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Skeletal System Overview

Sep 8, 2025

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Overview

This lecture explores the anatomy and physiology of the skeletal system, emphasizing bone structure, function, and the effects of microgravity on bone health. The year-long mission of astronauts Scott Kelly and Mikhail Kornienko aboard the International Space Station serves as a case study for understanding how extended time in space impacts bones.

Effects of Space on Bones

  • In microgravity, astronauts lose 1–2% of bone mass each month, a rate much higher than the 1–2% per year typically seen in elderly people on Earth.
  • Microgravity leads to increased bone resorption (breakdown) and decreased bone formation, resulting in significant bone loss.
  • Over a year in space, astronauts like Kelly and Kornienko can lose up to 20% of their bone mass. While most of this loss is reversible after returning to Earth, rehabilitation can take years and is a difficult process.
  • Astronauts are required to exercise at least 15 hours per week to slow bone loss, but this regimen cannot fully prevent the reduction in bone density.
  • The lack of gravity means osteocytes receive less mechanical stimulus, causing osteoclasts to increase bone breakdown while osteoblasts reduce bone formation. This imbalance accelerates bone loss.
  • The mission’s primary goal was to study the physical effects of long-term weightlessness, especially on the skeletal system.

Functions of Bones

  • Provide structural support and scaffolding for the body, allowing movement.
  • Protect vital organs (e.g., the skull protects the brain, the rib cage protects the heart and lungs).
  • Store essential minerals such as calcium and phosphate, which are necessary for neuron function and muscle contraction.
  • House bone marrow, which is responsible for hematopoiesis (production of blood cells) and energy storage as fat.
  • Regulate blood calcium levels and produce osteocalcin, a hormone that influences bone formation and helps protect against glucose intolerance and diabetes.
  • Bones are dynamic, living tissues that constantly break down, regenerate, and repair themselves. The human skeleton is essentially renewed every 7 to 10 years.

Bone Classification

  • The adult human body contains 206 bones, which are categorized by location and shape:
  • Axial skeleton: Includes bones along the body’s vertical axis—skull, vertebral column, and rib cage. These bones provide foundational support, carry other body parts, and protect organs.
  • Appendicular skeleton: Comprises the bones of the limbs and the girdles (pelvis, shoulder blades) that attach them to the axial skeleton, enabling movement.
  • Bones are also classified by shape:
    • Long bones: Longer than they are wide (e.g., femur, tibia, finger bones). These are the classic limb bones.
    • Short bones: Cube-shaped (e.g., talus and cuboid in the foot, scaphoid in the wrist).
    • Flat bones: Thin and often curved (e.g., sternum, scapulae, bones of the skull).
    • Irregular bones: Complex, unique shapes (e.g., vertebrae, pelvis).

Bone Structure

  • All bones share a similar internal structure:
    • Compact (cortical) bone: Dense, smooth outer layer that provides strength.
    • Spongy (trabecular) bone: Porous, honeycomb-like inner region made up of trabeculae, which help resist stress.
  • Bone marrow is found within the spongy bone:
    • Red marrow: Produces blood cells.
    • Yellow marrow: Stores energy as fat; a rich calorie source for predatory animals.
  • In long bones:
    • Epiphyses (ends): Contain spongy bone and red marrow.
    • Diaphysis (shaft): Surrounds a hollow medullary cavity filled with yellow marrow.
  • In flat, short, and irregular bones, spongy bone is sandwiched between layers of compact bone, resembling a “spongy bone sandwich.”
  • Despite their solid appearance, bones are filled with layered plates and small tunnels, making them intricate and highly organized at the microscopic level.

Bone Microanatomy

  • Osteons: The basic structural units of compact bone. These cylindrical, weight-bearing structures run parallel to the bone’s axis and are made up of concentric rings (lamellae).
    • Each lamella contains collagen fibers running in one direction, while adjacent lamellae have fibers running in the opposite direction, creating an alternating pattern that helps bones resist twisting (torsion) stress.
  • Central canals: Run through the center of each osteon, containing nerves and blood vessels that nourish bone tissue.
  • Lacunae: Small, oblong spaces between lamellae that house osteocytes (mature bone cells).
  • The arrangement of compact and spongy bone, as well as the distribution of marrow, varies depending on bone type, but the microanatomy is consistent across all bones.

Bone Cells and Remodeling

  • Osteocytes: Mature bone cells located in lacunae. They monitor and maintain the bone matrix, acting as supervisors for bone health.
  • Osteoblasts: Bone-building cells that secrete collagen and enzymes to absorb minerals from the blood, forming new bone tissue. They are especially active from fetal development until about age 25.
  • Osteoclasts: Bone-resorbing cells that break down bone tissue by digesting collagen and dissolving minerals, releasing them into the blood (a process called resorption).
  • Bone remodeling is a continuous process that balances bone breakdown and formation:
    • Osteocytes detect stress, strain, or microscopic damage and signal osteoclasts to the site.
    • Osteoclasts secrete enzymes and acids to break down old bone tissue, then undergo apoptosis (self-destruction).
    • Osteoblasts are recruited to rebuild bone, laying down new collagen and minerals.
    • The ratio of osteoclast to osteoblast activity changes with physical activity, injury, or environmental conditions.
  • Exercise and mechanical loading stimulate bone remodeling and increase bone strength, while lack of mechanical stimulus (as in microgravity) disrupts this balance.

Impact of Exercise and Space Conditions

  • Regular exercise stimulates bone remodeling, promoting both bone breakdown and formation, and ultimately increasing bone strength.
  • In microgravity, the lack of mechanical loading reduces bone formation and increases bone resorption, leading to rapid bone loss even with exercise.
  • Osteocytes in space receive less loading stimulus, causing osteoclasts to increase bone breakdown while osteoblasts decrease bone formation.
  • Maintaining bone health in space requires not only exercise but also strategies to counteract the imbalance in bone cell activity caused by low gravity.
  • The challenge for long-term space missions is to prevent the body from consuming its own skeleton due to this imbalance.

Key Terms & Definitions

  • Microgravity: An environment with very low gravity, such as in space.
  • Hematopoiesis: The production of blood cells in bone marrow.
  • Osteon: The cylindrical structural unit of compact bone.
  • Lamellae: Concentric layers within osteons.
  • Osteocyte: A mature bone cell that maintains bone tissue.
  • Osteoblast: A cell responsible for building new bone.
  • Osteoclast: A cell that breaks down bone tissue.
  • Resorption: The process of breaking down bone tissue and releasing minerals into the blood.
  • Epiphysis: The end part of a long bone.
  • Diaphysis: The shaft or central part of a long bone.
  • Medullary cavity: The hollow cavity within the diaphysis, containing bone marrow.
  • Trabeculae: The supporting structures in spongy bone.
  • Osteocalcin: A hormone produced by bones that regulates bone formation and helps protect against glucose intolerance and diabetes.

Action Items / Next Steps

  • Review bone classification, structure, and function for the upcoming quiz.
  • Read the textbook section on bone remodeling for a deeper understanding of the process and its regulation.
  • Consider how microgravity and exercise influence bone health, and be prepared to discuss these effects in detail.

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