Cell Identification and Cell Lineage Analysis - Caenorhabditis Elegans: Molecular Genetics and Development - page 327

relative position due to cell migration. In some places, such as at the anterior or
posterior poles of the early embryo, steric constraints prevent the two daughters from
remaining in strict anterior-posterior order, and their final positions are skewed
relative to the initial orientation of the spindle.
Avery small number of cells have variable ancestry. In several cases, a pair of cells
constitutes an ''equivalence group'' in which each member of the pair can give rise to
each fate. This is usually when pairs of cells formed on the left and right sides
migrate to the ventral midline to form a single anterior-posterior series. For example,
the cell ABplapaapp can become either of two ventral epidermal cells, P1 or P2,
depending on whether it migrates to a midline position anterior or posterior to its
contralateral homolog ABprapaapp. P1 is therefore denoted ABpl( l r )apaapp. Such
fate choices involve an interaction between the members of the cell pair.
V. Cell Identification and Lineage Analysis
A. Embryonic Cell Identification and Lineage Analysis
1. Manual Lineage Analysis
As all early embryonic blastomeres are very similar in morphology, early
embryonic cell identification relies on the small number of cells involved and
their invariant positions. Cell positions can be easily learnt up to the 28-cell stage
(see Fig. 1 ). Between the 28-cell stage (100 min) and late gastrulation most cells
can only be conclusively identified by following their lineage. After gastrulation
Fig. 1 Ventral view of wild-type embryo at 84 min/28-cell stage, redrawn from nuclear positions using
NucleiTracker4D software (anterior is to left). Eggshell outline is an approximation based on nuclei
filling in late embryogenesis. Depth is indicated by the thickness of the outline of each nucleus.