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Bone Demineralization in Microgravity

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Osteoblasts form new bone by using minerals in the blood stream (e.g., calcium) ... There is a primary increase in serum ionized calcium ... – PowerPoint PPT presentation

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Title: Bone Demineralization in Microgravity


1
Bone Demineralization in Microgravity
2
The Skeletal System
3
Bone Tissues
  • Bone consists of both living and non-living
    tissues
  • 5 living tissue types blood vessels, nerves,
    osteoblasts, osteoclasts, osteocytes
  • Non-living substances include minerals and
    crystalline salts gt hydroxyapatite crystals
  • Ca10PO46OH2
  • Collagen fibers and hydroxyapatite crystals form
    a complex support structure

4
Osteo-huh?
  • Osteoblasts form new bone by using minerals in
    the blood stream (e.g., calcium)
  • Osteocytes are Osteoblasts that have retired
  • Osteoclasts eat away small holes of old bone
    These holes are refilled by osteoblasts

5
Bone Structure
  • The bone is divided into two major parts
  • The epiphysis is the expanded portion on both
    ends of the bone which forms joints with other
    bones
  • The diaphysis is the main shaft of the bone

6
Bone Structure
  • Periosteum tough covering
  • Articular cartilage Covers epiphysis as a joint
    cushion
  • Compact bone - surrounds medullary cavity
  • Medullary cavity hollow tube in diaphysis
    filled with marrow
  • Yellow marrow - found in adult bones, acts as fat
    storage
  • Red marrow found in spongy bone produces
    blood cells
  • Spongy bone developing region of epiphysis

7
Bone Structure
  • Compact Bone Structure
  • Blood vessels supply of nutrients to the bone
  • Haversian canal passage for the blood vessel
  • Spongy bone surrounds the medullary canal

8
Bone Composition
  • The complex network of collagen fibers and
    crystals give great tensile and compressional
    strength
  • Elastic collagen fibers allow the bone to stretch
  • Hydroxyapatite crystals line up as bricks in a
    wall, supporting compressional forces
  • Bones tend to lack torsional strength

9
Growth Factors
  • Hormones
  • - Somatotropin (Growth Hormone)
  • - Parathyroid hormone regulates calcium
    levels
  • Nutrition
  • - Vitamins A, C, and D
  • - Calcium
  • Exercise
  • - Bone mass increases with compressional
  • load

10
Homeostasis
11
Methods for Experimentation
  • Space Flight
  • Impractical and Expensive
  • Parabolic Flight
  • Short Time Span
  • Bed Rest Studies
  • Head-Down Tilt
  • Twin Studies
  • Animal Studies
  • Hind Limb Suspension
  • Vibration Studies

Story Time...
12
Effects of Microgravity
  • Hypothesis 1
  • Gravity is necessary for normal development of
    bone structure and decreased gravity or skeletal
    unloading causes defective skeletal growth.
  • Hypothesis 2
  • The response of bone to space flight will be
    localized and will differ not only from bone to
    bone, but also at different sites within the same
    bone.
  • Hypothesis 3
  • The type of housing (group vs. individual) will
    influence the bone response to space flight and
    the recovery from space flight.

13
Effects of Microgravity
  • Tests on Rats
  • The bones of some rats never stop growing
  • Experimenters looked at changes in bone length,
    diameter, and mass
  • Both weight bearing and
  • and non-weight bearing
  • bones were examined

14
Sampling in Microgravity
15
Effects of Microgravity
  • Animal Enclosure Module Control
  • (AEM-C) 5 rats in a cage
  • Animal Enclosure Module flight
  • (AEM-F) 5 rats floating in a cage
  • VIVarium Control animals
  • (VIV-C) rat housed alone
  • Research Animal Holding Facility Flight.
  • (RAHF-F) - Preflight control to establish baseline

16
Effects of Microgravity
  • Bone Length Changes
  • Non-weight bearing bones show no major changes
  • Weight bearing bones show minimal change probably
    caused by ion/fluid shifts

17
Effects of Microgravity
  • Bone Mass Changes
  • The bone mass in the vertebra and rib were not
    affected
  • The bone mass of the femur of flight rats was
    significantly smaller than that of control rats

18
Effects of Microgravity
  • Bone Diameter Changes
  • Bone diameter was significantly smaller in flight
    rats

19
Summary of Animal Results Microgravity
  • What did these tests reveal?
  • Bones slowly lose mass in microgravity
  • Mass is not lost in length, but in width
  • Length isnt responsible for bearing weight but
    diameter is
  • Many believe that mass and growth are
    proportional to compressional load

20
All Right, What Questions Remain?
21
Summary of 30 years of bone loss studies
  • Bone mass decreases by about 1.5 per month in
    microgravity
  • No conclusions can be drawn regarding the
    duration of bone loss
  • Calcium balance is altered
  • Negative calcium balance is produced by reduction
    in intestinal absorption and increased excretion
    through gastrointestinal tract and kidneys
  • Osteoblast proliferation and function are
    reduced.
  • There is a primary increase in serum ionized
    calcium
  • Average male bone density in the spine is
    1.5g/cm2
  • Risk of fracture when the density approaches
    1g/cm2. This amount of bone loss is conceivable
    for a person living in microgravity for one year
    or more.

22
Summary of 30 years of bone loss studies
  • However, no definitive statements can be made
    regarding the effect of microgravity on bone
    resporption, intestinal calcium absorption,
    regulatory factors, or the transduction mechanism
    by which microgravity signals bone.
  • Centrigfugation appears to prevent
    demineralization in microgravity (Jankovich et.
    al.)
  • Increased bone density above normal 1g levels was
    NOT observed for continuous exposure to
    hypergravity (1.5g, 2g, 2.5g)
  • This suggests a threshold for demineralization
    possibly less than 1g, which has implications as
    to the rotational speed necessary for an
    artificial gravity habitat.
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