CHAPTER SIX
Content Review
1. Hyaline cartilage has a “glassy” extracellular matrix with no visible protein fibers.
Fibrocartilage has an extracellular matrix that displays numerous thick collagen
fibers. Elastic cartilage exhibits extensively branched elastic fibers within its
extracellular matrix.
2. The periosteum is a tough dense irregular connective tissue sheath that covers the
outer surface of the bone, except where articular cartilage is present. It isolates and
protects the bone from surrounding tissues, anchors blood vessels and nerves to the
surface of the bone, and provides stem cells (osteoprogenitor cells and osteoblasts)
for bone width growth and fracture repair.
3. Compact bone is organized into cylindrical, columnlike structures called osteons. In
cross-section, it has a distinct target like appearance: The bull’s-eye is the central
canal containing blood vessels and nerves; concentric lamellae are the rings of one
around the central canal; lacunae are spaces that house osteocytes (bone cells); and
canaliculi are tiny channels that connect the osteocytes within the lacunae.
4. Trabeculae of spongy bone form a meshwork of crisscrossing bars of bone. This
structure provides great strength against stresses applied in many directions without
requiring a large mass by distributing the stress throughout the entire framework.
5. Ossification is the formation and development of bone connective tissue. It begins in
the embryo and continues as the skeleton grows during both childhood and
adolescence. In adulthood, it continues as the bones undergo remodeling.
6. The steps of endochondral ossification are: development and growth of the hyaline
cartilage model; calcification of cartilage and formation of the periosteal bone collar
around the diaphysis; formation of the primary ossification center in the diaphysis;
formation of the secondary ossification centers in the epiphyses; replacement of
cartilage in most areas of bone, except epiphyseal plates and articular cartilage; and
ossification of the epiphyseal plate (formation of the epiphyseal line).
7. There are five zones of cartilage in the epiphyseal plate. (1) Facing the epiphysis is
the zone of resting cartilage, where the bone connective tissue of the epiphysis is
connected to the epiphyseal plate. (2) The zone of proliferating cartilage, which is
adjacent to the resting cartilage zone but nearer the diaphysis, contains chondrocytes
undergoing rapid division. (3) In the hypertrophic cartilage zone, chondrocytes cease
dividing and begin to hypertrophy (enlarge). The walls of the lacunae become thin as
the chondrocytes resorb matrix during their hypertrophy. (4) The calcified cartilage
zone has an opaque matrix resulting from mineral deposition in the matrix between
the columns of lacunae; this calcification kills the chondrocytes. (5) The final zone is
the ossification zone, where new bone matrix is produced and deposited on the
calcified cartilage matrix that remains. Thus, cartilage is produced on the epiphyseal
side of the epiphyseal plate; the matrix of dying chondrocytes is ossified, and bone
replaces the cartilage on the diaphyseal end of the plate.
8. Calcitonin is produced by the thyroid gland in response to elevated levels of calcium
in the blood. It inhibits the activity of osteoclasts, thus reducing bone resorption. This
suppression of osteoclast activity allows calcium to be deposited in the organic matrix
of bone produced by osteoblasts. Calcitonin also reduces blood calcium levels by
increasing the rate of calcium loss in urine. Parathyroid hormone is secreted and
released by the parathyroid glands in response to reduced calcium levels in the blood.
Its ultimate effect is to increase the blood calcium level by stimulating osteoclasts to
resorb bone. Parathyroid hormone also reduces the rate at which calcium is lost in the
urine. Finally, parathyroid hormone increases calcium absorption across the lining of
the small intestine, so calcium is actively absorbed from ingested food and drink.
9. In response to mechanical stress, bone has the limited ability to increase its strength.
Usually, applications of stress strengthen bone tissue over a period of time by
increasing the amounts of mineral salts deposited and collagen fibers synthesized.
Stress in the form of exercise also increases the production of the hormone calcitonin
to help inhibit bone resorption.
10. The steps involved are: (1) fracture hematoma formation; (2) fibrocartilaginous (soft)
callus formation; (3) hard (bony) callus formation; and (4) bone remodeling.