vitamin D synthesis in the body?

UV-stimulated synthesis in the skin Signup and view all the answers

What is the primary role of Parathyroid Hormone (PTH) in the body?

Regulate free Ca2+ levels Signup and view all the answers

How does acidosis affect the levels of free Ca2+ in the body?

Increases free Ca2+ levels Signup and view all the answers

Which of the following organs is responsible for the absorption of dietary calcium?

Gastrointestinal Tract Signup and view all the answers

What is the effect of low plasma Ca2+ levels on the release of PTH?

It stimulates PTH release Signup and view all the answers

Which ion is primarily involved in the regulation of calcium levels in the body?

Ca2+ Signup and view all the answers

What type of bone is characterized by high porosity and a spongy structure?

Trabecular bone Signup and view all the answers

What happens to urinary excretion of calcium when dietary intake is at 1000 mg?

It stays constant at 200 mg Signup and view all the answers

Which cells in the parathyroid glands are responsible for the release of PTH?

Chief cells Signup and view all the answers

Which mechanism primarily controls the elongation of long bones during childhood?

Growth hormone and IGF-1 Signup and view all the answers

What is the primary function of osteoclasts in bone tissue?

Promote bone reabsorption Signup and view all the answers

What is the consequence of alkalosis on free Ca2+ levels?

Free Ca2+ levels decrease Signup and view all the answers

What role does vitamin D3 (1,25 dihydroxycholecalciferol) play in calcium regulation?

Enhances calcium absorption Signup and view all the answers

What comprises most of the human skeleton, approximately 80%?

Cortical bone Signup and view all the answers

What is the unit of bone growth that consists of concentric circles around a blood vessel?

Osteon Signup and view all the answers

What is the main effect of PTH on osteoclasts?

Stimulates bone reabsorption Signup and view all the answers

Which phase of bone remodelling involves the preparation of the resorbed bone surface for new deposition?

Reversal phase Signup and view all the answers

Which hormone is indirectly stimulated by parathyroid hormone (PTH) to promote bone reabsorption?

Vitamin D Signup and view all the answers

Which component is NOT a direct function of osteocytes?

Promoting bone reabsorption Signup and view all the answers

What is the primary mineral that is deposited during the ossification of osteoid?

Hydroxyapatite Signup and view all the answers

What process describes the change of cartilage to mineralized bone during fetal development?

Ossification Signup and view all the answers

Which cell type is responsible for laying down the bone matrix?

Osteoblasts Signup and view all the answers

After bone growth ceases, what is the ongoing process that maintains bone health?

Bone remodelling Signup and view all the answers

What is the purpose of calcium movement across membranes in the body?

Neuromuscular functions Signup and view all the answers

In what form is calcium predominantly found in the blood?

Ionized calcium Signup and view all the answers

What role do proteoglycans play in the structure of bone?

Providing compressive strength Signup and view all the answers

What is the primary cause of osteoporosis?

Defective bone remodelling Signup and view all the answers

Which statement accurately describes the structure of bones?

Bone is composed of an organic matrix largely made up of type 1 collagen. Signup and view all the answers

Which biological function of bones primarily aids in locomotion?

Movement Signup and view all the answers

What type of bone is characterized by its spongy structure and high porosity?

Trabecular bone Signup and view all the answers

What effect does PTH have on calcium and phosphate levels in the kidneys?

Inhibits calcium reabsorption and promotes phosphate excretion Signup and view all the answers

Which process is not involved in the activation of vitamin D?

Cytokine mediation for vitamin D activation Signup and view all the answers

Where does the crucial 1α-hydroxylation of vitamin D occur?

In the kidney Signup and view all the answers

What is the primary role of active vitamin D in the intestine?

Increases calcium absorption Signup and view all the answers

Which statement accurately describes the synthesis of the active form of vitamin D?

It requires two hydroxylation steps and is not rate-limited by the liver step. Signup and view all the answers

How does PTH influence the plasma concentration of calcium and phosphate?

Increases both through enhanced bone reabsorption Signup and view all the answers

What is the role of calcium-binding proteins in the intestine?

Transport calcium into circulatory system Signup and view all the answers

Which of the following statements about hydroxyapatite is true?

It contains calcium and phosphate in its structure. Signup and view all the answers

What effect does an increase in hydrogen ion concentration ([H+]) have on the levels of free calcium ions (Ca2+) in the body?

