MCAT Biology › Bone Development and Growth
What type of bone cell is responsible for resorbing bone tissue?
Osteoclasts
Osteoblasts
Cartilage
Lacunae
Osteocytes
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.
The diaphysis is able to grow in length because of thin layers of cartilage cells located in which of the following regions?
Epiphyseal plate
Periosteum
Medullary cavity
Haversian canal
Marrow
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.
A doctor oberves a teenager who noticed that one of his legs is about two inches longer than the other. He also sees that the patient had broken one of his legs when he was 8 years old, but the patient's notes don't indicate which leg. What conclusion is the doctor likely to come to?
The patient broke the shorter leg on the epiphyseal plate.
The patient broke the longer leg on the epiphyseal plate.
The patient broke the shorter leg on the diaphysis.
The patient broke the longer leg on the periosteum.
The patient broke the longer leg in the diaphysis.
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.
An adolescent girl is suffering from reduced longitudinal bone growth in her humerus. Which of the following bone regions should be examined closely in order to determine the cause of the growth deficiency?
The epiphyseal plate
Articular cartilage
The diaphysis
The periosteum
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.
How do bones maintain a functional weight as they grow?
Osteoclasts lining the medullary cavity remove interior layers of matrix
Osteoblasts creates new lacuna to surround bone cells
Osteocytes form long fibers around the medullary cavity
Osteoblasts secrete acidic compounds, which dissolve some of the adjoining matrix
Osteocytes create new Haversian systems to limit the bone weight
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.
In childbirth, the child’s skull is flexible due to fontanelles, or soft regions that are composed of connective tissue. Through what process does the connective tissue of the infant’s skull become bone?
Intramembranous ossification
Endochondral ossification
Bony secretion
Compact bone resorption
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.
Where are osteocytes located?
The lacunae of Haversian systems
The lining of the medullary cavity
Surrounding the growth plates
Beneath the periosteum
Throughout the hydroxyapatite matrix
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.
Which of the following is true relating cartilage to the skeletal system?
Cartilage is an intermediate that is stiffer than muscle, but softer than bone
Cartilage is produced via the breakdown of muscle and bone
Cartilage has the ability to regenerate
There are two types of cartilage
Cartilage is not found in joints and vertebral discs
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.
A fetal rat's femoral cartilage is injected with a marker prior to endochondral ossification. The marker becomes deactivated as the cartilage is converted to bone. In which part of the femur would we expect to see a high concentration of the marker once the rat reaches adulthood?
On the articular surface of the bone
In the spongy bone of the diaphysis
In the compact bone of the diaphysis
Within the medullary cavity
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.