Skeletal Sytem

Question 1

1. Understand the gross anatomy of long bones

Answer 1

• Shaft/body of a long bone is called the diaphysis (long middle sheath)
• Proximal and distal ends are called the epiphyses
  o Proximal en d of our thigh bone is at the hip
  o Distal end is at the knee (closer to the origin)
• The area between the epiphysis and the diaphysis is called the metaphysis
  o In a growing bone, the metaphysis contains the epiphysial growth plate
• Except for areas that are covered by articular cartilage, long bones are surrounded on their external surface by periosteum
  o Periosteum is a tough connective tissue sheet that wraps around the bone with the exception of the area where the bones is covered by cartilage
  o Periosteum nourishes the external aspects of the skeletal tissue
  o Its capable of laying down more bone
  o Has an active role in things like a bone fracture
  o The periosteum is attached to the underlying bone by perforating fibres of collagen
• Within the diaphysis of a long bone, there’s a hollow cylindrical space (called the medullar cavity)
  o Acts as a highway for blood vessels
  o The medullary cavity itself is lined by endosteum, which is a thin connective tissue membrane
• Nutrient artery: located outside of the periosteum but heads into the centre of the bone towards the medullary cavity

Question 2

2. Classify bones by their shapes into long bones, short bones, flat bones, and irregular bones

Answer 2

1. Long bones
   o Longer than they are wide , are tubular
   o Generally have hollow middles where the bone marrow resides
   o Bear a lot of weight (e.g. femur of the thigh, radius and ulna (two bones in the forearm), humerus)
2. Short bones
   o Somewhat cube-shaped
   o Nearly equal in length and width
   o Consist of spongy bone except at the surface where they have that thin layer of compact bone
   o E.g. carpel bones of the wrist
   > Sesamoid – subcategory
    Means shaped like a sesame seed
    Develop in certain tendons where there’s considerable friction, tension or physical stressors
    E.g. the patella
    Also have sesamoid bones in the palms of the hands and the soles of the feet (these are irregular in number and some people don’t have them at all)
3. Flat bones
   o Thin, flattened, usually a bit curved
   o Composed of two nearly parallel plates of compact bones enclosing spongy bones; they can be wide but are usually very thin
   o Found in areas where there are underlying structures that are pretty important, ones that require considerable protection, e.g. cranial bones or bones of skull that protect brain, the sternum that protects the heart, ribs that wrap around thorax and protect all the viscera (liver, kidneys, spleen, lungs etc)
   o Protect vital organs , provide surfaces for large muscle attachment (e.g. sternum, skull bones)
4. Irregular bones
   o Don’t fit into other categories
   o Complicated shapes (e.g. vertebrae)
   o Vary in the amount of compact vs spongy bone composition

Question 3

3. Summarise the functional properties of trabecular and cancellous bone

Answer 3

• structural support, bone marrow, bone remodelling, cushions stress

Question 4

4. Understand the neurovascular supply of bones

Answer 4

• Bone tissue + periosteum has rich blood supply
• Periosteal arteries supply periosteum and outer compact bone, enter through many small canals
• Large nutrient artery enters via nutrient foramen at centre of diaphysis
  o Enters medullary cavity, courses toward epiphyses
• Ends of long bone supplied by epiphyseal and metaphyseal arteries
• Blood vessels are especially abundant in portions of the bone that contain that red bone marrow
  o Red bone marrow produces red blood cells, white blood cells for the immune system and platelets for clotting
• Long bones have one or more nutrient arteries which also have blood vessels and passes through the compact bone at the external surface of the long bone, via a small hole or space called the nutrient or vascular foramen at the diaphysis
• Within the medullary cavity (space within the long bone), the nutrient artery divides into medullary branches that extend toward proximal and distal epiphyses and supply the marrow, spongy bone, and the deeper portions of the compact bone
• Veins carry blood away from the bone, accompany arteries
• Rich sensory nerve supply to periosteum, very sensitive to tearing and tension
• Nerves accompany blood vessels

Question 5

5. Describe the role of osteoblasts and osteoclasts

Answer 5

1. Osteoblasts:
   o Bone-building cells
   o Synthesise and secrete collagen fibres, organic components to build extracellular matrix of bone
   o Initiate calcification
2. Osteoclasts:
   o Break down bone ECM (bone resorption)
   o Releases lysosomal enzymes + acids to digest protein and mineral components of ECM

Question 6

6. Describe the process of intramembranous ossification

Answer 6

INTRAMEMBRANOUS OSSIFICATION - directly from mesenchyme
*All bones are derived from mesenchyme
- Occurs in fetal development
- mesenchyme condenses
-
1. Development of ossification centre:
- Osteoblasts start synthesising osteiod (organic extracellular matrix before mineralisation has occured)

2. Calcification:
   - calcium and other mineral salts are deposited and the extracellular matrix calcifies (hardens)
3. Formation of trabeculae:
   - ECM develops into trabeculae that fuse to form spongy bone
   - As the osteoblasts form the matrix, some get trapped forming osteocytes in the lacunae
   - They have cytoplasmic processes that sit in canalculi - this is an ossification centre and multiple of these form and fuse
   - The bone matrix organises into rods and plates forming the trabeculae of cancellous bone around blood vessels
4. Development of the periosteum:
   - mesenchyme at the periphery of the bone becomes periosteum
   - Between the trabeculae is hematopoetic tissue where mesenchymal cells become red bone marroow
   - lining in the bone marrow medullary cavity forms the endosteum

