Bone Physiology and Pathology

Question 1

What dietary and regulatory factors are required for bone growth?

Answer 1

Bone growth requires adequate dietary calcium and protein - they are the building blocksk of bone tissue.

Bone growth is highly regulated by hormonal systems:

• GH and IGF required for protein and cell division
• Thyroid hormones have a permissive role in bone growth
• Insulin supports growth and provides glucose-derived energy
• Sex steroids

Question 2

Discuss bone structure

Answer 2

Bone is a dynamic living tissue with substantial **ECM **

Calcium phosphate crystals precipitate and attach to the ECM collagen lattice - giving bone density and strength. The most common calcium phosphate is hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂).

A significant vascular supply of bone exists to supply blood to bone cells.

There are three different types of bone tissue:

1. Compact/Cortical Bone
   * Outer layer of bone providing strength and structure
2. Trabecular/Cancellous Bone
   * Inner spongy layer that gives substance to the bone. It contains open, cell-filled spaces between the struts of calcified lattice
3. Central Bone Marrow
   * Is present in only some bones (typically long bones) and is responsible for haemopiesis

Question 3

What three factors are central to the regulation of calcium within the body?

Answer 3

Parathyroid Hormone (PTH)

Vitamin D3 (Calcitriol)

Calcitonin

Question 4

What are the key roles of calcium in human body?

Answer 4

1. Intracellular Signalling
2. Calcified matrix of bone
3. Active at tight junctions between cells
4. Cofactor in blood coagulation
5. Excitability of neurons and muscle

Question 5

Where in the body is calcium found?

Answer 5

There are three pools of calcium within the body:

1. Bone Matrix (99%)
2. Extracellular Fluid (0.1%)

• ionised calcium
• cement for tight junctions
• NT release in excitable cells
• myocardial and smooth muscle contraction
• cofactor in coagulation

3. Intracellular (0.9%)

• Free Ca²⁺
• Signal in 2nd messenger pathways
• Muscle contraction

Question 6

Illustrate the regulation of calcium

Answer 6

Question 7

Discuss the actions of Parathyroid Hormone (PTH)

Answer 7

PTH produced and secreted from the parathyroid glands located on the posterior surface of the thyroid gland.

PTH is secreted in response to low plasma calcium levels.

PTH has several effects:

1. Actions on bone

• Increases bone resorption
  
  • Elevates the expression of RANKL and reduces expression of OPG on osteoblasts
• Increases the calcium and phosphate release from bone into plasma
• Bone effects within 2-3 hours of stimulation

2. Actions on kidney distal nephron

• Increases calcium reabsorption
• Decreases phosphate reabsorption
• Kidney effects are immediate

3. Actions on Intestine

• PTH indirectly enhances both calcium and phosphate absorption from the intestines by increasing the formation in the kidneys of 1,25-dihydroxycholecalciferol from vitamin D
• Intestinal effects occur after 1-2 days

Question 8

Discuss the effects of calcitonin

Answer 8

Calcitonin is released only in response to extremely high levels of calcium in plasma

Calcitonin:

• reduces bone resporption
• increases calcium excretion

Has no physiological role normally - only functions in extreme hypercalcemia

Question 9

Illustrate the relationship between PTH and Vitamin D3 in calcium metabolism

Answer 9

Question 10

Compare and contrast the conditions of hyper- and hypoparathyroidism

Answer 10

Hyperparathyroidism

• Most commonly the result of a tumour
• Hypercalcemia and hypophosphatemia
• Variable effects on health

Hypoparathyroidism

• Rare
• Tended to result from inadvertent parathyroidectomy during thyroid surgery or autoimmune destruction of parathyroid glands
• Hypocalcemia and hyperphosphatemia
• Leads to neurovascular excitability

Question 11

What are the effects of Vitamin D3 deficiency?

Answer 11

Vitamin D3 deficiency results in impaired intestinal calcium absorption.

As a result in sufficient Ca²⁺aquisition from the diet, PTH maintains Ca²⁺at the expense of bone - > leading to conditions of bone demineralisation:

Rickets in children and osteomalacia in adults

Question 12

Describe the condition of osteoporosis

Answer 12

Osteoporosis develops from a **long term imbalance between bone resorption > bone formation **

Unknown cause

It is particularly prevalent in post-menopausal women -> thought to be a result of estrogen withdrawl.

Osteoporosis poses a significant fracture risk

Preventable risk factors include low dietary calcium, smoking and lack of exercise.

