ASEN 5016 Lecture 12: Skeletal System

Objectives

1. Describe (top level) function / structure of the human skeletal system

2. Explain the process by which ‘dynamic homeostasis’ is maintained in bone

3. Describe the differential effects of space flight on the skeletal system

4. Distinguish characteristic bone differences between rodents and humans

5. Explain basics of countermeasure principles intended to minimize bone loss

1. Function and Structure of Skeletal System

Protect and support body mass
Facilitate movement
Provide leverage for muscles
Calcium and phosphate reservoir (needed for ATP production)
Protective milieu for immune system

Structure and makeup

Component	Volume	Mass
Water	1/3	15%
Organic	1/3	20%
Mineral	1/3	65%

· Organic material à 90-95 % Type I collagen (Type II = cartilage)

· Mineral à ~70% hydroxyapatite and ~30% other (Calcium Phosphates)

Overall density is ~1.2 g/ml

Skeletal System load distribution ‘optimized’ for upright posture in 1g

2. Dynamic Homeostasis

Skeleton has a "Service Life" of ~80 years

By contrast… skyscrapers are designed to last > 80 years, but they don’t move!

Aircraft typically have a design lifetime of ~30 years, as motion induces creep and fatigue

Design close to yield strength à shorter lifetime

Skyscrapers built with factors of safety around 5x

Aircraft have to be light, so they require a lot of structural maintenance

Bones are similarly undergoing maintenance, or "remodeling", to minimize weight and maximize lifetime

"Modeling" (OB>OC) only occurs through adolescence in humans

· Osteoclasts (OC) absorb old bone – damaged, fractured, fatigued

· Osteoblasts (OB) lay down new bone

“Remodeling” - OB:OC activity ~balanced in early adulthood

“Loss” - OC>OB as you age

Bone is a composite material analogous to concrete

· Steel: Collagen à tensile strength and elasticity

· Mortar: Mineral à compressive strength

Organic content is slightly higher in children (~35-40%)

Minerals replace collagen as we age à bones become more brittle

Compression increases mineralization, not size

A size increase is an injury response

Wolff’s Law (~1880)

Mechanical environment affects long and spongeous bone structure (not cranial) à forces acting on the bone stimulate "remodeling"

Flexure creates local tensile and compressive forces and the resultant bending causes a preferential shift of negative charges allowing Na⁺, Ca⁺⁺ and K⁺ ions to be channeled

The mechanically induced signal squeeze fluid through the channels, resulting in cellular messenger and charge change which causes osteoblast stimulation

Stress generated (streaming) potential ~ piezoelectric response

3. Space Flight Effects

Overall bone loss rate is ~0.5-2% / month

Space flight effects are not evenly distributed on the skeleton

Trabecular (spongy) vs. Cortical bone (compact)

Trabecular – high rate of turnover, predominantly the bones that carry most of the loading,
consequently, represent most of what is lost in space -- e.g. heel bone (os calcis), hip joints, patella

Ca⁺⁺ excretion shows no signs of abatement after 84 days on Skylab

Long term, Ca⁺⁺balance becomes increasingly negative

Increases risk to kidney stones and fracture potential

Bone loss not seen to be self-limiting as with the cardiovascular system, fluid shift, and muscle loss phenomena

4. Rodents vs. Humans

Tail suspension models for mice and rats

· Rodents are constantly "modeling"

· Undergo a reduced rate of increase in growth (decreased OB activity or formation) in space

· Similar to children, not adults

· Adult humans are "remodeling"

· Undergo an increased rate of resorption (increased OC activity) in space

· Haversian Canals in humans allow OB/OC cells to get to a greater surface area of bone

· Rodents do not have these à OB/OC activity is only at surface

5. Countermeasure Principles

Weight loading (impact) or artificial gravity would be best

Treadmill vs. cycle ergometer

Exercise – increases mineralization

EMF – electrically stimulates bone growth

Research indicates whole body stimulation causes degradation, but local stimulation can enhance growth

Vibration Therapy – similar to loading (~ac rather than ~dc signal)

Pharmaceutics – similar to clinical osteoporosis treatment, although whether or not full bone mass recovery postflight is possible is unclear

Nutritional factors

Current research is primarily aimed at identifying the fundamental mechanisms causing bone loss in order to develop effective countermeasures