Biology Reference
In-Depth Information
with fibronectin, or other extracellular matrix, are directly involved in
mediolateral cell intercalation.
The special function of bipolarity
A critical element of the intercalation of the deep mesodermal cells appears to
be balanced, bipolar protrusive activity. It appears that cells can intercalate
two ways: they can use the neural mode consisting of a strong bias of traction
consistently in one direction, or they can use the mesodermal, bipolar mode,
but if the latter is used, the bipolar protrusive activity, the cells must minimize
any imbalance of protrusive activity and traction on the two ends of the cells at
any given time in order for intercalation to occur. If the cell is biased first in the
lateral direction, and then in the medial direction, while nearby cells similarly
alternate from one polarity to the other, the cells would be expected to
exchange places but produce little convergent extension. This type of exchange
of places is seen during intercalation of neural deep cells in explants in which
the midline notochord and notoplate have been removed and the monopolar
mode lost. Under these conditions, the deep neural cells show bipolar
protrusive activity, when activity is averaged over time, and in fact they
intercalate, in the sense that a given cell moves first one way, either medially or
laterally, and then back again, and other cells will do the same, resulting in a
high degree of cell mixing. But this promiscuous mixing only involves exchange
of mediolateral positions and results in only weak convergence and extension
results (Elul et al., 1997; Elul and Keller, 2000). In contrast, both the
monopolar neural mode and the balanced bipolar mesodermal mode produce
conservative intercalation that is productive in producing convergent extension
(Elul and Keller, 2000; Shih and Keller, 1992a,b). Biomechanical modeling
studies should be done in order to evaluate the tissue-level effects of balanced
mediolaterally orientated traction, strongly biased traction directed toward the
midline, and various patterns of alternating directionality of traction. The
mechanism of establishing bipolarity also deserves more attention. Most cells
translocate by polarizing their motility, and their traction on the substrate, in
one direction such that one edge advances and the other retracts, forming a
'tail' (Trinkaus, 1976; Euteneuer and Schliwa, 1986; Dormann et al., 2002). But
a bipolar cell must maintain traction at opposite ends of the cell, and in
addition, it appears that balancing this traction is important. It is not
understood how cells maintain a balanced bipolarity.
References
Adler, P., 2002. Planar signaling and morphogenesis in Drosophila. Dev. Cell 2: 525-535.
