110
UNIT 1
Organization of the Body
3
the major signals may be nothing more than slight differences in
oxygen and carbon dioxide concentrations between the more
superficial and the deeper cells. But as development continues,
cells release chemicals that influence development of neighboring
cells by triggering processes that switch some genes “off” and oth-
ers “on.” Some genes are active in all cells. For example, genes for
rRNA and ATP synthesis are “on” in all cells, but genes for synthe-
sizing the enzymes needed to produce thyroxine are “on” only in
cells that are going to be part of the thyroid gland. Hence, the story
of cell specialization lies in the kinds of proteins made and reflects
the activation of different genes in different cell types.
Cell specialization leads to
structural
variation—different or-
ganelles come to predominate in different cells. For example,
muscle cells make large amounts of actin and myosin, and their
cytoplasm fills with microfilaments. Liver and phagocytic cells
produce more lysosomes. ±e development of specific and dis-
tinctive features in cells is called
cell differentiation
.
Apoptosis and Modified Rates
of Cell Division
During early development, cell death and destruction are nor-
mal events. Nature takes few chances. More cells than needed
are produced, and excesses are eliminated later in a type of
programmed cell death called
apoptosis
(ap
0
o-to
9
sis; “falling
away”). Apoptosis is particularly common in the developing
nervous system. It is also responsible for “carving out” fingers
and toes from their embryonic webbed precursors. ±is pro-
cess of controlled cellular suicide also eliminates cells that are
stressed, no longer needed, injured, or aged.
How does apoptosis work? In response to internal cellular dam-
age or to some extracellular signal, mitochondrial membranes be-
come permeable, allowing cytochrome c and other factors to leak
into the cytosol. ±ese chemicals trigger apoptosis by activating
intracellular enzymes called
caspases
. ±e activated caspases un-
leash a torrent of digestive activity within the cell, destroying the
cell’s DNA, cytoskeleton, and so on, producing a quick, neat death.
±e apoptotic cell shrinks without leaking its contents into the sur-
rounding tissue, detaches from other cells, and rounds up. Because
the dying cell releases a chemical that attracts macrophages, and
sprouts “eat me” signals, it is immediately phagocytized.
Most organs are well formed and functional long before
birth, but the body continues to grow and enlarge by forming
new cells throughout childhood and adolescence. Once we
reach adult size, cell division is important mainly to replace
short-lived cells and repair wounds.
During young adulthood, cell numbers remain fairly con-
stant. However, local changes in the rate of cell division are
common. For example, when a person is anemic, his or her
bone marrow undergoes
hyperplasia
(hi
0
per-pla
9
ze-ah), or ac-
celerated growth (
hyper
5
over;
plas
5
grow), to produce red
blood cells at a faster rate. If the anemia is remedied, the exces-
sive marrow activity ceases.
Atrophy
(at
9
ro-fe), a decrease in
size of an organ or body tissue, can result from loss of normal
stimulation or from diseases like muscular dystrophy. Muscles
that lose their nerve supply atrophy and waste away, and lack of
exercise leads to thinned, brittle bones.
Proteins called
ubiquitins
(u-bĭ
9
kwĭ-tinz) mark doomed
proteins for attack (proteolysis) by attaching to them in an ATP-
dependent reaction. ±e tagged proteins are then hydrolyzed
to small peptides by soluble enzymes or by
proteasomes
, gi-
ant “waste disposal” complexes composed of protein-digesting
enzymes, and the ubiquitin is recycled. Proteasome activity is
critical during starvation when these complexes degrade preex-
isting proteins to provide amino acids for synthesis of new and
needed proteins.
Extracellular Materials
Name and describe the composition of extracellular materials.
Extracellular materials
are any substances contributing to
body mass that are found outside the cells. Classes of extracel-
lular materials include
Body fluids
, mainly interstitial fluid, blood plasma, and cere-
brospinal fluid. ±ese fluids are important transport and dis-
solving media.
Cellular secretions
, such as substances that aid in digestion
(intestinal and gastric fluids) and some that act as lubricants
(saliva, mucus, and serous fluids).
±e
extracellular matrix
, the most abundant extracellular ma-
terial. Most body cells are in contact with a jellylike substance
composed of proteins and polysaccharides. Secreted by the cells,
these molecules self-assemble into an organized mesh in the ex-
tracellular space, where they serve as a universal “cell glue” that
helps to hold body cells together. As described in Chapter 4,
the extracellular matrix is particularly abundant in connective
tissues—in some cases so abundant that it (rather than living
cells) accounts for the bulk of that tissue type. Depending on the
structure to be formed, the extracellular matrix in connective
tissue ranges from so² to rock-hard.
Check Your Understanding
32.
What is the importance of ubiquitin in the life of a cell?
33.
What are two body fluids that inhabit the extracellular space
and what role does each play in the body?
For answers, see Appendix H.
Developmental Aspects of Cells
Discuss some theories of cell differentiation and aging.
Indicate the value of apoptosis to the body.
We all begin life as a single cell, the fertilized egg, and all the cells
of our body arise from it. Very early in development, cells begin
to specialize, some becoming liver cells, some nerve cells, and so
on. All our cells carry the same genes, so how can one cell be-
come so different from another? ±is is a fascinating question.
Apparently, cells in various regions of the embryo are exposed
to different chemical signals that channel them into specific path-
ways of development. When the embryo consists of just a few cells,
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