Bone formation and growth

Chondrocytes sit in lacunae in cartilage matrix. Cartilage has no blood vessels, so nutrients reach them by diffusing through the matrix from the perichondrium.

• Correct: Chondrocytes, by diffusion through the matrix:
• Osteocytes, through canaliculi:
• Fibroblasts, from capillaries in the matrix:

Each widened end is an epiphysis; the shaft is the diaphysis, and the flared zone between them is the metaphysis.

• The diaphysis:
• Correct: The epiphysis:
• The medullary cavity:

Osteoblasts secrete osteoid, which then mineralizes. Osteoclasts resorb bone, and osteocytes maintain it.

• Osteoclast:
• Osteocyte:
• Correct: Osteoblast:

The flat bones of the skull roof form by intramembranous ossification: osteoblasts differentiate from mesenchymal cells within a sheet of mesenchyme and lay down bone directly, with no cartilage stage.

• Hyaline cartilage models: Cartilage models are the starting point for endochondral ossification, which forms the limb bones, vertebrae and skull base.
• Fibrocartilage: Fibrocartilage is not a template for bone formation.
• Correct: Sheets of mesenchyme: Correct. Skull roof bones form directly in mesenchyme.
• Epiphyseal plates: Epiphyseal plates are found in growing long bones, not in flat skull bones.

The flat bones of the skull dome form directly in mesenchyme and grow at their edges, without cartilage models or epiphyseal plates. A defect in plate chondrocytes shortens the long bones, which lengthen at the plates, and spares bones that never use them.

• Correct: It forms by intramembranous ossification, with no epiphyseal plates: Correct. No cartilage, no plates, so a plate defect cannot slow them.
• Its epiphyseal plates close before birth, ending growth early: The skull dome never has epiphyseal plates at all; they are features of long bones.
• Skull bones have no osteoblasts: All bone, including skull bone, is laid down by osteoblasts.
• Its cartilage model is replaced by bone faster than in the limbs: The skull dome has no cartilage model to replace; that is the point of intramembranous ossification.

Growth in length starts with cell division in the proliferative zone. Block it and less cartilage is made, lengthening slows, and the plate thins because replacement by bone continues. Growth in width uses a different mechanism and continues.

• Number of new chondrocytes made in the plate: down. The proliferative zone is where plate chondrocytes divide; blocking division stops new cells being made.
• Rate of growth in length: down. Fewer new cells means less cartilage added to push the epiphysis away, so there is less cartilage for bone to replace and the diaphysis lengthens more slowly.
• Appositional growth at the periosteum: no change. Width growth depends on osteoblasts under the periosteum, which the drug does not target.
• Thickness of the epiphyseal plate: down. Bone keeps replacing cartilage on the diaphysis side while little new cartilage is added on the epiphysis side, so the plate thins.

A cartilage model forms first. Its middle calcifies and many of its chondrocytes die, while the perichondrium becomes periosteum and lays a bone collar. A nutrient artery brings in bone-forming cells, creating the primary ossification center; osteoclasts then hollow out the medullary cavity. Around birth and in childhood, the epiphyses ossify from secondary centers.

• Correct order: 1. Chondrocytes build a hyaline cartilage model 2. The middle of the model calcifies as a bone collar forms around it 3. A nutrient artery invades and a primary ossification center forms 4. Osteoclasts open the medullary cavity 5. Secondary ossification centers form in the epiphyses

Those thin lines are epiphyseal lines. His plates have closed and been replaced by bone, so there is no cartilage left to grow interstitially. No supplement can add length. His bones can still thicken slowly by appositional growth.

• His bones can still grow in length slowly: Growth in length needs an open plate of cartilage. The lines show his plates have closed.
• His bones can grow in length if the line is stimulated: An epiphyseal line is bone, and bone cannot grow from within, so there is nothing to stimulate.
• Correct: His bones can no longer grow in length: Correct. Closed plates mean no more growth in length.
• His bones can no longer grow in width: Appositional growth at the periosteum continues through life, so width can still increase slowly.

Cartilage does not harden into bone. Its matrix calcifies, many of its chondrocytes die, and invading osteoblasts lay bone on the leftover calcified columns, which osteoclasts later clear away. Bone replaces the cartilage.

• It is converted into fibrocartilage: The model is hyaline cartilage, and bone replaces it; it does not turn into another kind of cartilage.
• It hardens into bone as minerals are added: Calcified cartilage is still cartilage; it only serves as a temporary scaffold for new bone.
• It stays inside the bone as a core: Only the articular cartilage and, until growth ends, the epiphyseal plate remain. The rest is removed.
• Correct: It is broken down and replaced by bone: Correct. The model is a template that bone replaces.

Appositional growth adds new bone under the periosteum, on the outside. Osteoclasts on the endosteum remove bone from the inside, widening the medullary cavity. The bone widens without its wall becoming ever thicker and heavier.

• Correct: Osteoblasts add bone at the periosteum while osteoclasts remove it at the endosteum: Correct. Build outside, remove inside.
• Chondrocytes divide in the shaft wall while osteoblasts add bone at the endosteum: There is no cartilage in the shaft wall to divide, and adding bone inside would narrow the cavity.
• Osteoclasts remove bone at the periosteum while osteoblasts add bone at the endosteum: That reversal would make the bone narrower, not wider.
• The epiphyseal plate adds bone to the outside of the shaft: The epiphyseal plate adds length at the ends; it does not add bone to the shaft's surface.