It increases the levels of free calcium ions. Signup and view all the answers

Which of the following describes the primary function of parathyroid hormone (PTH) in regulating plasma calcium levels?

Stimulating bone reabsorption. Signup and view all the answers

How does alkalosis affect free calcium ion levels in the body?

It results in a decrease in free calcium ions. Signup and view all the answers

What is the source of parathyroid hormone (PTH) in the body?

Chief cells of the parathyroid glands. Signup and view all the answers

What physiological condition is indicated by low plasma calcium levels in relation to PTH?

Hypocalcemia. Signup and view all the answers

What role does calcium play in relation to parathyroid hormone's activity?

It inhibits PTH release. Signup and view all the answers

What is the daily dietary intake of calcium required for maintaining health?

1000 mg. Signup and view all the answers

In bone metabolism, what is the primary action of osteoclasts as stimulated by PTH?

Stimulating bone reabsorption. Signup and view all the answers

What is a direct consequence of bone reabsorption stimulated by PTH?

Release of calcium and phosphate into the bloodstream. Signup and view all the answers

Which form of vitamin D is involved in enhancing calcium absorption in the body?

Calcitriol. Signup and view all the answers

What characterizes compact bone compared to trabecular bone?

Dense and stiff structure with low porosity Signup and view all the answers

Which of the following best describes the role of osteocytes?

Transfer minerals from the inner regions of bone to growth surfaces Signup and view all the answers

Which phase of bone remodelling is primarily focused on the release of growth factors?

Formation phase Signup and view all the answers

What is the primary function of osteoclasts in bone tissue?

Promoting bone resorption and removing mineralization Signup and view all the answers

What initiates the elongation of long bones during childhood and adolescence?

Cartilage proliferation at the growth plate Signup and view all the answers

Which protein framework is laid down by osteoblasts during bone formation?

Osteoid Signup and view all the answers

What happens to bone mass approximately every 10 years in adults?

It undergoes complete remodelling Signup and view all the answers

Which of these factors directly stimulates osteoclasts to promote bone resorption?

Parathyroid hormone acting indirectly through osteoblasts Signup and view all the answers

During which phase of bone remodelling are osteoclasts attracted to the bone surfaces?

Resorption phase Signup and view all the answers

What cellular mechanism underlies the sensing of mechanical load by bone?

Cytoplasmic projections of osteocytes Signup and view all the answers

What is the primary purpose of the epiphyseal growth plate during bone development?

Enables lengthening of long bones Signup and view all the answers

What distinguishes trabecular bone from compact bone?

Spongy, lightweight structure with high porosity Signup and view all the answers

How does bone remodelling adapt to mechanical loading?

Through a balance of osteoblast and osteoclast activity Signup and view all the answers

What condition is characterized by low bone mass and increased susceptibility to fractures, particularly in post-menopausal women?

Osteoporosis Signup and view all the answers

Which vitamin is essential for promoting calcium and phosphate absorption in the gut?

Vitamin D Signup and view all the answers

What is the primary consequence of vitamin D deficiency in adults?

Osteomalacia Signup and view all the answers

Which mineral is actively pumped out at the basolateral side of epithelial cells after absorption?

Calcium Signup and view all the answers

Which physiological change is associated with the increased risk of osteoporosis in aging populations?

Decreased bone remodeling Signup and view all the answers

What mechanism primarily allows continuous regulation of serum calcium levels?

Parathyroid hormone (PTH) and calcitonin Signup and view all the answers

What defines the clinical condition of rickets in children resulting from vitamin D deficiency?

Unmineralised osteoid leading to bowing of long bones Signup and view all the answers

Which process in the kidneys is promoted by vitamin D regarding phosphate?

Enhanced reabsorption of phosphate Signup and view all the answers

What is the primary structural component of the bone matrix that provides tensile strength?

Type 1 collagen Signup and view all the answers

Which of the following roles best describes osteocytes within the bone tissue?

Regulating mineral content Signup and view all the answers

Which term refers to the resistance of a material or structure to deformation under compressive loads?

Compressive strength Signup and view all the answers

What condition arises from a deficiency in bone remodeling, leading to decreased bone density and increased fracture risk?

Osteoporosis Signup and view all the answers

Which substance acts as a store for calcium and phosphate in the body, playing a crucial role in maintaining mineral balance?

