1. Even though most osteocytes have no direct contact with a blood vessel, they can pass blood-borne chemicals from cell to cell by way of the osteocyte processes in the canaliculi of the bone.

2. When the first few crystals of hyroxyapatite form, they act as nuclei or seed crystals on which more calcium phosphate is deposited. The more hydroxyapatite that forms, the faster these minerals precipitate out of the blood. Thus mineralization is a self-accelerating positive feedback process.

3. A drop in plasma calcium concentration stimulates the secretion of parathyroid hormone, which works through several mechanisms to raise the calcium concentration. In correcting the initial change, this exemplifies negative feedback.

4. It is rather difficult to explain bone elongation without referring to cells. At best, one could say that in puberty there is a sudden thickening of the cartilage plates between the diaphysis and epiphysis, and this corresponds to a growth in the individual's height. At the end of adolescence, this cartilage is gone, the epiphysis and diaphysis are fused together by bone, and growth ceases. One can see that this explanation is not nearly as informative or insightful as an explanation based on proliferation and hypertrophy of cartilage cells in the epiphyseal plate and calcification of the cartilage trabeculae by osteoblasts. This is one of the innumerable processes in human physiology that relies on cell theory for meaningful explanation.

5. The trabeculae are not randomly arranged, but are organized around lines of stress applied to the bone. Thus the microscopic form of spongy bone is based on its function–to provide maximum stress resistance with minimum weight.

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