106
UNIT ONE
The term “blast” is used to describe fl edgling differentiated
cells, such as osteoblast and myoblast. The osteoblast does
not produce contractile proteins, just as the myoblast does not
produce mineral-binding proteins and alkaline phosphatase.
From Science to Technology 3.1 looks at how our mas-
ter builder cells—the stem cells that perpetuate the entire
genetic instruction manual—are being investigated for use in
health care.
PRACTICE
33
Distinguish between a stem cell and a progenitor cell.
34
Distinguish between totipotent and pluripotent.
35
How do cells dif
erentiate?
3.7
CELL DEATH
A cell that does not divide or differentiate has another
option—death.
Apoptosis
(ap
o-to
sis) is a form of cell
death. It is also called “programmed cell death” because it is
a normal part of development. Apoptosis sculpts organs from
tissues that naturally overgrow. In the fetus, apoptosis carves
away webbing between developing ±
ngers and toes, prunes
extra brain cells, and preserves only those immune system
cells that recognize the body’s cell surface. If it weren’t for
apoptosis, a child’s lung or liver couldn’t grow to adult size
and maintain its characteristic form. Apoptosis is also pro-
tective. After a sunburn, this form of cell death peels away
damaged skin cells that might otherwise turn cancerous.
Apoptosis is a fast, orderly, contained destruction that
packages cellular remnants into membrane-enclosed pieces
that are then removed. It is a little like packaging up the con-
tent of a messy room into plastic bags. In contrast is necrosis,
a disordered form of cell death associated with infl
ammation
and injury.
Like mitosis, apoptosis is a continuous, stepwise process.
It begins when a “death receptor” on the doomed cell’s cell
membrane receives a signal to die. Within seconds, enzymes
called caspases are activated inside the cell, where they cut
up various cell components. These enzymes:
Destroy enzymes that replicate and repair DNA.
Activate enzymes that cut DNA into similarly-sized pieces.
Dismantle the cytoskeletal threads that support the
nucleus, which collapses, condensing the DNA within.
Fracture mitochondria, which release molecules that
trigger further caspase activity, cut off the cell’s energy
supply, and destroy other organelles.
Abolish the cell’s ability to adhere to other cells.
Transport certain phospholipids from the inner face of
the cell membrane to the outside, where they attract
phagocytes that break down debris.
A cell dying from apoptosis has a characteristic appear-
ance
(f g. 3.42)
. It rounds up as contacts with other cells are
cut off, and the cell membrane undulates, forming bulges
stem cells present later in development as well as progenitor
cells are
pluripotent,
which means that their daughter cells
can follow any of several pathways, but not all of them.
Researchers are discovering that many, if not all, of the
organs in an adult human body harbor very small popula-
tions of stem or progenitor cells activated when injury or
illness occurs. For example, one in 10,000 to 15,000 bone
marrow cells is a hematopoietic stem cell, which can give
rise to blood and several other cell types. Stem cells in the
adult body may have been set aside in the embryo or fetus,
as repositories of future healing. Alternatively, or perhaps
also, stem cells or progenitor cells may travel from bone mar-
row to replace damaged or dead cells in response to signals
sent from injured or diseased tissues.
All cells in the human body (except red blood cells, which
expel their nuclei), have the same set of genetic instructions,
but as cells specialize, they use some genes and ignore others.
For example, an immature bone cell (osteoblast) forms from
a progenitor cell by manufacturing proteins necessary to bind
bone mineral, as well as alkaline phosphatase, an enzyme
required for bone formation. An immature muscle cell (myo-
blast), in contrast, forms from a muscle progenitor cell and
accumulates the contractile proteins that de± ne a muscle cell.
Stem cell
(hematopoietic stem cell)
Stem cell
Self-
renewal
Progenitor cell
(e.g., myeloid progenitor cell)
Specialized cells
(white blood cells)
FIGURE 3.40
Stem cells and progenitor cells. A true stem cell
divides mitotically to yield two stem cell daughters, or a stem cell and
a progenitor cell, which may show the beginnings oF dif
erentiation.
Progenitor cells give rise to progenitors or more dif
erentiated cells oF a
restricted lineage.
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