Bone tissue Signup and view all the answers

What is the primary action of PTH on the kidneys in relation to calcium?

Promotes calcium reabsorption in the thick ascending limb Signup and view all the answers

Which enzyme is crucial for the conversion of 25-hydroxyvitamin D to its active form in the kidney?

1α-hydroxylase Signup and view all the answers

How does active vitamin D primarily facilitate calcium absorption in the intestine?

Through calcium-binding proteins and channels Signup and view all the answers

Which of the following statements about the production of active vitamin D is NOT true?

25-hydroxylation in the liver is a rate-limiting step. Signup and view all the answers

What minerals are increased in plasma as a result of bone resorption stimulated by PTH?

Both calcium and phosphate Signup and view all the answers

What is the primary regulatory site for the synthesis of the active form of vitamin D?

Kidney Signup and view all the answers

Which step is NOT involved in the formation of active vitamin D?

Phosphate excretion in the gut Signup and view all the answers

Which action does vitamin D have in terms of bone health?

Stimulates mineralization of the bone Signup and view all the answers

What significant action does vitamin D promote in the kidneys?

Increased calcium and phosphate reabsorption Signup and view all the answers

What happens to the levels of free Ca2+ in the presence of increased concentrations of hydrogen ions (H+)?

Free Ca2+ levels increase Signup and view all the answers

Which age group is most at risk for osteoporosis-related fractures?

Women over 50 years Signup and view all the answers

What distinguishes osteomalacia from rickets?

Osteomalacia occurs in adults, whereas rickets occurs in children Signup and view all the answers

Which physiological condition promotes the binding of Ca2+ to proteins, resulting in decreased free Ca2+ levels?

Alkalosis Signup and view all the answers

What triggers the release of Parathyroid Hormone (PTH) from the parathyroid glands?

Low plasma calcium levels Signup and view all the answers

How does vitamin D deficiency affect the structure of the bone?

It causes abnormal amounts of unmineralised osteoid Signup and view all the answers

How does PTH contribute to calcium regulation in the body?

By stimulating osteoclasts to increase bone reabsorption Signup and view all the answers

What process continues in bones throughout life after growth stops?

Bone turnover mediated by specific cells Signup and view all the answers

Which of the following is a direct consequence of aging on bone health?

Reduction in calcium absorption efficiency Signup and view all the answers

What is the primary effect of Parathyroid Hormone (PTH) on phosphate levels in the kidneys?

Decreases reabsorption of phosphate Signup and view all the answers

What is the role of calbindin in calcium metabolism?

Facilitates calcium absorption across the gastrointestinal tract Signup and view all the answers

What role does 1,25 dihydroxycholecalciferol (vitamin D3) play in calcium regulation?

Enhances calcium absorption in the intestines Signup and view all the answers

What is the effect of low plasma calcium on the activity of osteoclasts?

Increases osteoclast activity to release calcium from bone Signup and view all the answers

What is a potential consequence of excessive activation of osteoclasts?

Bone fragility and higher fracture risk Signup and view all the answers

What substance is primarily involved in the detection of calcium levels by the parathyroid glands?

G-protein coupled receptors Signup and view all the answers

During acidosis, what is the effect on the binding of Ca2+ to proteins?

Decreases the binding of Ca2+ to proteins Signup and view all the answers

What is the daily dietary intake requirement for calcium?

1000 mg Signup and view all the answers

What occurs at the epiphyseal growth plate until approximately age 18?

Cartilage proliferation and elongation occur. Signup and view all the answers

What is the primary composition of trabecular bone?

High porosity structure. Signup and view all the answers

What role do osteocytes primarily play in bone physiology?

They sense mechanical load and communicate with osteoblasts. Signup and view all the answers

During which phase of bone remodeling do osteoclasts create resorption pits?

Resorption phase. Signup and view all the answers

What triggers osteoclast differentiation in bone remodeling?

Osteocyte signalling and PTH signaling. Signup and view all the answers

Which type of bone accounts for approximately 80% of human bone mass?

Compact bone. Signup and view all the answers

What initiates the process of endochondral ossification during fetal life?

Formation of ossification centers. Signup and view all the answers

What cellular process occurs after the growth plate fuses?

Vascularization and ossification of the mature bone. Signup and view all the answers

How does PTH influence the activity of osteoclasts?

