Longitudinal bone growth occurs at the epiphyseal plate through the process of endochondral ossification. Cartilage cells undergo rapid mitosis in this region forming the structure that is later replaced by bone tissue.

Intramembranous ossification is the process in which bones are formed within dermal tissue. This process is responsible for forming the flat bones of the skull, as well as the clavicle. Other bones of the body are formed by the process of endochondral ossification, in which cartilage is replaced by bone tissue.

Bones consist of two types of cells that regulate bone growth: osteoblasts and osteoclasts. Osteoblasts lay down collagen and other important organic substances that are required to synthesize bone tissue, whereas osteoclasts reabsorb existing bone tissue. The activity of both cells is important for repair, growth, and maintenance of bone tissue.

Note that a third type of cell, osteocytes, is also found in bone, but does not play as much of an active role in maintaining bone structure.

The question states that the unknown hormone has the opposite effect of parathyroid hormone. Recall that parathyroid hormone functions to increase blood calcium levels; therefore, this unknown hormone must decrease blood calcium levels.

When blood calcium levels increase due to parathyroid hormone, osteoclasts in bones break down the bone matrix and release the calcium into the blood. This means the activity of the unknown hormone must inhibit the activity of osteoclasts to decrease blood calcium levels. This hormone is most likely released to prevent bone loss.

The unknown hormone in this question would most likely be calcitonin. Calcitonin decreases blood calcium levels and is very important in menopausal and pregnant women who have a higher risk of having excessive bone loss.

The question states that the cell is breaking down its nuclear membrane. Recall that the nuclear membrane is broken down during prophase of mitosis or prophase I of meiosis; therefore, the cell must be undergoing either mitosis or meiosis. One of the key characteristics of osteoblasts, osteocytes, and osteoclasts is that they do not undergo mitosis. Also, remember that only germ cells undergo meiosis. Since the observed cell is undergoing a division, the researcher can conclude that the cell is not an osteoblast or an osteoclast.

Osteogenic cells, however, can undergo mitosis. Osteogenic cells are cells that differentiate into osteoblasts. Once differentiated, the osteogenic cells can no longer undergo mitosis.

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.