Bone Structure and Growth

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Questions and Answers

What is the primary mineral component of bone, represented by the formula Ca10(PO4)6(OH)2?

Bicarbonate of lime

Calcium sulfate

Calcium carbonate

Hydroxyapatite (correct)

Which part of the kidney does parathyroid hormone (PTH) specifically promote calcium reabsorption?

Thick ascending limb of the loop of Henle (correct)

Proximal tubule

Collecting duct

Distal convoluted tubule

Which enzyme in the kidney is responsible for converting 25-hydroxyvitamin D into its active form?

25-hydroxylase

Hydroxylase

1α-hydroxylase (correct)

Dehydrogenase

What effect does active vitamin D (calcitriol) have on calcium absorption?

It promotes calcium absorption from the gut. Signup and view all the answers

What is one of the direct effects of parathyroid hormone (PTH) on phosphate in the kidney?

Inhibits phosphate reabsorption Signup and view all the answers

Which vitamin undergoes 25-hydroxylation in the liver before being converted to its active form in the kidney?

Vitamin D Signup and view all the answers

What role does calcium play in bone mineralization?

It is the primary structural mineral in bones. Signup and view all the answers

What structure in the intestines is primarily involved in the absorption of Ca2+?

Duodenum Signup and view all the answers

Which action does PTH NOT perform in the kidney?

Promotes potassium retention Signup and view all the answers

What is the active form of vitamin D also referred to as?

Calcitriol Signup and view all the answers

What characterizes trabecular bone compared to compact bone?

Lower density and higher porosity Signup and view all the answers

What is the main role of osteoblasts in bone growth?

Lay down osteoid and initiate mineralization Signup and view all the answers

Which factor primarily stimulates the activation of osteoclasts for bone resorption?

Parathyroid Hormone (PTH) signaling from osteoblasts Signup and view all the answers

Which condition is characterized by decreased bone density and increased fracture risk?

Osteoporosis Signup and view all the answers

How is the majority of calcium in the body found?

Mostly bound to proteins Signup and view all the answers

Which condition would result in increased levels of free calcium in the plasma?

Acidosis Signup and view all the answers

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

Stimulates bone resorption to raise plasma Ca2+ levels Signup and view all the answers

How does the body's acid-base status affect the levels of bound calcium?

Decreased acidity promotes calcium displacement from proteins Signup and view all the answers

What triggers the release of Parathyroid Hormone (PTH)?

Low plasma [Ca2+] Signup and view all the answers

Which of the following processes does PTH NOT promote?

Bone mineralization Signup and view all the answers

In the context of osteoporosis, which of the following factors is least likely to contribute to its development?

Excessive calcium intake Signup and view all the answers

What is the effect of Parathyroid Hormone on osteoclasts?

Stimulates the proliferation of osteoclasts Signup and view all the answers

What is the main consequence of excessive PTH release over time?

Increased risk of fractures Signup and view all the answers

What characterizes osteoporosis?

Low bone mass and increased fragility Signup and view all the answers

Which of the following is NOT a consequence of vitamin D deficiency in adults?

Bow legged appearance Signup and view all the answers

How does vitamin D affect calcium levels in the kidneys?

Increases calcium reabsorption Signup and view all the answers

What is the overall effect of vitamin D on bone?

Increases flux of calcium and phosphate into bone Signup and view all the answers

What happens to bone growth once the epiphyseal plate fuses?

Bone growth stops completely Signup and view all the answers

Which factor contributes to an increased risk of osteoporosis in women post-menopause?

Declining bone mineral density Signup and view all the answers

Which type of bone primarily receives impact stress during physical activities?

Cortical bone Signup and view all the answers

What is the main regulatory hormone for calcium homeostasis alongside vitamin D?

Parathyroid hormone (PTH) Signup and view all the answers

What process do osteoblasts primarily facilitate?

Bone mineralization Signup and view all the answers

In rickets, what visible deformity is commonly seen in children?

