Bone cells and bone tissue

An ion is an atom or group of atoms that has gained or lost electrons, so it carries a charge. Calcium ions (Ca2+) and phosphate ions are the ions of bone mineral.

• Correct: An atom or molecule with an electric charge:
• A molecule made of amino acids:
• A neutral atom with a full outer shell:

Collagen fibers are thick bundles of collagen protein with great tensile strength. They are the main fiber of bone matrix.

• Elastic fibers:
• Correct: Collagen fibers:
• Reticular fibers:

Through childhood, yellow marrow replaces red marrow in the limb shafts. Adult red marrow remains in the flat bones, vertebrae, hip bones and the proximal epiphyses of the thigh and upper arm bones.

• In the shafts of the long bones of the limbs:
• In the articular cartilage:
• Correct: In the flat bones, vertebrae, hip bones and the upper ends of the thigh and upper arm bones:

The osteoclast is a very large cell with several nuclei and a frilled membrane pressed against the bone. It resorbs bone.

• Osteoblast: Osteoblasts are medium-sized cells with one nucleus that sit in rows on bone surfaces.
• Osteocyte: An osteocyte is a small cell with long branching arms, inside the matrix.
• Osteogenic cell: An osteogenic cell is a flat stem cell with one nucleus.
• Correct: Osteoclast: Correct. Huge, many nuclei, frilled edge on the bone: an osteoclast.

Vinegar is an acid, and acid dissolves the calcium phosphate (hydroxyapatite) crystals, just as the acid from osteoclasts does. What remains is the collagen framework, which is flexible but cannot resist squeezing.

• The collagen and ground substance: If the collagen and ground substance had gone, the bone would be hard and brittle, as it is after baking. The bone bends because they remain.
• The osteocytes: Removing cells would not change how hard or flexible the matrix is; the matrix sets those properties.
• Correct: The calcium phosphate crystals: Correct. The acid dissolved the mineral and left the flexible collagen.
• The periosteum: The periosteum is a thin outer membrane. Its loss would not make the whole bone rubbery.

Collagen resists pulling and gives bone slight flexibility. Mineral without good collagen is like chalk: hard but brittle, so it snaps instead of bending. This inherited condition is called brittle bone disease.

• Her bones cannot store calcium: The question says her bone is fully mineralized, so storing mineral is not the problem.
• Correct: Her bone lacks the flexibility to bend under a load: Correct. Faulty collagen leaves hard but brittle bone.
• Her osteoclasts cannot resorb old or damaged bone: Osteoclasts are not affected by a collagen defect, and failing to resorb bone would not in itself make bones snap.
• Her bones have too few lacunae for osteocytes: Lacunae hold osteocytes. A collagen defect does not remove them, and it is the matrix that sets strength.

Osteoclasts form when several precursor cells from the blood-forming line in red marrow, the line that also makes macrophages, fuse. That is why they have several nuclei. They are not part of the osteogenic cell to osteoblast to osteocyte line.

• Osteoblasts that change their job: Osteoblasts become osteocytes, not osteoclasts. The two cell types come from different lines.
• Correct: Fused precursor cells of the macrophage line: Correct. Osteoclasts are fused cells of the macrophage line.
• Osteocytes released from their lacunae: Osteocytes stay in their lacunae for life. They do not become osteoclasts.
• Osteogenic cells of the periosteum: Osteogenic cells make osteoblasts. Osteoclasts come from a separate, blood-forming line.

Mineralized matrix is almost impermeable. Osteocytes get oxygen and nutrients only from vessels in the central canal, passed along through canaliculi. Block the canaliculi and the cells starve.

• They carry on normally: They cannot carry on normally: nutrients cannot diffuse through mineralized matrix, which is why canaliculi exist.
• They divide to make new canaliculi: Osteocytes do not divide.
• They turn into osteoclasts: Osteocytes do not become osteoclasts; the two come from different cell lines.
• Correct: They die, cut off from oxygen and nutrients: Correct. The canaliculi are their only supply line.

Spongy bone is a lattice of trabeculae with marrow in the spaces between them. It fills the ends of long bones and the inside of other bones, and its osteocytes sit in lacunae like those of compact bone.

• Correct: Built of trabeculae: Correct. Trabeculae are its struts.
• Organized into osteons with central canals: Osteons with central canals are the units of compact bone. Trabeculae are thin enough not to need a central canal.
• Correct: Spaces that can hold red marrow: Correct. Its spaces hold marrow, red in some bones.
• Correct: Fills the ends of long bones: Correct. The epiphyses are filled with spongy bone.
• Makes up about 80% of the skeleton's mass: About 80% of the skeleton's mass is compact bone. Spongy bone is about 20%.
• Correct: Osteocytes in lacunae: Correct. Osteocytes of spongy bone also live in lacunae, linked by canaliculi.

The calcium and phosphate freed from the matrix move into the blood. That is how bone works as a mineral bank: osteoclasts withdraw minerals from it.

• The osteoclast seals its frilled membrane onto the bone surface: This step is right. The sealed space keeps the acid and enzymes against the bone.
• It pumps hydrogen ions onto the bone: This step is right. Pumped hydrogen ions acidify the sealed space.
• The acid dissolves the calcium phosphate crystals: This step is right. Acid dissolves hydroxyapatite.
• Enzymes from its lysosomes digest the collagen: This step is right. Once the mineral is gone, lysosomal enzymes digest the exposed collagen.
• Correct: The released calcium and phosphate are locked away inside the osteoclast: This is the error. The released minerals pass into the blood rather than being stored in the cell.