Question 13

Characterise hyaline cartilage

Answer 13

Hyaline cartilage is associated with the articular surfaces of joints, ribs and trachea

It consists of:

• collagen type II
• aggrecans (large GAGs)
• hyaluronic acid
• chondronectin

Water comprises 70% of articular cartilage by weight

Hyaline / articular cartilage is avascular but is perfused by synovial fluid during compression/decompression of the cartilage - i.e exercise

Question 14

What is the difference between elastic and hyaline cartilage?

Answer 14

Elastic cartilage is hyaline cartilage with addition of elastin

Elastic cartilage is found in ears, ear canals, epiglottis and larynx

It is very flexible and springs back to maintain normal shapes

Question 15

Characterise fibrocartilage

Answer 15

Binds solid joints, forms minisci and invertebral discs

Composed of a mixture of dense connective tissue, type I collagen, and isolated islands of cartilage

No perichondrium is present

Chondrocytes which produce cartilage differentiate from fibroblast cell lineages

Question 16

Discuss the organisation of trabecular bone

Answer 16

Trabecular bone is less organised than cortical bone.

Trabecular bone comprises of lamellae struts with continuous spaces occupied by marrow and blood vessels.

Osteocytes within the trabecular bone have processes that extend out of the bone and communicate with the marrow cavity to obtain nutrients.

Question 17

Characterise the medullary cavity of bone

Answer 17

The medullary cavity is a feature of predominatly long bones of the human body; it contains **bone marrow **

Bone marrow can be **red bone marrow (haemopoietic cells) **or yellow bone marrow (fat cells)

Red marrow is prevalent early in life and is replaced by yellow marrow with age

• Yellow marrow preserves some haemopoietic cells capable of becoming active haemopoietic tissue if needed.

The blood vessels inside the medullary cavity are sinusoids rather than capillaries. Sinusoids have a larger diameter and can form pores that give newly produced haemopoietic cells a pathway to the blood circulation.

Question 18

Characterise cortical bone

Answer 18

Cortical bone consists of two layers:

**Periosteum **

• Outer fibrous layer containing fibroblasts, blood vessels and collagen
• Inner cellular layer of osteoprogenitor cells which give rise to osteoblasts

**Endosteum **

• Layer lining the inner marrow cavity
• Thinner that periosteum and still has osteoprogenitor cells

Question 19

What are Sharpey’s fibres?

Answer 19

Sharpey’s fibres is a histological term describing the penetration of tendon or ligament collagen fibres through the bone surface to anchor into the underlying bony ECM.

This means that bone collagen is contninuous with tendon collagen at sites of ligament or tendon insertion

Question 20

Where would you not expect endosteum or periosteum to be present in bone?

Answer 20

1. Insertions and origins of tendons or ligaments
2. Sites of bone articulation

Question 21

Describe the general blood and nerve supply of bones

Answer 21

Arteries supply bone at discrete points:

• Arteries supply the diaphysis (shaft) and epiphysis (ends) independently
  
  • Penetrating arteries branch within the marrow cavity
• The periosteum is seperately supplied by non-penetrating arteries on the bone’s surface

Nerves follow the blood vessels; their function is still unknown

Question 22

What type and function does the cartilage overlying articular surfaces (articular cartilage) have?

Answer 22

Articular cartilage is hyaline cartilage.

It si slippery, smooth and resistant to compression

IMPORTANT = there is no perichondrium within joints -> makes repair of articular cartilage difficult.

Question 23

Characterise the synovial membrane and synovial space

Answer 23

The synovial space is full of **synovial fluid **that lubricates and provides nutrients to avascular structures within the joint such as articular cartilage. The fluid is an ultrafiltrate of synovial blood vessels and proteoglycans

The **synovial space is lined by synovial membrane **

The synovial membrane is not an epithelium making it one of the few spaces in the body where there is no epithlium lining a fluid cavity or lumen. Because of this, it lacks a BM, tight junction or desmosomes and is very leaky and unable to regulate the flux of fluid effectively

Two layers of synovial membrane:

1. Intima (surface layer)

• two-to-three cells thick
• mix of fibroblast-like cells (Type A) and macrophage-like cells (Type B)

2. Sub-initma
   * fibrous connective tissue layer

Question 24

Characterise osteoprogenitor cells

Answer 24

Osteoprogenitor cells are localised to the **periosteum and endosteum **

They are flattened cells that are difficult to see microscopically

Osteoprogenitor cells are usually quiescent but capable of activating to give rise to osteoblasts for bone growth and repair

Are derived and renewed from mesenchymal stem cells in bone marrow

Question 25

Characterise osteoblast cells

Answer 25

Osteoblasts deposit new bone over the top of existing external bone layers. They are located ***lining the surfaces of bone at the periosteum and endosteum*** Osteoblasts produces **osteoid** - the ECM of bone - which contains **type I collagen and bone matrix proteins.** These bone matrix proteins include: * Osteocalcin/Osteonectin (Ca²⁺ binding proteins) * Adhesive proteins * Proteoglycans * Alkaline phosphatase *Note: Osteocalcin and alkaline phosphatase are used as markers of osteoblast activity in the blood* Inactive osteoblasts become quiescent

