Bones and their Structure

Question 1

Describe the two major divisions of the skeleton.

Answer 1

The skeletal system is made up of the axial and appendicular divisions.

• Axial: consists of the bones of the skull, thorax, and vertebral column.
  
  • Forms the longitudinal axis of the body.
• Appendicular: includes the bones of the limbs and the pectoral and pelvic girdles that attach the limbs to the axial skeleton.

Question 2

List the major functions of the skeletal system. [5]

Answer 2

1. Support: provides structural support for the entire body; individual bones or groups of bones provide a framework for the attachment of soft tissues and organs.
2. Store minerals and lipids: calcium salts in bone are a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in blood; calcium is the most abundant mineral in the body; bones also contain adipose tissue, which stores lipids as energy reserves
3. Produce blood cells: red blood cells, white blood cells, and platelets are produced in the red bone marrow, which fills the internal cavities of many bones
4. Protection: the skeleton surrounds delicate tissues and organs; the ribs protect the heart and lunbs; the skull encloses the brain; the vertebrae shield the spinal cord; the pelvis cradles digestive and reproductive organs
5. Leverage: many bones of the skeleton function as levers that can change the magnitude and direction of the forces skeletal muscles generate; the movement produced range from motions of a fingertip to powerful changes in the position of the entire body

Question 3

Identify the six broad categories for classifying a bone according to shape.

Answer 3

• Flat: have thin, roughly parallel surfaces (e.g., parietal bone)
• Sutural: small, flat, irregularly shaped bones between flat bones of the skull akin to a jigsaw puzzle piece
• Long: elongated and slender (e.g., femur)
• Irregular: complex shapes with short, flat, noticed, or ridged surfaces (e.g., spinal vertebrae)
• Sesamoid: generally small, flat, and shaped somewhat like a sesame seed; developed inside tendons (e.g., patella)
• Short: small and boxy (e.g., carpal/tarsal bones)

Question 4

What are bone markings?

Answer 4

Surfaces of bones have characteristic surface features (i.e., bone markings) which are projections, depressions, and openings related to particular functions.

• Projections: bumps that form where muscles, tendons, and ligaments attach and where adjacent bones form joints.
• Depressions and openings: sites where blood vessels or nerves lie alongside or penetrate the bone.

Question 5

Long bones transmit forces along the shaft and have a rich blood supply. Recall that typical long bones consist of both compact and spongy bone.

Identify the parts of a typical long bone and describe its internal structure.

Answer 5

• The epiphysis is an expanded area found at each end of the bone.
  
  • Consists largely of spongy bone (a.k.a. trabecular bone).
• The metaphysis is a narrow zone that connects the epiphysis to the shaft of the bone.
• The diaphysis (shaft) is long and tubular.
  
  • Relatively dense compact bone makes up the wall of the diaphysis; forms sturdy protective layer around central space
• The medullary cavity (i.e., marrow cavity) is a space within the hollow shaft filled with two types of bone marrow:
  
  1. Red bone marrow: highly vascular; involved in production of blood cells
  2. Yellow bone marrow: adipose tissue important as an energy reserve

Question 6

Where is articular cartilage found and how is it nourished?

Answer 6

• Articular cartilage covers portions of the epiphysis that articulate with other bones.
  
  • The cartilage is avascular and relies on diffusion from synovial fluid within the joint to obtain oxygen and nutrients and eliminate wastes.

Question 7

Why are bone injuries usually painful?

Answer 7

A fibrous periosteum covers the outer bone surfaces except at joints. It contains an extensive network of blood vessels, lymphatic vessels, and sensory nerves. The sensory nerves travel with the brances of nutrient arteries and the metaphyseal arteries to innervate the diaphysis, medullary cavity, and epiphyses.

Question 8

Identify the types of cells in bones and list their major functions.

Answer 8

Bone has a calcified matrix maintained and altered by osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

• Osteogenic cells: stem cells that divide to produce daughter cells that differentiate into osteoblasts to maintain their populations.
• Osteoblasts: produce new bone matrix by ossification/osteogenesis
• Osteocytes: maintain the bone matrix; continually recycle the protein and minerals of surrounding matrix; turnover rate varies from bone to bone; cannot divide, and a lacuna (pocket sandwiches between layers of matrix) never contains more than one osteocyte.
• Osteoclasts: remove bone matrix by osteolysis; large cells with 50 or more nuclei; secrete acids and protein-digesting enzymes that dissolve the matrix and release stored minerals.

Question 9

Describe the structure and function of compact bone.

Answer 9

Compact bone consists of parallel osteons (the basic functional unit of mature compact bone).

Osteons in the diaphysis of a long bone are parallel to the long axis of the shaft; thus, the shaft does not bend, even with extreme force. The femur can withstand 10 - 15 times the body’s weight without breaking. Yet a much smaller force applied to the side of the shaft can break any long bone, even the femur.

Question 10

Describe the structure and function of spongy bone.

Answer 10

Spongy bone consists of a network of trabeculae; found where bones are not heavily stressed or where stresses arrive from many directions.

Lamellae are not arranged in osteons; the matrix forms struts and plates called trabeculae. May contain red or yellow bone marrow which contain blood vessels that deliver nutrients to the trabeculae and remove wastes generated by osteocytes.

Question 11

Describe the process of appositional bone growth.