Question 7

7. Understand the process of bone remodelling and describe the factors that affect bone growth and remodelling over a lifetime

Question 8

8. Be familiar with the anatomical terminology of bone markings and apply this terminology to specific bony landmarks

Question 9

State the functions of the skeletal system

Answer 9

1. Support – structural framework for the body, attachment point for tendons of muscles
2. Protection – encloses visceral organs
3. Movement – skeletal muscles attach to bone, and contraction pulls on bone to produce movement
4. Mineral homeostasis – stores minerals (NaCl, Ca2+), can be released into blood on demand
5. Blood cell production – red bone marrow within bones produces RBCs
6. Triglyceride storage – yellow bone marrow stores triglycerides

Question 10

Describe the anatomy of the skeletal system

Answer 10

• Bones of the adult skeleton are grouped into two principal divisions;
• Axial skeleton: bones of head, neck & trunk
   Consists of about 80 bones
   Comprises of bones of the head, the neck, the trunk (includes the ribs and sternum), the vertebral column, sacrum
• Appendicular skeleton: bones of limbs (including pectoral and pectal girdle)
   126 bones
   Consists of bones of the limbs, including the pelvic and pectoral girdles (hips and shoulders), contains the clavicle, innominate bones
• Bones are vascular and innervated

Question 11

Understand bone composition

Answer 11

• Bone is a form of connective tissue
• Extracellular matrix surrounding widely separated cells
• ECM consists of:
  o 45% Organic components
   30% Collagen
   15% water
  o 55% crystallised mineral salts
   Calcium phosphate, and calcium carbonate – these are contained amongst ions of magnesium, fluoride, potassium, and sulphate which are deposited in the collagen fibre framework of the ECM, they then harden and crystalise (in a process called calcification)
   Makes bones opaque – X-rays
• Strong and resilient: minerals resist compression, and collagen resists tension

Question 12

Briefly describe gross anatomy of bone

Answer 12

• Consists of compact bone sandwiching spongy bone
• All bones have a superficial thin layer of compact bone that surrounds the central mass of spongy bone
• The exception is where the spongy bone is replaced by a medullary/marrow cavity which happens in the long part (diaphysis) of a long bone

Question 13

Understand anatomy of compact bone

Answer 13

• Dense bone, forms outer shell of all bone, and surrounds spongy bone
• Hollow pillars of bone matrix containing nerves and blood vessels
• Strongest form of bone tissue and is found beneath the periosteum of all bones
• The periosteum is a dense layer of vascular connective tissue that envelops bones, except at the surface of joints
• The periosteum is like a rigid wrapping that goes around bones
• At the microscopic level, compact bone is composed of repeating structural units (osteons) that consist of concentric lamellae arranged around a central Havergian canal network
• Blood vessels and nerves are contained within the Havergian canal network
• Overall, the osteons resemble the growth rings on a tree and are aligned in the same direction, parallel to the length of a diaphysis of a bone (long portion of a bone

Question 14

Understand anatomy of spongy bone

Answer 14

• Always located in the interior of a bone; always protected by a covering of compact bone and compact bone is always protected by a covering of periosteum
• Spongy bone isn’t organised into osteons, it consists of lamellae which are arranged into irregular patterns of thin columns called trabeculae
• The microscopic spaces between these trabeculae are filled with red or yellow bone marrow depending on the bone
• Looks poorly organised, but trabeculae are aligned to assist with weight-bearing along the lines of compressive stress + lines of tensile forces to aid the transfer of force through the body without breaking
• Filled with red or yellow bone marrow (also medullary cavity)

Question 15

Describe anatomy of long bone

Answer 15

• Diaphysis: Forms the long axis of the bone
• Epiphysis: Proximal and distal ends
• Metaphysis: regions between epiphysis and diaphysis
  o Has epiphyseal growth plate of hyaline cartilage in growing bone, epiphyseal line in adult
• Articular cartilage: Hyaline cartilage covering the joint surface
• Periosteum: outer fibrous covering, double-layered membrane
  o The entire surface except the joints
• Marrow (medullary) cavity: hollow space within diaphysis; contains yellow (fat) marrow + blood vessels
• Endosteum: thin membrane, lines medullary cavity, internal bone surfaces, trabeculae, lines canals of compact bone

Question 16

Describe the process of endochondral ossification

Answer 16

ENDOCHONDRAL OSSIFICATION
- from cartilage derived mesenchyme
- cartilage model of bone from mesenchyme during fetal period - bone replaces cartilage

1. Development of cartilage model: mesenchymal cells develop into chondroblasts, which form the cartilage model
2. Growth of cartilage model, growth occurs by cell division of chondrocytes
3. Development of primary ossification center: in this region of the diaphysis, bone tissue has replaced most of the cartilage
4. Development of the medullary (marrow) cavity: bone breakdown by osteoclasts forms the medullary cavity
5. Development of secondary ossification centres: these occur in the epiphyses of the bone
6. Formation of articular cartilage and epiphyseal plate: both structures consist of hyaline cartilage