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🫀 Science · Human Anatomy and Physiology · Lesson 2 of 12 · 5 min

Skeletal system: bone types, axial vs appendicular, joints and bone cells

🦴 The adult skeleton has 206 bones, but the TEAS mostly asks how they are grouped, how they join, and which cells build and break them down.

💡 Key idea

Osteoblasts build, osteoclasts break down, and the axial skeleton is the body's central axis.

🧠 The idea

The skeleton supports and protects, works with muscles for movement, stores calcium and phosphate, and makes blood cells in red marrow (HEMATOPOIESIS). The AXIAL skeleton (80 bones) is the skull (including the ear ossicles), hyoid, vertebral column, ribs and sternum; the APPENDICULAR skeleton (126) is the limbs plus the pectoral and pelvic girdles. Bones are classed by shape: long (femur, humerus), short (carpals), flat (sternum, skull bones), irregular (vertebrae) and sesamoid (patella). A long bone has a shaft (diaphysis) with a marrow cavity and ends (epiphyses) filled with spongy bone; growth in length happens at the epiphyseal plate. Bone cells: OSTEOBLASTS build bone matrix, osteocytes maintain it from within lacunae, OSTEOCLASTS break it down. Compact bone is organized in osteons around central canals. JOINTS are classed by movement: synarthroses are immovable (skull sutures), amphiarthroses slightly movable (pubic symphysis), diarthroses freely movable synovial joints (hinge at the elbow, ball-and-socket at the hip, pivot between C1 and C2).

✏️ Worked example

Classify the scapula. It is part of the shoulder (pectoral) girdle, which connects the arm to the trunk, so it is APPENDICULAR, even though it sits on the back. Now the sternum: it is in the midline chest with the ribs, so it is AXIAL. Shape? Both are FLAT bones. Last, which joint lets the arm swing in a full circle? The shoulder is a ball-and-socket synovial joint, the most freely movable type.

📌 Takeaways

  • Axial = skull, hyoid, vertebrae, ribs, sternum; appendicular = limbs and girdles.
  • Osteoblasts build bone, osteoclasts resorb it, osteocytes maintain it.
  • Synovial joints (hinge, pivot, ball-and-socket) are the freely movable ones.

✅ Check yourself

Question 1 of 3

Which bone belongs to the appendicular skeleton?

  1. Sacrum
  2. Mandible
  3. Clavicle
  4. Hyoid bone
Show the answer and rationale

Answer: C

The clavicle is part of the pectoral girdle, which is appendicular. The hyoid, mandible (skull) and sacrum (vertebral column) are all axial bones.

Question 2 of 3

Which cells break down bone matrix and release calcium into the blood?

  1. Osteoblasts
  2. Chondrocytes
  3. Osteocytes
  4. Osteoclasts
Show the answer and rationale

Answer: D

Osteoclasts resorb bone and release calcium. Osteoblasts lay down new matrix, osteocytes are mature cells maintaining bone, and chondrocytes are cartilage cells.

Question 3 of 3

The knee and elbow are examples of which kind of joint?

  1. Pivot joint
  2. Hinge joint
  3. Saddle joint
  4. Suture
Show the answer and rationale

Answer: B

The knee and elbow move mainly in one plane, like a door hinge. A pivot joint allows rotation (C1 on C2), a saddle joint is at the base of the thumb, and sutures are immovable skull joints.

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