Bone - MCAT Biological and Biochemical Foundations of Living Systems

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.

Since red bone marrow is a site of red blood cell and platelet production, hypoxia (low oxygen) would result in an increase in red marrow and therefore RBC concentration. Yellow bone marrow (typcially adipocyte-filled) can be converted into red bone marrow under conditions of low oxygen or blood loss.

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.

The epiphyseal plate is the site of longitudinal growth in bones. The patient likely broke the leg along the plate, which caused it to grow less than the other leg.

The first important concept to understand for this question is the process of endochondral ossification. In this process, cartilage is converted into bone during the early life of an organism. Since the question specifies that the rat has reached adulthood, it must refer to the parts of the final bone product that remain as cartilage once endochondral ossification is complete.

Spongy bone and compact bone in the diaphysis have already ossified, and the medullary cavity contains bone marrow and adipose. To find cartilage in any of these regions would indicate a developmental abnormality.

The ends of bones that are in contact with other bones are protected from frictional damage by articular cartilage. Articular cartilage is essential to maintaining healthy joint function. Deterioration of this cartilage results in arthritis, or inflammation in the joints.

Cartilage is a flexible, avascular connective tissue. It is less flexible than muscle, but softer and more flexible than bone. These properties make it an ideal candidate for joints, providing a medium between the muscles and bones that enact forces on the joint. Cartilage is found on the epiphyses of long bones and between certain bones, such as vertebrae, to cushion the motion of the joints.

Due to its avascular nature, cartilage does not easily regenerate. It is formed by chondroblasts (cartilage cells) in a chondrin matrix. In endochondral ossification, cartilage can be used as a precursor to bone, but will never be formed as a result of bone breakdown. Finally, there are three types of cartilage: elastic cartilage, hyaline cartilage, and fibrous cartilage.

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.

Osteoclasts are bone cells that are responsible for resorbing—or breaking down—bone tissue. Osteoblasts, on the other hand, deposit bone tissue.

Cartilage is not a type of bone cell at all; it is a type of connective tissue consisting of chondrocytes suspended in an avascular matrix. Lacunae are small cavities within the bone matrix that house osteocytes; osteocytes are mature bone cells.

Osteoclasts dissolve bony matrix and repatriate calcium as the bone grows. This expands the meduallary cavity and maintains a manageable mass for the bones, while allowing the body to recycle valuable calcium deposits.

Osteoytes, the long-lived star-shaped cells found in established bones, are primarily found within Haversian systems—the target-shaped tubes of bone matrix. They are encased in a bubble of interstitial fluid known as a lacuna.

Thin layers of cartilage cells in the epiphyseal plate enable the diaphysis (bone shaft) to grow in length. The epiphyseal line forms when growth stops and ossification occurs, permanently fusing the diaphysis and epiphysis.

The periosteum is a tough connective tissue sheath that covers the outer surface of bones. The medullary cavity is a hollow cylinder inside the diaphysis. The medullary cavity contains bone marrow, which contains blood cells in different stages of development. The Haversian canals perforate bony structure and contain blood vessels, lymph vessels, and nerves.

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.

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.

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.

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.