Biology Reference
In-Depth Information
CHAPTER
18
Invagination and Evagination:
The Making and Shaping of Folds
and Tubes
Epithelial sheets are essentially two-dimensional structures. They come to occupy and
frequently to dominate three-dimensional space by a process of controlled bending and
deformation, a biological origami made far more powerful than its paper-based equivalent
by the fact that living sheets are not limited to folding but can stretch and shrink within their
planes as well. This feature allows two-dimensional sheets to be transformed into tubes by
invagination or evagination; these transformations are the basis of three of the five ways in
which tubes can be made (
Figure 18.1
).
INVAGINATION
Invagination
)
is the production of a tube by local in-pushing of a surface, as when an
extended finger is pressed radially against a partially inflated balloon. There are two main
forms of invagination: axial and orthogonal. Axial invagination occurs at a point and can
only produce a dent or a tube; the surface pushes inwards directly along the axis of the
tube that will form. Orthogonal invagination occurs along a line rather than at a single point
and produces a trough, the axis of which is parallel to the original surface and therefore at
right angles to the direction of invagination (
Figure 18.1
).
Broadly, the mechanisms suggested as drivers of invaginations (axial and orthogonal) fall
into two classes: those based on the cytoskeleton, and those based on extracellular matrix.
Each mechanism has its champions and each appears to reign supreme in its own special
set of exemplar experiments. Despite this, a few recent experiments are beginning to suggest
that in real situations both mechanisms may operate at once, their relative importance
)
'Invagination' derives directly from the Latin in meaning 'in' and vagina meaning 'sheath', and does not
imply any analogy with development of the vagina (birth canal). Ironically, the vagina is an example of a tube
that does not develop by invagination but rather by a complex series of fusions in the cloacal area. It gains its
name directly from Latin (sheath), probably because Roman soldiers' slang for penis was gladius (sword).
