Biomedical Engineering Reference
In-Depth Information
1994; see embryonal rest theory of cancer later). In simplistic terms, the cellular
processes that occur during embryonal development consist of two phases:
expansion and determination (Fig. 1A). Following fertilization of the ovum,
the primordial embryonal stem cell divides symmetrically for the first 5-6 divi-
sions. During these symmetric divisions (Fig. 1B), each embryonal cell produces
two daughter cells, which are also able to divide. Thus, the first phase of
embryonal development results in exponential expansion of the embryonal cells
(Sell, 2004b). The primordial stem cell, the fertilized egg, and its embryonic stem
cell progeny are totipotent; they have the potential to produce progeny for all
embryonic and adult
tissues. During the second phase of development,
A. STEM CELLS AND
ORGANOGENESIS
E
C
T
O
D
E
R
M
E
N
D
O
D
E
R
M
M
E
S
O
D
E
R
M
P
L
A
C
E
N
T
A
B. SYMMETRIC DIVISION
EXPANSION
BRAIN
SKIN
GI TRACT
LUNGS
MUSCLE
C. ASYMMETRIC DIVISION
DETERMINATION
BONE
BLOOD
TISSUE STEM CELL
TRANSIT AMPLIFYING CELLS
M
E
M
B
R
A
N
E
S
TERMINALLY
DIFFERENTIATED
CELL
EXPANSION
DETERMINATION
Fig. 1 Proliferation and determination of embryonic stem cells during early development.
A Embryonic stem cells and organogenesis; B symmetric division (expansion); C asymmetric
division (determination). During the first stage of embryogenesis, the primordial stem cell and
each embryonic stem cell divide, to produce two equivalent daughter cells each (symmetric
division). This results in an exponential expansion of the number of cells in the embryo. Then,
the type of cell division changes to asymmetric. During asymmetric division, one daughter
cell remains a stem cell, whereas the other daughter cell begins the process of differentiation,
to produce mature cells that carry out the specialized function mature organs. In adult tissue,
normal tissue renewal is carried out through asymmetric division of tissue-specific transit-
amplifying cells. The number of transit-amplifying cells that divide is equal to the number that
differentiates, so the number of cells in adult tissue remains constant. However, in cancer
tissues some of the transit-amplifying cells divide symmetrically, cell maturation is delayed,
and cancer cells increase in number
