Osteoblasts form bone by crystalizing calcium phosphate around collagen.

Osteoclasts are responsible for bone resorption and osteoblasts are responsible for bone re-formation. While bed-ridden, this man will have a decrease in osteoblast production since he was not moving during the three weeks he was in a coma. His osteoclast production, however, would probably stay the same as it was before the accident (and therefore the man would experience atrophy during his time of being immobile).

Osteoclasts are responsible for resorbing bone matrix and releasing key minerals, including calcium, back into the bloodstream. They are stimulated by parathyroid hormone when blood calcium levels are low.

Osteoblasts build bone matrix and later differentiate into osteocytes, which preserve the bone tissue.

This question requires a strong understanding of factors affecting bone resorption and bone re-formation.

Bone re-formation takes place when osteoblasts use calcium and phosphate from the blood to form bone. Calcitonin stimulates osteoblasts to form bone. Parathyroid hormone (PTH), on the other hand, stimulates bone resorption and causes osteoclasts to break down bone, causing blood calcium and phosphate levels to increase.

Of the possible answers, only increased osteoclast activity would result in higher blood calcium levels. Increased osteoblast activity, decreased parathyroid hormone, and increased calcitonin would all result in lower blood calcium levels.

In the event of hypercalcemia the body has elevated blood calcium levels. As a result, osteoclasts, which are responsible for the resorption of bone matrix and the release of calcium into the bloodstream, would experience reduced function.

Osteoblasts, in contrast, help to synthesize bone and would be stimulated by high blood calcium levels to remove calcium from the blood and sequester it in bone. Osteocytes, sometimes known as osteogenitor cells, are the progenitor cells to osteoblasts.

After a fracture, osteoblasts—bone forming cells—start to produce new bone through the process of osteogenesis. They produce compact bone and fuse the bone segments together.

An osteocyte is a mature bone cell. Osteoclasts are cells that dissolve the bony matrix. An osteon is a unit of hard bone with embedded osteocytes that surround a central (Haversian) canal containing a capillary. Osteodentin is calcification resembling bone that forms very rapidly in response to trauma, such that cells and blood vessels are incorporated. 

Volkmann canals connect different Haversian systems, allowing the osteocytes within their lacuna to communicate via chemical and cellular signalling.

Canaliculi form a "spiderweb" of tiny channels to facilitate communication between osteocytes within a single Haversian system, but do not permit communication between different osteons.

The primary function of red bone marrow is to make red blood cells in the process known as erythropoiesis. If all bone marrow is destroyed, then an individual will lose the ability to make red blood cells.

Myogenesis is performed by muscle fibers and satellite cells. Osteogenesis is performed by osteoblasts. Neurogenesis primarily occurs during early development and is performed by neural stem cells.

Red bone marrow is primarily located in flat bones (such as the sternum and pelvis) and in the epiphyses of long bones. It is responsible for producing red blood cells, a process known as erythropoiesis. At birth, all bones of the human skeleton carry out erythropoesis, but many bones stop this function as the newborn ages.

It is important to note that yellow bone marrow is found in the medullary cavity within the diaphyses of long bones and assists in fat storage.

Red bone marrow is filled with hematopoietic stem cells. Red bone marrow is found in the heads, or epiphyses, of long bones. Yellow marrow fills the medullary cavity and consists mostly of fats. The diaphysis contains the medullary cavity and therefore contains no red marrow. The periosteum has no marrow in it at all.