Bow legged appearance Signup and view all the answers

Study Notes

Bone Structure

Bone is a living tissue that provides support, protection, and movement for the body
Compact bone, also known as cortical bone, is dense and stiff with low porosity (5-25%) making up the majority of human bone (~80%)
Trabecular bone (also known as cancellous or spongy bone) is light and spongy with high porosity (up to 70%)
Long bones have a diaphysis (shaft) made of compact bone, and epiphyses (ends) made of trabecular bone
The epiphyseal growth plate is responsible for bone growth and fuses with the metaphysis around age 18, ending growth

Bone Growth

Ossification is the process of bone formation, starting in cartilage during fetal development
Bone growth occurs at the growth plate in childhood/adolescence through cartilage proliferation, controlled by growth hormone and insulin-like growth hormone (IGF-1)
Once the growth plate fuses, cartilage proliferation and elongation cease, and bone is vascularized and ossified
The osteon is the basic unit of bone growth, consisting of concentric circles of collagen deposition around a blood vessel, known as a Haversian canal
Collagen is mineralized with hydroxyapatite (Ca10(PO4)6(OH)2) during ossification

Bone Cells

Osteoblasts promote bone formation, laying down osteoid and initiating mineralization
Osteoclasts promote bone reabsorption, removing mineralization and liberating calcium and phosphate
Osteocytes originate from osteoblasts and facilitate the transfer of mineral from inside bone to growth surfaces
Osteocytes have cytoplasmic projections that sense mechanical load on bone and communicate with osteoblasts

Bone Remodeling

Bone turnover continues throughout life, remodeling the adult skeleton every 10 years with 1 million Bone Metabolic Units (BMUs) operating at any one time
Bone remodeling balances mineralization through activation of osteoblasts and osteoclasts, enabling adaptation to mechanical loading, fracture healing, and preventing bone fatigue
Osteocytes communicate mechanical strain to osteoblasts and PTH signaling, triggering a chain reaction leading to osteoclast activation
Growth factors stimulate osteoblast formation, depositing new osteoid for mineralization, resulting in no net loss of bone over the 120-day bone remodeling cycle
Bone remodeling involves four distinct phases: resorption, reversal, formation, and quiescence

Calcium Regulation

Calcium is essential for many physiological processes including neurotransmitter release, muscle contraction, hormone and enzyme secretion
Calcium exists in three forms in body fluids: ionized (free), protein-bound, and anion-bound (phosphate, citrate, and oxalate)
Ionized calcium (Ca2+) mediates most physiological functions
Acid-base status influences bound Ca2+ levels: acidosis increases free Ca2+ and alkalosis decreases free Ca2+ through protein binding

Parathyroid Hormone (PTH)

PTH is the primary regulator of free Ca2+ in the body
Released from the parathyroid glands by chief cells when plasma Ca2+ is low
Detected by a membrane-bound G-protein coupled receptor, which regulates cAMP levels
Decreases in Ca2+ increase cAMP and stimulate PTH release, while increases in Ca2+ inhibit PTH release

Vitamin D

The active form of vitamin D is 1,25 dihydroxycholecalciferol (DHCC), also known as calcitriol
Synthesized from precursor vitamin D3 (cholecalciferol) through two hydroxylation steps
25-hydroxylation occurs in the liver, catalyzed by 25-hydroxylase
1α-hydroxylation in the kidney, catalyzed by 1α-hydroxylase, is the rate-limiting step and a crucial regulatory site
Vitamin D acts on intracellular receptors as a steroid-like structure

Vitamin D Action

Vitamin D promotes Ca2+ absorption in the duodenum through Ca2+ channels and binding proteins (calbindin)
It also stimulates the synthesis of components involved in Ca2+ and phosphate absorption in the gut
In the kidney tubules, vitamin D promotes Ca2+ and PO43- reabsorption
Overall effect of vitamin D is to increase the flux of Ca2+ and phosphate into bone

Osteoporosis

A systemic skeletal disease characterized by low bone mass and weakened bone microarchitecture, leading to increased fracture risk
Affects 33% of women after age 50 and 20% of men after age 50
Increases in the aging population escalate the risk
Hip fractures are more common in women than breast cancer

Vitamin D Deficiency

Rickets in children, characterized by abnormal amounts of unmineralized osteoid causing bowing of long bones
Osteomalacia in adults, characterized by bone weakness due to unmineralized osteoid, without bowing

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Description

This quiz explores the essential aspects of bone structure and growth. It covers the types of bones, their functions, and the process of ossification. Test your knowledge on how bones develop and the factors influencing their growth.