206 Anatomy, Histology, and Cell Biology
after birth (answer d).Fetal development of long bones occurs by the
process of endochondral ossification in which a cartilage model is replaced
by bone. Before birth, growth in length of the long bone occurs primarily
through the proliferation of chondroblasts within the diaphysis of the car-
tilage model (primary ossification center). Growth in the width of the long
bone occurs by the addition of osteoblasts from the periosteum and depo-
sition of a periosteal collar (answer a).This is appositional growth without
a cartilage intermediate (intramembranous ossification). It is one of the
best examples of intramembranous ossification, even though it occurs in
the development of a long bone. The action of osteoblasts is to deposit
bone matrix and secrete alkaline phosphatase; they do not proliferate in
either the primary or the secondary ossification centers.
106.The answer is e.(Alberts, pp 1093–1094. Junqueira, pp 113–114.)
Hydration of the glycosaminoglycans plays an important role in shock
absorption and enhances the resiliency of the cartilage. This role is particularly
important in the articular cartilages, which receive pressure during joint
movement and are required to resist strong compressive forces. Proteoglycans
are the major component of the ground substance of cartilage. They possess a
large anionic charge because of the presence of sulfate, hydroxyl, and carboxyl
groups within the glycosaminoglycans, which join to form proteoglycan sub-
units by linking with a core protein (answer c).The proteoglycan subunits
(monomers) subsequently form an aggregate by linking noncovalently to
hyaluronic acid (answer c).Those aggregates establish the rigidity of hyaline
cartilage by reacting electrostatically with type II collagen (answer a),proba-
bly through the sulfate groups of the glycosaminoglycans. The negative charge
of the glycosaminoglycans facilitates the binding of cations (answer b)and
the transport of electrolytes and water within the matrix. This is an important
aspect of cartilage metabolism because the chondrocytes depend on diffusion
to obtain nutrients or to dispose of waste products. Glycoproteins (answer d)
are not a major constituent of the cartilage matrix.
107.The answer is c.(Young, pp 184–187.)The child in the vignette is suf-
fering from Turner’s syndrome, gonadal dysgenesis in which the XO kary-
otype results in multiple medical problems. The short stature has recently
been attributed to the short stature homeobox gene SHOX which can affect
various stages of endochondral development. The light micrograph illus-
trates a developing long bone. The zone shown is the region of chondrocyte