Tissues and movement

1 Cartilage starts to gain calcium (gives bone hardness)

2 Blood vessel pushes its way inside the cartilage, bringing with it osteoblasts

3 Blood vessels move into the diaphysis of long bones and release some of the osteoblasts, turning some of the cartilage into bone

4 Most of the diaphysis will be turned into bone tissue; it is hollowed in the middle in order to have room for blood vessels and marrow

What are the main steps (4)in endochondral ossification?

• The diaphysis is the primary ossification centre in long bone, while the epiphyses remain cartilage.

• The epiphyses are the secondary ossification centres; they undergo the same ossification process as the primary centres.

• They are separated from the diaphysis by an epiphyseal plate / growth plate (made of cartilage).

What are the primary and secondary ossification centres in long bone development, and how are they separate

• The lengthwise growth of bones is enabled by epiphyseal plates, meaning that the inner part of the bone lengthens rather than the ends. These are the ‘growth plates’ formed of cartilage.

• This begins at puberty, when epiphyses start to fuse with diaphyses.

How does lengthwise bone growth occur and what happens at puberty?

• Width wise growth is proportional to lengthwise growth (appositional).

• Osteoblast activity will produce circumferential lamellae (layers on the outside), while osteoblasts will mould the bone shape, forming a medullary cavity.

How does bone grow in width and what ensures it doesn’t get too heavy?

1. Bones articulate at joints.

2. Hold bones together

3. Touch and move - involve bone ends

4. Connect and cover - involve soft tissues

5. Allow control of movement, through muscle attachment

What is the function of joints in the body (5)?

1. Cartilage is formed by chondrocytes found in lacuna.


2. DFCT is made of fibroblasts that make collagen fibres (for strength) and some elastin fibres (for stretch).

What are the two key soft tissues in joints and what are they made of?

• Contains collagen fibres embedded in ground substance (a jelly-like matrix).

• Avascular (no blood vessels), making nutrient supply difficult.

• Nutrients are diffused through the matrix by joint loading (pressure from movement).

• Cartilage is hard to repair due to limited nutrition.

What are the characteristics of cartilage in joints (4)?

1. Hyaline cartilage has a lot of ground substance making it good at trapping water / resisting compression, however it has few collagen fibres. It degrades with age and moulds to bone surfaces at articulation, creating a smooth frictionless surface.

2. Fibrocartilage has many collagen fibres in bundles and less ground substance, making it good at resisting both tension and compression. The fibre orientation aligns with stresses to act as a shock absorber by distributing force, and it deepens articular surfaces. It is useful in joints that experience both compression and tension.

What are the two types of cartilage, and how do they differ (9)?

• DFCT is tightly packed, slow to heal, and contains few blood vessels or ground substance, making it good at resisting tension.

• Three kinds of DFCT:


1. Ligaments (bone-bone): Made of collagen and elastin; resist tension while allowing some stretch and recoil; restrict movement if bone moves incorrectly


2. Tendons (muscle-bone): More collagen than elastin; facilitate and control movement; transmit muscle contraction


3. Joint capsules: Enclose and support joints

What are the properties (4) and types (3) of dense fibrous connective tissue (DFCT)?

• Bone congruence is the sum of bone surfaces that form articulation.


• Less congruence = more soft tissue support is needed

• More congruence = higher surface area contact

What is bone congruence and why is it important?

1. Fibrous joints:


   • Made of dense fibrous connective tissue ligaments


   • Function to limit movement (none or very little) and provide stability


   • Examples: cranial sutures

2. Cartilaginous joints:


   • Made of fibrocartilage tissue


   • Allow limited movement


   • Examples: pubic symphysis

3. Synovial joints:


   • Made of many tissues and structures


   • Allow lots of movement


   • Most joints in the appendicular skeleton are synovial

What are the three joint classifications, their makeup, their levels of movement, and an example?

• Bone is a living tissue that changes throughout your life.

• It has cells (that form bones, remove bones, and change bones throughout your life), as well as a calcified extra-cellular matrix.

What is bone made of (2) and how does it behave over time?

• Your bone changes depending on how you use your body, it can get thicker or thinner in response to external forces.