It stimulates osteoblasts which indirectly activates osteoclasts. Signup and view all the answers

What limits osteocyte function in bone tissue?

Their low metabolic activity. Signup and view all the answers

What physiological role does calcium ion movement support?

Facilitation of neurotransmitter release at synapses. Signup and view all the answers

Which stage of bone remodeling has the longest duration?

Formation phase. Signup and view all the answers

What triggers the activation of circulating monocytes during bone remodeling?

Increased mechanical strain detected by osteocytes. Signup and view all the answers

Which structure is formed as a result of resorption of bone by osteoclasts?

Resorption pits. Signup and view all the answers

Study Notes

Bone Structure & Function

Bones provide support, protection, movement, calcium and phosphate stores, and red blood marrow production.
Bone is a living tissue composed of an organic matrix made of Type 1 collagen fibres (~90-95%) and proteoglycans (~5%).
Collagen provides tensile strength while proteoglycans provide compressive strength.
Compact bone (cortical bone) is dense and stiff with a low porosity (~5-25%).
Trabecular bone (cancellous/spongy bone) is spongy, light, and has a high porosity (up to 70%).

Bone Growth

During fetal development, bones are modelled in cartilage and then ossified.
During childhood/adolescence, cartilage proliferates at the growth plate, elongating long bones.
Growth is controlled by Growth Hormone and Insulin-like Growth Factor 1 (IGF-1).
Once the epiphyseal growth plate fuses, cartilage proliferation and elongation stop.
The growth plate is located between the epiphysis and the metaphysis.

Bone Remodelling

Bone is constantly being remodelled, with the entire adult skeleton undergoing turnover every 10 years.
Bone remodelling is a dynamic process involving osteoblasts, osteoclasts, and osteocytes.
Osteoclasts resorb mineralized bone, releasing calcium and phosphate into the blood.
Osteoblasts deposit new bone matrix.
Osteocytes sense mechanical loading and relay information to osteoblasts.
Bone remodelling is essential for bone adaptation to mechanical loading, fracture healing, and preventing bone fatigue.

Calcium Homeostasis

Calcium movement across membranes is crucial for many physiological processes, including neurotransmitter release, muscle contraction, and hormone secretion.
Calcium exists in blood in three forms: free (ionized), bound to protein, and bound to small anions.
Ionized calcium is the most physiologically active form.
Parathyroid Hormone (PTH), secreted by parathyroid glands, is the primary regulator of free calcium levels.
PTH release is triggered by low plasma calcium concentrations.
PTH acts to increase calcium levels by stimulating bone resorption, calcium reabsorption in the kidneys, and activating vitamin D production.

Vitamin D

Vitamin D is a steroid-like molecule, produced by the skin in response to sunlight and ingested in the diet.
The active form, 1,25 dihydroxycholecalciferol, is synthesized in the kidneys from inactive precursors.
Vitamin D promotes calcium absorption from the intestines and calcium and phosphate reabsorption in the kidneys.
Vitamin D deficiencies can lead to reduced bone mineralisation, resulting in rickets in children and osteomalacia in adults.

Osteoporosis

Osteoporosis is characterized by low bone mass and microarchitectural deterioration of bone tissue.
It leads to increased bone fragility and a higher risk of fractures.
Osteoporosis is more common in women, particularly after menopause, than in men.
Post-menopausal women are more likely to suffer a hip fracture than develop breast cancer.
The ageing population is facing an increase in the prevalence of osteoporosis.

Bone Structure and Growth

Bones provide support, protection, movement, a calcium and phosphate store, and house bone marrow.
Bone is a mineralised organic matrix, with collagen providing tensile strength and proteoglycans providing compressive strength.
Compact bone (cortical bone) is dense and stiff with low porosity, making up most of human bone.
Trabecular bone (cancellous or spongy bone) is spongy and light with high porosity.
In long bones, cortical bone forms the shaft (diaphysis), while trabecular bone is found at the ends (epiphysis and metaphysis).
The epiphyseal growth plate is located between the epiphysis and metaphysis.
Long bone growth occurs at the growth plate until about 18 years of age, when it fuses with the metaphysis.
Bone growth during fetal life involves cartilage modelling and mineralisation (ossification).
During childhood and adolescence, cartilage proliferates at the growth plate, elongating long bones, controlled by growth hormone and insulin-like growth hormone (IGF-1).
Once the growth plate fuses, cartilage proliferation and elongation stop, and the bone undergoes vascularisation and ossification.
The basic unit of bone growth is the osteon, concentric circles of collagen deposited around a blood vessel.
Blood vessels run through Haversian canals within the osteon.
The collagen in the osteon is then mineralised with hydroxyapatite (Ca10(PO4)6(OH)2).

