Overview
This lecture explores the anatomy and function of the skeletal system, focusing on bone structure, bone remodeling, and the effects of microgravity on bone health, especially in astronauts.
Effects of Space on Bones
- Astronauts like Scott Kelly and Mikhail Kornienko spent a year on the ISS to study effects of microgravity on bones.
- In microgravity, astronauts lose 1–2% of their bone mass per month, compared to 1–2% per year in elderly people.
- Bone loss in space is partly reversible, but rehabilitation can take years.
Bone Structure and Function
- Bones are living, dynamic organs that constantly regenerate and repair throughout life.
- Human bodies have 206 bones, divided into axial (skull, vertebral column, rib cage) and appendicular (limbs, pelvis, shoulder blades) groups.
- Bones provide support, movement, mineral storage, blood cell production (hematopoiesis), energy storage, and hormone production.
Classification and Internal Anatomy of Bones
- Bones are classified by shape: long (femur), short (wrist bones), flat (sternum), irregular (vertebrae).
- All bones have a dense outer layer of compact bone and an inner layer of spongy bone.
- Spongy bone contains trabeculae for support and bone marrow (red for blood cells, yellow for fat storage).
- Long bones have spongy bone at ends (epiphyses) and a medullary cavity with yellow marrow in the shaft (diaphysis).
Bone Microanatomy and Remodeling
- Osteons are cylindrical structural units of compact bone, composed of concentric lamellae with alternating collagen directions for strength.
- Lacunae in lamellae house osteocytes, mature cells that monitor and maintain bone tissue.
- Osteoblasts build new bone by producing collagen and absorbing minerals.
- Osteoclasts break down bone by dissolving minerals and digesting collagen (resorption).
- Bone remodeling is a balance of osteoblast and osteoclast activity, triggered by osteocytes in response to stress or injury.
Exercise, Microgravity, and Bone Health
- Physical activity increases bone remodeling and strength, while microgravity reduces bone formation and increases bone breakdown.
- Space crews need to exercise ~15 hours per week to slow bone loss, but this does not stop density loss entirely.
Key Terms & Definitions
- Axial skeleton — Bones along the body's vertical axis (skull, spine, rib cage).
- Appendicular skeleton — Bones of limbs and attachments (shoulder blades, pelvis).
- Compact (cortical) bone — Dense outer bone layer.
- Spongy bone — Porous inner bone with trabeculae and marrow.
- Osteon — Structural unit of compact bone.
- Lamellae — Concentric rings in osteons.
- Lacunae — Small spaces in bone containing osteocytes.
- Osteocyte — Mature bone cell that regulates bone maintenance.
- Osteoblast — Bone-building cell.
- Osteoclast — Bone-destroying (resorbing) cell.
- Bone remodeling — Process of resorption and formation to renew bone tissue.
- Hematopoiesis — Production of blood cells in bone marrow.
Action Items / Next Steps
- Review the shapes and functions of different types of bones.
- Study the process and importance of bone remodeling.
- Prepare for questions on how microgravity affects bone health.