Biology Reference
In-Depth Information
some spreading in the mediolateral orientation as a result of the resistance to
extension in the anterior-posterior orientation. As the cells intercalate and
come under compressive force in the anterior-posterior direction, they would
tend to be forced between mediolateral neighbours as well, unless there were a
countering process. Could it be that cryptic mediolateral intercalation has
already begun during radial intercalation? Or is it the case that some other
process resists the tendency for uniform spreading? In any case, radial
intercalation produces thinning and extension and provides an example of
why extension is not necessarily linked to convergence. The cellular and
mechanical basis of the channelling of the forces generated in one axis to the
perpendicular axes deserves further investigation.
Extension by shape change, orientated division and growth
Extension may occur by processes other than cell intercalation. For example,
if cuboidal cells divide in an orientated fashion without growth, the linear
dimensions of the array will increase in the axis of division and decrease in the
transverse axis. Moreover, if the cells grow (increased in volume) as well, the
effect of orientated division would be enhanced. Such a process would
produce extension but not convergence. There is evidence that orientated
division plays a role in elongation of the primitive streak (Wei and Mikawa,
2000) as well as in extension of the axial tissue in amniotes (Schoenwolf and
Alvarez, 1989; Sausedo and Schoenwolf, 1993, 1994; Sausedo et al., 1997).
Bipolar traction
In the bipolar mode of cell intercalation, each cell is presumed exert traction
on adjacent, lateral and medial cells, using the large lamelliform protrusions
found at the medial and lateral ends. This bipolar traction is thought first to
stretch the cells mediolaterally, thus elongating them, and then, when the cell
resistance to stretch balances the resistance to intercalation, the traction pulls
the cells between one another along the mediolateral axis (Keller et al., 1991,
1992a, 2000). There is no direct evidence that the large medially and laterally
orientated lamelliform protrusions actually exert traction. However, similar
protrusions of cells cultured on elastic membranes pull wrinkles in these
membranes, demonstrating that they are the primary tractive organelles of the
cell (Harris et al., 1980). The regions of the cell between such large protrusions
are under tension (Kolega, 1986). These facts support the idea that the large
lamelliform protrusions generate the tension that elongates the cells
mediolaterally, and that these protrusions pull between one another along
this axis.