Bone Cells and Remodelling

Bone contains three main cell types: osteoblasts, osteoclasts, and osteocytes.
Osteoblasts promote bone formation, laying down osteoid and initiating mineralisation.
Osteoclasts promote bone reabsorption, removing mineralisation and releasing calcium and phosphate.
Osteocytes transfer mineral from bone's inner regions to the growth surfaces.
Osteoblasts are derived from mesenchymal stem cells and lay down osteoid (type 1 collagen matrix) to facilitate ossification.
Osteocytes are derived from osteoblasts, have cytoplasmic projections that sense mechanical load, and relay information to osteoblasts.
Osteoclasts are derived from the macrophage lineage, resorb mineralized bone, and create resorption pits.
Osteoclasts solubilise mineral at low pH, phagocytose the organic matrix, and are indirectly stimulated by PTH to promote bone reabsorption.
Bone remodelling is a continuous process in adults, with the entire skeleton being remodelled every 10 years.
Bone remodelling involves a balance between osteoblast and osteoclast activity, which enables adaptation to mechanical loading, fracture healing, and prevents "bone fatigue".
Osteocyte detected mechanical strain is relayed to osteoblasts, triggering NFκB signalling in circulating monocytes.
NFκB signalling activates monocytes into osteoclasts, which move to the region needing reabsorption.
Growth factors stimulate osteoblast formation, leading to new osteoid deposition and mineralisation.
Bone remodelling consists of four phases: resorption, reversal, formation, and quiescence (resting).

Calcium and Phosphate Regulation

Calcium movement across membranes is crucial for various physiological mechanisms.
Calcium exists in three forms in body fluids: ionised (free), bound to proteins, and bound to small anions.
The ionised form (Ca2+) is responsible for most physiological functions.
Acid-base status influences bound calcium levels; acidosis increases free calcium, while alkalosis decreases free calcium.
Parathyroid hormone (PTH) is the major controller of free calcium in the body.
PTH is released from the parathyroid glands when plasma calcium levels are low.
PTH stimulates bone reabsorption, leading to an increase in plasma calcium and phosphate levels.
PTH promotes calcium reabsorption in the kidneys and inhibits phosphate reabsorption, promoting phosphate excretion.
PTH stimulates 1α-hydroxylase, the key enzyme in the synthesis of active vitamin D.
Active vitamin D (1,25 dihydroxycholecalciferol or calcitriol) is produced through 25-hydroxylation in the liver and 1α-hydroxylation in the kidneys.
Vitamin D promotes calcium and phosphate absorption in the gut, calcium and phosphate reabsorption in the kidneys, and mineralisation in bone.

Bone Disorders

Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration, leading to bone fragility and fracture risk.
Vitamin D deficiency in children leads to rickets, characterized by abnormal unmineralised osteoid and bone bowing.
Vitamin D deficiency in adults leads to osteomalacia, characterized by bone weakness due to unmineralised osteoid but without bone bowing.

Bone Functions & Structure

Bones provide support, protection, movement, a calcium and phosphate store, and a bone marrow store.
Bone is a mineralised organic matrix.
The matrix is mainly type 1 collagen fibres (90-95%) with some proteoglycans (5%).
Collagen provides tensile strength while proteoglycans provide compressive strength.

Bone Types

Two main types of bone: compact bone (cortical bone) and trabecular bone (cancellous or spongy bone).
Compact bone is dense, stiff with low porosity (5-25%).
Trabecular bone is spongy, light with high porosity (up to 70%).
The combination of these two types provides mechanical strength despite being lightweight.

Bone Growth

Bones are modelled in cartilage during fetal life and then mineralised through ossification.
Cartilage proliferates at the growth plate elongating bones during childhood/adolescence.
Bone growth is controlled by growth hormone and insulin-like growth hormone (IGF-1).
Growth plate fuses around age 18, stopping cartilage proliferation and elongation.
Vascularisation and ossification of the bone occur after the growth plate fuses.