• Every year your bone changes by around 10%, it remodels and can change shape to reflect how you are using your skeleton.

How does bone respond to use and stress throughout life?

• Bone can repair itself, as bone cells will respond to trauma to unite broken parts.

• Bone tissue is the only tissue with a hard feel (other than dental enamel), it is made of connective tissue.

• The role of connective tissue is to support organs/other tissues, maintain shape of body, and join things within the body.

Can bone repair itself, and what kind of tissue is it?

• 33% is organic and consists of

  1. Collagen: flexible protein macromolecule in long fibres that give bones flexibility and the ability to resist tension.

  2. Proteoglycans: ground substance – watery mass of proteins surrounding the collagen

• 67% is inorganic and consists of

  1. Hydroxyapatite

  2. Other mineral salts (Ca minerals). The minerals make bone hard and resistance to compression, allowing us to have strong rigid bones.

• Without the collagen, bones are brittle and break very easily. Without the inorganic compounds, bone is extremely flexible.

What are the two extracellular components of bone (4) and their roles?

• Bone cells take up 2% of bone by weight and there are four types of bone cells:


1. Osteogenic cells (stem cells – become other cells, typically osteoblasts)


2. Osteoblasts (produce new bone matrix)


3. Osteocytes (mature bone cells – sit within the bone, communicators, recycle proteins and minerals from the matrix)


4. Osteoclasts (remove bone matrix)

What proportion of bone is made of cells, and what types are there (4)?

• There are two types of bone tissue, compact and cancellous.

• These have got the same material in the same ratio, however, have different structures.

• Compact bones have an osteon structure (ring of bone), whereas cancellous bones have a trabecular structure (thin structures of bone).

What are the two types of bone tissue and how do they differ?

• Compact bone is dense with not many gaps (other than foramina that blood vessels go through in order to supply nutrients for cells in bones).

• Macroscopically: Outer surfaces seem dense and impenetrable (periosteum) / foramina for blood supply.

• Microscopically: Made up of circumferential lamellae and osteons.

What are the macroscopic and microscopic features of compact bone?

Osteons are a longitudinal (lengthwise) unit within compact bone that provide a pathway for nutrients to get to cells in the ECM.

• They consist of a central canal, lamellae, lacunae, and canaliculi.

  1. Central canal: contains blood vessels and nerves.

  2. Lamellae: series of cylinders formed of ECM around the central canal. They form the shape of the osteon and collagen fibres within lamellae resist forces.

  3. Lacunae: ‘lakes’ for osteocytes.

  4. Canaliculi: channels for nutrients to travel to osteocytes through the ECM.

What structures make up an osteon in compact bone (4)?

• Trabeculae structure

• Marrow in cavities between trabecula.

• No central canal as they are thinner therefore can diffuse nutrients from marrow.

• Osteocytes: in lacunae in between lamellae/on surface.

• Organisation of trabeculae: resists force from multiple directions and directs force from body weight in a single direction down shaft

• More fibres can transmit force, allowing force to be spread out distally due to density of bone.

• Upper-lower body: trabeculae channel weight around the ilia into femora.

What is the structure (4) and function of cancellous bone?

• Bone remodelling is when osteoblasts (in the outer layer) receive signals and add bone matrix in lamellae, at the same time osteoclasts (located internally) remove bone from the medullary cavity.

• As we age our long bones get wider in order to support the increasing weight, this also happens as a response to stress.

What happens during bone remodelling and why is it important?

• Osteoporosis is a clinically significant imbalance in OB/OC activity when osteoclastic activity > osteoblastic activity.

• This results in both cancellous and compact bones thinning out.

• It is a loss of cortical bone, the trabeculae in cancellous bones become thinner, and holes develop, meaning that bones can’t absorb much shock and you are at great risk of fractures (typically compression fractures of vertebrae).

• Biological females are more at risk due to a loss of estrogen post-menopause.

• Lifestyle factors can also contribute such as: lack of exercise, smoking, drinking, bad nutrition/lack of calcium, and too much salt.

• You reach peak bone mass in your 20’s, and if this is low then you’re also more at risk.

What is osteoporosis and what are its causes (7) and consequences (4)?