site-specific delivery Anapproach to target a DRUG
to a specific tissue using PRODRUGs or antibody recog-
nition systems.
skeletal muscle Striated muscle attached to the
skeleton; contraction is under voluntary control. A sin-
gle skeletal muscle is attached at its origin to a large
area of bone, while at the other end the insertion tapers
into a tendon.
skeleton Aset of bones or cartilage, ligaments, and
tendons that acts as a frame and holds the body and its
organs together while also allowing locomotion.
Endoskeletons are within the body, while exoskeletons
surround the outside of the body.
sliding filament model An explanation as to why
muscular contraction occurs. During muscular contrac-
tion, thin (actin) filaments within the sarcomere of a
myofibril are pulled toward the center of the sarcomere
(called the H zone) by the thick (myosin) filaments; the
two myofilaments slide past each other, increasing over-
lap. During this process, the sarcomere’s length short-
ens, the myofibril shortens, and the muscle contracts.
slugs Snails without shells. Slugs are mollusks belong-
ing to the class Gastropoda. Several species of slugs are
frequently damaging to gardens and flowers, including
thegray garden slug (Peroceras reticulatum), the band-
ed slug (Limax poirieri), and the greenhouse slug (Milax
gagates). Slugs are hermaphrodites, and there are at
least 40 species of slugs in the United States.
small nuclear ribonucleoprotein(snRNP) Acom-
bination of nuclear RNA and protein that is part of the
spliceosome, the intron-removing structure in the nuclei
of eukaryotes. Introns are DNA segments within a gene
that interrupt the coding sequence of the gene. SnRNPs
are not fully understood.
smooth ER Smooth ER is part of the endoplasmic
reticulum that does not contain ribosomes and is a
308 site-specific delivery
Computer artwork of the human skeletal system. The 206 bones of
the skeleton provide protection and support, and their joints
allow locomotion. The skull (at top) protects the brain. The ribs of
the chest (at upper center) enclose the heart and lungs. The pelvis
(at center) protects the lower abdominal organs. The flexible
backbone runs from the skull to the pelvis and protects the spinal
cord. The long leg and arm bones provide support. The humerus
(upper-arm bone) articulates with the radius and ulna (lower-arm
bones) at the elbow. The femur (thigh bone) articulates with the
fibula and tibia (lower-leg bones) at the knee.(Courtesy © Roger
Harris/Photo Researchers, Inc.)