Answer 11

Involves the periosteum and the endosteum; osteogenic cells in the inner layer of the periosteum produce daughter cells that differentiate into osteoblasts and add bone matrix to the surface. This adds successive layers of circumferential lamellae to the outer surface of the bone. Osteoblasts trapped between these lamellae differentiate into osteocytes. Over time, deeper lamellae are recycled and replaced with the osteons of a typical compact bone. As bone is being added through appositional growth, osteoclasts are removing and recyling lamellae at the inner surface; the medullary cavity gradually enlarges as the bone increases in diameter. Appositional bone growth is important in increasing diameters of existing bone but does not form the original bone.

Bone matrix is removed by osteoclasts.

Bone deposited by superficial osteoblasts.

Question 12

Distinguish between the periosteum and the endosteum.

Answer 12

• The periosteum (1) isolates bone from surrounding tissues, (2) provides a route for blood and nervous supply, and (3) actively participates in bone growth and repair.
• The endosteum is an incomplete cellular layer that lines the medullary cavity; active during bone growth, repair, and remodelling.

Question 13

Endochondral ossification replaces a cartilage model with bone.

Describe the process of endochondral ossification.

Answer 13

1. As the cartilage enlarges, chondrocytes near the centre of the shaft increase in size. The matrix is reduced. Condrocytes disintegrate, leaving cavities within the cartilage.
2. Blood vessels grow around the cartilage. The cells of the inner layer of the perichondrium differentiate into osteoblasts. The shaft of cartilage then becomes ensheathed in a superficial layer of bone.
3. Blood vessels penetrate the cartilage. Fibroblasts migrate in the blood and differentiate into osteoblasts and begin producing spongy bone at a primary ossification centre. Bone formation spreads along the shaft towards both ends.
4. Remodeling occurs as growth continues, creating a medullary cavity. The bone of the shaft becomes thicker, and the cartilage near each epiphysis is replaced by shafts of bone. Growth involves increases in length and diameter.
5. Capillaries and osteoblasts migrate into the epiphyses creating secondary ossification centres. Interstitial growth takes place at each metaphysis.
6. Epiphyses are filled with spongy bone. Articular cartilage remains exposed to joint cavity; reduced over time to superficial layer. At each metaphysis, an epiphyseal plate separates the epiphysis from the diaphysis.
7. Chondrocytes on epiphyseal side continually add new cartilage. Chondrocytes degenerate on the diaphyseal side. Osteoblasts are continuously invading this zone and replacing it with bone, moving towards the epiphysis.
8. Completion of epiphyseal growth (puberty induced) in epiphyseal closure when osteoblasts produce bone faster than chondrocytes produce cartilage.

Question 14

What is intramembranous ossification?

Answer 14

Forms bone without a prior cartilage model.

Question 15

List the minerals stored in bone.

Answer 15

Calcium (most abundant)

Potassium

Sodium

Magnesium

Carbonate

Phosphate

Question 16

Identify the organs involved in calcium homeostasis.

Answer 16

• In the intestines, calcium and phosphate ions are absorbed from the diet; hormonally regulated.
• In the kidenys, the levels of calcium and phosphate ions lost in the urine are hormonally regulated.
• Within the skeleton, osteoblasts continually deposit new bone matrix. At the same time osteoclasts erode existing matrix, releasing calcium and phosphate ions into the circulation. The balance between osteoblast and osteoclast activity is hormonally regulated.

Question 17

Discuss the effects of hormones on bone development, and explain the homeostatic mechanisms involved when there is a low calcium ion level in the blood.

Answer 17

The primary hormones regulating calcium ion metabolism are parathyroid hormone, calcitriol, and calcitonin.

• When blood calcium ion concentration falls below normal, cells of the parathyroid glands, embedded in the thyroid gland in the neck, release parathyroid hormone (PTH) into the bloodstream.
• Bone response: Osteoclasts do not have PTH receptors. PTH binds adjacent osteoblasts, which causes them to release a hormone (RANKL) that stimulates immature osteoclasts to differentiate into mature osteoclasts, which erode the bone matrix, releasing stored calcium ions.
  
  • Calcium released into bloodstream.
• Intestinal response: PTH enhances the calcium-absorbing effects of calcitriol on the intestines; rate of absorption increased
  
  • Calcium absorbed quickly
• Kidney response: PTH increases renal production of calcitriol which stimulates calcium reabsorption by the kidneys and calcium absorption in the intestines
  
  • Conserves calcium (less lost in urine)

Question 18

Discuss the effects of hormones on bone development, and explain the homeostatic mechanisms involved when there is a high calcium ion level in the blood.

Answer 18

The primary hormones regulating calcium ion metabolism are parathyroid hormone, calcitriol, and calcitonin.

• When blood calcium ion concentration rises above normal, C cells of the thyroid gland secrete calcitonin.
• Bone response: calcitonin decreases osteoclast activity but does not affect osteoblasts, which continue to deposit calcium ions within the bone matrix.
  
  • Calcium release slowed
• Intestinal response: decreasing PTH or calcitriol level results in a decrease in the rate of calcium ion absorption by the intestines
  
  • Calcium absorbed slowly
• Kidney response: increased calcitonin levels has an inhibitory effect on the kidneys and suppresses calcium ion reabsorption.
  
  • Calcium excreted in urine.