of proteins provides mechanical support to the sarcolemma during muscle
contraction.
The blood contains about 5 q 1012 erythrocytes per dm^3 (Chapter 13). Their
major function is to carry dioxygen from the lungs to the general body tissues.
The unique biconcave shape of erythrocytes is maintained by a cytoskeleton
composed of five major proteins that form a network lining the inner sur-
face of their plasma membranes (Figure 16.17). The spectrin–actin complex
is thought to act in a manner that resembles that of the dystrophin–actin
complex of skeletal muscle and provides mechanical support to the plasma
membrane preventing its lysis during circulation. The network of proteins
also allows erythrocytes to deform and spring back into shape as they are
pumped through the narrow capillaries of the vascular system. The numbers
of erythrocytes are maintained by a constant production in the bone marrow
and the destruction of worn out or misshapen erythrocytes by the spleen. This
destruction releases bilirubin, which is converted to bile salts in the liver and
released into the gastrointestinal tract in bile (Chapter 11). The iron from the
hemoglobin is largely retained and reused by the body.
Muscular Dystrophies
Mutations involving the genes for dystrophin, its associated transmembrane
complex proteins or the @2 laminin of the basal lamina lead to genetic
CYTOSKELETAL DISORDERSCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )+,
Figure 16.17 (A) Simplified schematic of the
cytoskeleton of an erythrocyte. (B) Schematic of
the separation of proteins of the cytoskeleton
of a red blood cell by SDS polyacrylamide gel
electrophoresis.A)
@ and A spectrinsAnkyrinBand 3 proteinBand 4.1 proteinBand 4.9 proteinActin@ spectrinA spectrinAnkyrinBand 3 proteinBand 4.1 proteinBand 4.2 proteinBand 4.9 proteinActinB)