157
CHAPTER FIVE
Tissues
M
uch of the human body consists of the
protein collagen. It accounts for more
than 60% of the protein in bone and
cartilage and provides 50% to 90% of the dry
weight of skin, ligaments, tendons, and the den-
tin of teeth. Collagen is in the eyes, blood vessel
linings, basement membranes, and connective
tissue. It is not surprising that defects in collagen
cause a variety of medical problems.
Collagen abnormalities are devastating
because this protein has an extremely precise
structure that is easily disrupted, even by slight
alterations that might exert little noticeable eF
ect
in other proteins. Collagen is sculpted from a pre-
cursor molecule called procollagen. Three procol-
lagen chains coil and entwine to form a regular
triple helix.
Triple helices form as the procollagen is syn-
thesized, but once secreted from the cell, the heli-
ces are trimmed. The collagen fibrils continue to
associate outside the cell, building the networks
that hold the body together. Collagen is rapidly syn-
thesized and assembled into its rigid architecture.
Many types of mutations can disrupt the protein’s
structure, including missing procollagen chains,
kinks in the triple helix, failure to cut mature colla-
gen, and defects in aggregation outside the cell.
Table 5A details some collagen disorders.
Knowing which specific mutations cause disor-
ders oF
ers a way to identify the condition before
symptoms arise. This can be helpful if early treat-
ment can follow. A woman who has a high risk of
developing hereditary osteoporosis, for example,
might take calcium supplements before symptoms
appear. Aortic aneurysm is a more serious connec-
tive tissue disorder that can be presymptomatically
detected if the underlying mutation is discovered.
In aortic aneurysm, a weakened aorta (the largest
blood vessel in the body, which emerges from the
heart) bursts. Knowing that the mutant gene has
not been inherited can ease worries—and know-
ing that it has been inherited can warn affected
individuals to have frequent ultrasound exams
so that aortic weakening can be detected early
enough to correct with surgery.
5.2
CLINICAL APPLICATION
Abnormalities of Collagen
TABLE
5A
|
Collagen Disorders
Disorder
Molecular Defect
Signs and Symptoms
Chondrodysplasia
Collagen chains are too wide and asymmetric
Stunted growth; deformed joints
Dystrophic epidermolysis bullosa
Breakdown of collagen ±
brils that attach skin
layers to each other
Stretchy, easily scarred skin; lax joints
Hereditary osteoarthritis
Substituted amino acid in collagen chain alters
shape
Painful joints
Marfan syndrome
Too little ±
brillin, an elastic connective tissue
protein
Long limbs, sunken chest, lens dislocation, spindly ±
ngers,
weakened aorta
Osteogenesis imperfecta type I
Too few collagen triple helices
Easily broken bones; deafness; blue sclera (whites of the eyes)
Stickler syndrome
Short collagen chains
Joint pain; degeneration of retina and ²
uid around it
Elastic
fiber
(a)
(b)
Collagenous
fiber
Fibroblast
Ground
substance
FIGURE 5.18
Areolar tissue contains numerous ±
broblasts that produce collagenous and elastic ±
bers (800×).
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