Bone Cells

Three main cell types in bone: osteoblasts, osteoclasts, and osteocytes.
Osteoblasts promote bone formation, laying down osteoid and initiating mineralisation.
Osteoclasts promote bone reabsorption, removing mineralisation and liberating calcium and phosphate.
Osteocytes transfer mineral from inner regions of bone to the growth surfaces.

Osteoblasts

Modified fibroblasts derived from mesenchymal stem cells.
Lay down osteoid (type 1 collagen matrix) and facilitate ossification.
Some osteoblasts become embedded in the lining of the new bone and become osteocytes.

Osteocytes

Derived from osteoblasts.
Transfer mineral from inner regions of bone to the growth surfaces.
Sense mechanical load on the bone.
Cytoplasmic projections into the bone pass information to osteoblasts.

Osteoclasts

Derived from the macrophage lineage of cells.
Attracted to and resorb mineralized bone, creating resorption pits.
Solubilise mineral at low pH and phagocytose the organic matrix.
Indirectly stimulated by PTH, which promotes bone reabsorption.

Bone Remodelling

Constant process of bone turnover throughout life (adults remodel every 10 years).
Balances bone mineralisation through osteoblast and osteoclast activation.
Enables adaptation to mechanical loading, fracture healing, and prevents "bone fatigue" by renewing bone matrix.

Bone Remodelling Phases

Resorption phase (2 weeks): bone lining cells pull away, osteoclasts are attracted, pockets of bone resorbed, resorption pits, osteoclast apoptosis.
Reversal phase (2 weeks): resorbed bone surface prepared for bone deposition, cement line formation.
Formation phase (13 weeks): resorption of bone releases growth factors, osteoblasts attracted, osteoid deposited, mineralises, osteoblasts become osteocytes or bone-lining cells.

Calcium Regulation

Calcium movement across membranes triggers physiological mechanisms like neurotransmitter release, smooth muscle contraction, heart muscle contraction, and hormone and enzyme secretions.
Ca2+ found in three forms: ionised (free), bound to protein, and bound to small anions.
Most physiological functions are mediated by ionised Ca2+.
Acid-base status can affect levels of bound Ca2+.

Parathyroid Hormone (PTH)

Major controller of free Ca2+ in the body.
Released from chief cells in the parathyroid glands at low plasma Ca2+.
Detected by a membrane-bound G-protein receptor coupled to cAMP.
Decreases cAMP and inhibits PTH release.

PTH & Negative Feedback

PTH release is regulated by negative feedback: High plasma Ca2+ decreases PTH secretion, while low plasma Ca2+ increases secretion. This mechanism ensures calcium homeostasis.

PTH Actions

PTH stimulates bone reabsorption by activating osteoclasts, increasing plasma Ca2+ and phosphate.
Promotes Ca2+ reabsorption in the kidneys (thick ascending limb of the loop of Henle).
Inhibits phosphate reabsorption in the proximal and distal tubules (promotes phosphate excretion).
Stimulates 1α-hydroxylase enzyme in the kidneys, crucial for the synthesis of active vitamin D.

Active Vitamin D Production

Active form is 1,25 dihydroxycholecalciferol (DHCC).
Synthesised through two hydroxylation steps: 25-hydroxylation in the liver and 1α-hydroxylation in the kidney.
1α-hydroxylation is the rate-limiting step and is regulated by PTH.
Vitamin D acts by binding to intracellular receptors.

Vitamin D Actions

Promotes calcium absorption in the duodenum by increasing synthesis of calcium channels and binding proteins.
Promotes phosphate absorption in the gut.
In the kidneys, vitamin D promotes Ca2+ and phosphate reabsorption.
Overall effect is to increase the flux of calcium and phosphate into bone.

Osteoporosis

Systemic skeletal disease characterized by low bone mass and microarchitectural deterioration.
Leads to bone fragility and susceptibility to fractures.
Affects 33% of females after age 50 and 20% of males.

Vitamin D Deficiency

Rickets (in children): abnormal amounts of unmineralised osteoid, bowing of long leg bones.
Osteomalacia (in adults): bone weakness due to unmineralised osteoid, but no bowing of the legs.

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Description

Test your knowledge on the structure, function, and growth of bones. This quiz covers important concepts such as bone composition, types, and the growth process during development. Challenge yourself with questions related to bone remodeling and their roles in the body.