Biology Reference
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suggests the existence of a ''developmental clock'' controlling the rate of embryo-
genesis; the overall standard deviation in this developmental clock has been esti-
mated at 4.5% (
Bao et al., 2008
).
Earlier lineage studies suggested that relative cell positions could show variability
in midembryogenesis (
Schnabel et al., 1997
), although such variability decreases by
the premorphogenetic stages. More recent studies have suggested that some of this
variability may result from the slight compression introduced when an embryo is
mounted on an agar pad or between slide and coverslip using beads (
Hench et al.,
2009
). Compressed embryos display two stereotypical rotations. First, during gas-
trulation, the embryo turns from a left-right aspect to a dorsal-ventral aspect. Then,
following epidermal enclosure, the embryo turns once again to display the left-right
aspect and its comma shape. The increased variability in cell positions in such
compressed embryos may reflect increased migration displacements in the flattened
eggshell in conjunction with rotational movements. Unlike compressed embryos,
freely mounted embryos attached to polylysine coated coverslips show less variabil-
ity in cell positions (
Hench et al., 2009; Schnabel et al., 2006
) and do not display the
typical left-right/dorsal-ventral rotations. However, the increased depth of the
uncompressed embryo leads to a slight loss of optical quality, and many laboratories
continue to use slightly compressed embryos for optimal imaging (see chapter by
Hardin).
B. Postembryonic Cell Identification and Lineage Analysis
Cell identification in larval and adult stages is facilitated by the increased sepa-
ration of nuclei and the differentiation of cell types. However as development
proceeds nuclei tend to have slightly less stereotyped positions. Accurate identifi-
cation of cells and nuclei is also complicated by the tendency of worms to move out
of the field of view; at present there is no anesthetic or physical restraint that is
compatible with long-term development.
Most cell types are readily identified by position and nuclear morphology.
Complex cell groups such as the anterior ganglia require practice and tracing of cell
positions frommultiple animals. To begin identifying cells it is essential to start with
simple easily recognized stages and tissues such as the 12-cell stage of vulval
development. A novel approach to identifying new expression patterns is to analyze
their intersection with previously characterized patterns using ''split GFP''
(
Zhang et al., 2004
).
Paralleling the automated lineaging efforts in the embryo,
Long et al. (2009)
have recently constructed a 3D atlas of nuclear positions in L1 larvae. Generating
a standard 3D representation of the L1 larvae nuclei is instrumental for mapping
gene expression patterns or high-throughput computer-controlled functional
screens. Atlas building depends on (1) reliable identification of larval nuclei,
(2) registration of multiple larval samples into the same standard representation,
and (3) mapping of novel samples onto this standard representation. To achieve
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