C h a p t e r 1
Cell Structure and Function
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different molecules in other cells, providing for interaction
between the same cell types or different cell types.
Cadherins.
Cadherins are the major CAMs responsible
for calcium-dependent cell-to-cell adhesion junctions.
The word
cadherin
is derived from the term “calcium-
dependent adhesion protein.” There are over 90
members of the cadherin superfamily. The first three cad-
herins that were discovered were named according to the
main tissues in which they were found: the E-cadherins,
which are present in many types of epithelial cells; the
N-cadherins, which are present in nerve and muscle
cells; and the P-cadherins, which are found on cells in the
placenta and epidermis. All are found in various other
tissues; N-cadherin, for example, is expressed in fibro-
blasts, and E-cadherin is expressed in parts of the brain.
Most cadherins function as transmembrane adhesive
proteins that indirectly link the actin cytoskeletons of
cells they join together, an arrangement that occurs in
adhering junctions. Cadherins also form desmosomes
that interact with intermediate filaments of the cyto-
skeleton, rather than the actin filaments. Cell-to-cell
interactions mediated by cadherins play a major role in
regulating cell motility, proliferation, and differentiation.
Selectins.
Selectins are cell surface carbohydrate-binding proteins (lectins) that mediate a variety of
transient, cell-to-cell adhesion interactions in the blood-
stream. They are found on activated endothelial cells of
blood vessels, on leukocytes, and on platelets. Selectins,
together with integrins and immunoglobulins, partici-
pate in leukocyte movement through the endothelial lin-
ing of blood vessels during inflammation.
Integrins.
Integrins usually assist in attaching epi-
thelial cells to the underlying basement membrane.
Extracellularly, they are attached to fibronectin and lam-
inin, the two major components of the basement mem-
brane. Like the cadherins, their intracellular portion is
linked to actin. One group of integrins is associated with
hemidesmosomes, whereas others are associated with the
surface of white blood cells, macrophages, and platelets.
Integrins usually have a weak affinity for their ligands
unless they are associated with cellular focal contacts and
hemidesmosomes. This allows some movement between
cells except where a firm attachment is required to attach
epithelial cells to the underlying connective tissue.
Certain integrins play an important role in allowing
white blood cells to pass through the vessel wall, a process
called
transmigration
. Persons affected with leukocyte
adhesion deficiency are unable to synthesize appropriate
integrin molecules. As a result, they experience repeated
bacterial infections because their white blood cells are
not able to transmigrate through vessel walls.
Immunoglobulin Superfamily.
The Ig superfamily
consists of a group of one or more immunoglobulin-
like adhesion proteins that are similar structurally to
those of antibody molecules. They have many func-
tions outside the immune system that are unrelated
to immune defenses. The best-studied example of Ig
superfamily proteins are the neural cell adhesion mol-
ecules (N-CAMs), which are expressed in a variety of
cells, including most nerve cells. During early develop-
ment, N-CAMs play an important role in connecting the
neurons of the developing nervous system.
SUMMARY CONCEPTS
■■
Body cells are organized into four basic tissue
types: epithelial, connective, muscle, and
nervous.The epithelium covers and lines the body
surfaces and forms the functional components of
glandular structures. Epithelial tissue is classified
into three types according to the shape of the
cells and the number of layers that are present:
simple, stratified, and pseudostratified.
■■
Connective tissue supports and connects body
structures; it forms the bones and cartilage,
the joint structures, the dermis of the skin, the
sheaths of blood vessels and nerves, adipose
tissue, lymphatic tissues, and blood. Fibroblasts
are the most abundant connective tissue cells.
They are responsible for the synthesis of collagen,
elastic, and reticular fibers and the gel-like ground
substance that fills the intercellular spaces.
■■
Muscle tissue is a specialized tissue designed
for contractility.Three types of muscle tissue
exist: skeletal, cardiac, and smooth. Actin and
myosin filaments interact to produce muscle
shortening, a process activated by the presence
of calcium. In skeletal muscle, calcium is released
from the sarcoplasmic reticulum in response
to an action potential. Smooth muscle is often
called involuntary muscle because it contracts
spontaneously or through the activity of the
autonomic nervous system. It differs from
skeletal muscle in that its sarcoplasmic reticulum
is less defined and it depends on the entry of
extracellular calcium ions for muscle contraction.
■■
Nervous tissue is designed for communication
purposes and includes the neurons, the supporting
neural structures, and the ependymal cells that
line the ventricles of the brain and the spinal canal.
■■
Within tissues, cells are held together by cell
junctions, which are especially plentiful in
epithelial tissues.There are three basic types
of cell junctions: tight junctions, adhering or
adhesive-type junctions, and gap junctions.The
ability of cells to adhere together (cell to cell)
or to components of the extracellular matrix
(cell to matrix) is mediated by cell adhesion
molecules (cadherins, selectins, integrins, and the
immunoglobulin superfamily of proteins).
