The Skeletal System
The skeletal system domain tests bone as living tissue, the organization of the axial and appendicular skeleton, joint structure and movement, and the skeleton’s roles in support, protection, mineral storage and blood cell production. Expect questions that link bone cells, growth, remodeling and calcium balance.
Core rules and concepts
- Bone matrix is about one-third organic material, mainly type I collagen, and two-thirds mineral, mainly hydroxyapatite. Collagen gives tensile strength; mineral gives compressive strength.
- Osteoblasts form bone, osteocytes maintain matrix and sense mechanical load, and osteoclasts resorb bone. Parathyroid hormone raises blood calcium by increasing resorption and renal calcium retention; calcitonin lowers it.
- Compact bone is organized into osteons with concentric lamellae around a central canal. Spongy bone has trabeculae and often contains red marrow.
- Intramembranous ossification forms flat bones directly from mesenchyme. Endochondral ossification uses a cartilage model and produces most long bones.
- Long bones grow in length at the epiphyseal plate and in width by appositional growth under the periosteum. The epiphyseal line is the remnant after plate closure.
- Synovial joints have a capsule, synovial fluid and hyaline cartilage. Ball-and-socket joints allow multiaxial movement; hinge joints allow flexion and extension.
Worked example
A radiograph of an adolescent shows a radiolucent band at the distal femur, while an adult shows a thin dense line in the same location. The adolescent band is the epiphyseal plate, cartilage that permits longitudinal growth. The adult line is the epiphyseal line, the remodeled remnant after closure. This distinction matters because a fracture through an open plate can disturb growth.
Traps that cost marks
Do not reverse osteoblast and osteoclast roles. Do not say compact bone lacks blood supply. Do not confuse the epiphyseal plate with the epiphyseal line. Remember that parathyroid hormone raises blood calcium, even though it acts on bone by increasing resorption. Synovial fluid nourishes cartilage and lubricates; it is not the same as lymph.
Worked example
If a fracture occurs through the epiphyseal plate, growth can be impaired because the cartilage template is damaged.