Overview
This lecture covers the structure and function of bones, effects of microgravity on bone health, and the cellular process of bone remodeling, with reference to long-term space missions.
Effects of Microgravity on Bones
- Astronauts experience 1-2% bone loss per month in space, compared to 1-2% per year in elderly people.
- Bone loss in space is mostly reversible, but recovery takes years and significant rehabilitation.
- Exercise is critical for astronauts but does not fully prevent bone density loss in microgravity.
Functions and Structure of Bones
- Bones provide body support, protect organs, and enable movement.
- Bones store calcium, phosphate, and other minerals necessary for nerve and muscle function.
- Bone marrow produces blood cells (hematopoiesis) and stores energy as fat.
- Bones help maintain homeostasis by regulating blood calcium levels and producing the hormone osteocalcin.
- Human adults have 206 bones, grouped into axial (skull, vertebral column, rib cage) and appendicular (limbs, pelvis, shoulder blades) skeletons.
Classification and Internal Structure of Bones
- Bones are classified by shape: long, short, flat, and irregular.
- All bones have an outer compact (cortical) bone layer surrounding spongy bone.
- Spongy bone has a trabecular structure that resists stress and contains bone marrow (red for blood production, yellow for fat storage).
- Long bones have spongy bone at the ends (epiphyses) and a hollow medullary cavity in the shaft (diaphysis) filled with yellow marrow.
Bone Microanatomy and Remodeling
- Osteons are cylindrical units in compact bone, made of concentric lamellae with alternating collagen fiber directions for strength.
- Central canals in osteons carry nerves and blood vessels.
- Osteocytes (mature bone cells) live in lacunae and regulate bone maintenance.
- Osteoblasts build bone by laying down collagen and minerals; osteoclasts break it down (resorption).
- Bone remodeling is a balance of osteoblast and osteoclast activity, triggered by stress or injury, overseen by osteocytes.
- Exercise increases bone remodeling and bone strength.
Key Terms & Definitions
- Microgravity — environment with very low gravity, like space.
- Hematopoiesis — blood cell production in bone marrow.
- Osteon — basic structural unit of compact bone.
- Osteocyte — mature bone cell managing bone maintenance.
- Osteoblast — bone-building cell.
- Osteoclast — bone-resorbing (breaking down) cell.
- Bone remodeling — ongoing replacement of old bone tissue with new tissue.
- Epiphysis — end part of a long bone.
- Diaphysis — shaft of a long bone.
- Lamellae — concentric rings in osteons.
Action Items / Next Steps
- Review bone classification and structure.
- Understand the roles of osteoblasts, osteoclasts, and osteocytes.
- Prepare for questions on bone remodeling and effects of microgravity on bones.