Specialized Connective
Tissues: Bone and
Cartilage
Answers
104.The answer is e. (Junqueira, pp 141–147. Ross and Pawlina, pp
232–235.)The difference between endochondral and intramembranous ossi-
fication is the microenvironment in which bone formation occurs. In both
cases, bone development occurs by essentially the same process, the synthesis
of collagen and other matrix components by osteoblasts (answer a)and the
calcification of the matrix through the action of alkaline phosphatase from
osteoblasts. Bone development occurs in two different locations, which differ
in the presence or absence of cartilage models of the bones. For example, in
the flat bones of the skull, bone formation occurs through the differentiation
of osteoprogenitor cells from mesoderm and is accompanied by vasculariza-
tion. This is known as intramembranous ossification. In the other form of
ossification (endochondral), chondrocytes establish a cartilage model of the
long bone that is subsequently replaced by bone. This method occurs in
bones such as the humerus and femur. The cartilage model of each bone is
used as a scaffolding for bone formation. Bone formed by the two methods
cannot be distinguished with the light or electron microscope (answers b and
c).In both endochondral and intramembranous ossification, the first bone
formed is woven bone (answer d),also known as primary bone. This bone is
replaced by adult, lamellar bone through a remodeling process.
105.The answer is e.(Sadler, pp 129–131.)In achondroplasia, the com-
mon mutations cause a gain of function of the FGFR3gene, resulting in
decreased endochondral ossification, inhibited proliferation of chondrob-
lasts in growth plate cartilage, decreased cellular hypertrophy, and
decreased cartilage matrix production (answers b and c).Growth in the
length of long bones after birth (postnatally) occurs through cell prolifera-
tion of chondroblasts (immature chandrocytes) in the secondary ossifica-
tion centers of the epiphyses. The primary ossification centers “close” soon
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