Overview
This lecture explains the anatomy of the human skeletal system, its functions, bone classification and structure, and the process of bone remodeling, emphasizing its importance during long-term space missions.
Effects of Space on Bones
- Astronauts Scott Kelly and Mikhail Kornienko spent a year on the ISS to study long-term microgravity effects on the body.
- Microgravity causes astronauts to lose 1–2% of bone mass every month.
- Rehabilitation after space missions can take years due to significant bone loss.
Functions of Bones
- Bones provide structural support, enable movement, and protect vital organs.
- Bones store essential minerals like calcium and phosphate, needed for nerve and muscle function.
- Bone marrow in bones produces blood cells (hematopoiesis) and stores fat for energy.
- Bones help regulate blood calcium levels and produce hormones like osteocalcin, which affects metabolism.
Bone Classification and Structure
- The human body has 206 bones, divided into axial (skull, spine, rib cage) and appendicular (limbs, pelvis, shoulder blades) groups.
- Bones are classified by shape: long (limb bones), short (wrist, ankle bones), flat (skull, sternum), and irregular (vertebrae, pelvis).
- All bones have a compact (cortical) outer layer and spongy bone inside, containing bone marrow (red for blood cells, yellow for fat).
Microanatomy of Bones
- Osteons are cylindrical structures running parallel to bone’s axis, made of concentric lamellae (rings).
- Lamellae have alternating collagen fiber directions for added strength and resistance to twisting.
- Osteons have central canals with blood vessels and nerves; lacunae in lamellae house osteocytes (mature bone cells).
Bone Cells and Remodeling
- Osteocytes monitor bone health and trigger repair when stressed or damaged.
- Osteoblasts build new bone matrix by depositing collagen and minerals.
- Osteoclasts break down bone tissue via enzymes and acid in a process called resorption.
- Bone remodeling is a balance between osteoblast and osteoclast activity and is stimulated by physical activity.
Bone Health in Microgravity
- In space, osteoclast activity increases and osteoblast activity decreases, leading to net bone loss.
- Astronauts must exercise regularly in space to reduce bone loss, but it does not fully prevent it.
Key Terms & Definitions
- Microgravity — a condition in which objects appear to be weightless.
- Axial skeleton — bones along the body's central axis (skull, spine, ribs).
- Appendicular skeleton — bones of the limbs and girdles (pelvis, shoulders) that attach to the axial skeleton.
- Compact (cortical) bone — dense outer layer of bone.
- Spongy bone — porous, internal bone tissue containing bone marrow.
- Osteon — cylindrical structural unit in compact bone.
- Lamellae — concentric layers of bone matrix within an osteon.
- Osteocyte — mature bone cell maintaining bone tissue.
- Osteoblast — bone-forming cell.
- Osteoclast — bone-resorbing (breaking down) cell.
- Bone remodeling — process of bone renewal and repair.
- Hematopoiesis — formation of blood cells in bone marrow.
Action Items / Next Steps
- Review bone classifications, structures, and cell functions.
- Prepare for questions about bone remodeling and the effects of microgravity on bone health.