Healthcare and Medicine Reference
In-Depth Information
1 Day
survival
Retrovirus infects
a single progenitor cell
Chick
embryo
Inject
virus
Mature
spinal chord
Viral genes integrate
into cell DNA
Viral genes are
expressed, marking
the infected cell and
all of its progeny (blue)
T
O
MN
FIGURE 3.5 Retroviral lineage tracing methods are used to
determine the relationships among the cell types. A retrovirus can
infect and incorporate its genome into a cell only in the S-phase of
the cell cycle. Thus, by engineering retroviruses to contain the gene
for an enzyme not normally expressed in animal cells, like the
bacterial beta-galactosidase gene or the jellyfish green fluorescent
protein gene, and infecting the cells of the developing brain, the only
cells that will contain the foreign genes are those that were prolifer-
ating at the time of the infection—that is, the progenitor cells. If only
a relatively few viral particles are used for the infection, only a few
progenitor cells will be labeled, and in this way all the progeny of a
single infection can be tracked (blue cells). In practice, viral particles
are diluted until only a few clones in each brain are observed.
A
MN
O
O
FIGURE 3.6 Clonal analysis of progenitor cells in the chick
neural tube. Injections of a retrovirus with a reporter gene are made
into the chick embryo neural tube. After either short or long postin-
jection survival periods, the spinal cord is sectioned and analyzed
for the labeled progeny of the few infected progenitor cells. In the
case shown, a single progenitor cell has been infected at this level of
the spinal cord, and it has gone through a single-cell division to give
rise to two daughter cells after one day. If the embryo is allowed to
survive to a point where the spinal cord is relatively mature, and
neurons and glial cells can be identified, the labeled cells can be
assigned to specific cell classes. In the case shown, the progeny of
the infected cell include motoneurons (MN), astrocytes (A), and
oligodendrocytes (O) (Leber et al., 1990).
ent stages of brain development has confirmed that
the number of progeny generated by a ventricular
zone cell declines over the periods of neurogenesis. For
example, retroviral infection of the progenitor cells in
the early embryonic brain results in very large clones
of labeled cells, but retroviral infection of progenitor
cells in the brains of late staged embryos gives much
smaller clones (Figure 3.5). The retroviral labeling
technique also allows the discrimination of the contri-
butions of neuronal and glial production to the total
cell number. In the cerebral cortex, for example, the
number of neurons per clone is typically less than 5,
while the number of astrocytes and oligodendrocytes
may be between 10 and 30 in the same aged embryo.
In the chick spinal cord, the distribution of clones has
been analyzed shortly after injections as well as in the
more mature spinal cord. Clonally related cells were
typically found in radially oriented arrays when ana-
lyzed shortly after the viral infections; however, when
similar clones were analyzed in the mature cord, the
cells were typically more dispersed. Many of the
clones contain both motoneurons and glial cells in
the white matter. In the example shown in Figure 3.6,
both astrocytes and oligodendrocytes are derived from
the same progenitor cell that gave rise to the motoneu-
rons (Leber et al., 1990).
 
 
 
Search Pocayo ::




Custom Search