Question 26

Characterise osteocytes

Answer 26

**Osteocytes** are derived from **entombed osteoblasts** as a result of bone deposition. ***Maintain bone in response to loading forces sensed by their mechanotransduction properties*** Osteocytes form small canal systems, known as **canaliculae**, between one another and to blood vessels to form a chain of nutrient supply. Loss of osteocytes leads to bone resorption Alternatively, osteocytes are also capable of destroying local surrounding bone to free calcium

Question 27

Characterise osteoclasts

Answer 27

Osteoclasts sit on the surface of bone to destroy and resorb it as part of growth, repair and normal turnover They are activated by the increased expression of **RANK-L** or **M-CSF** or the reduced expression of **osteoprotegrin** The **giant multinuclear cells seal themselves** to the bone around their cell edge and secrete a range of factors from their **ruffled border** including: * H⁺ * Cl^- * H₂CO₃ * Proteases Note: tartrate resistant acid phosphatase is a marker of osteoclast activity Osteoclast activity is **elevated in response to parathyroid hormone** and **reduces as a result of calcitonin** release. Osteoclasts are derived from bone marrow **granulocyte/macrophage cell lineage** - a completely seperate cell lineage from osteoblasts. Osteoclasts apoptose when not required

Question 28

Describe endochondral bone formation

Answer 28

**Endochrondral ossification **formation is the process of producing an initial cartilagenous skeleton which is later replaced by bone. This occurs in weight bearing bones and bone of the extremities. 1. A **bone collar** forms around the diaphysis 2. The cartilage beneath the collar degenerates due to bone restricting its blood supply 3. Blood vessels invade the collar and bring in bone cell progenitors forming a nucleus of ossification that spreads throughout the diaphysis 4. Secondary nucleuses of ossification occur in each of the **epiphysises** 5. The zones of ossification grow at the same rate but remain seperated by the cartilaginous **epiphyseal growth plate**

Question 29

Discuss the organisation of the **epiphyseal growth plate**

Answer 29

The **epiphyseal growth plates **enable long bones to grow and are present until the age of 21-22 years old before fusion. No more growth can occur after this time. There are five layers of the growth plate, from epiphyseal to diaphyseal: 1. **Resting zone: **Normal hyaline cartilage 2. **Proliferation zone**: Dividing chondrocytes 3. **Maturation zone:** Area of mature chondrocytes 4. **Hypertrophy zone**: Zone of hypertrophic/dying chondrocytes 5. **Ossification zone:** Degenerating cartilage is replaced by bone deposition

Question 30

What is woven bone?

Answer 30

**Woven bone** is a special type of new bone initially during development or repair It is more cellular, more cartilagenous and without Haversian systems compared to mature bone. It is remodelled by osteoclasts and osteoblasts into mature bone.

Question 31

Describe the process of remodelling via Haversian systems

Answer 31

Remodelling of bone via Haversian systems involves: ## Footnote 1. Osteoclasts tunnel and resorb bone along stress axis 2. Blood vessels invade behind osteoclasts 3. Endosteum invades and lines the inside of the tunnel 4. Endosteum gives rise to osteoblasts 5. Osteoblasts lay down sequential layers of bone with the collagen alternating in direction 6. The final layer leaves a small narrow space around the artery supplying the haversian system known as the **Haversian Canal**. 7. During this process, some osteoblasts are entombed within deposited bone and become osteocytes

Question 32

What structures are contained within the Haversian Canal?

Answer 32

Blood vessels Lymphatics Nerves Connective Tisssue

Question 33

What is the difference between woven and lamellar bone?

Answer 33

The bone matrix is synthesized in one of two histologic forms, woven or lamellar **Woven bone** is produced rapidly, such as during fetal development or fracture repair, but the haphazard arrangement of collagen fibers imparts **less structural integrity** than the parallel collagen fibers in slowly produced **lamellar bone**. In an adult, the presence of woven bone is always abnormal, but it is not specific for any particular bone disease since it can be found in a variety of pathologic settings.

Question 34

What is **intramembranous ossification**? What bones are formed by this process?

Answer 34

**Intramembranous ossification** is the production of osteoid by mesenchymal cells that have differentiate into osteoprogenitor cells. It is a process that does not involve cartilage - as opposed to endochondral ossification This process gives rise to the flat bones of the human body - particularly the skull