Epithelial Fusion - Mechanisms of Morphogenesis - page 220

FIGURE 19.2 The difference between (a) hole closure, in which an epithelium seals up lateral domain to lateral
domain, and fusion, in which epithelia meet either basal domain to basal domain (b), as in fusion of renal nephrons
and collecting ducts or apical domain to apical domain (c), as in the secondary palate.

the outside world is, however, an object of topological genus 1; it is mathematically impos-
sible to derive it from a hollow sphere without cutting and joining ( Figure 19.1 b). Many
examples of epithelial and endothelial fusion are necessitated by these laws of topology
but others, such as the production of an isolated excretory nephron and its subsequent
joining to a urine-collecting duct within the kidney, are not formally required by the laws
of mathematics and may instead be quirks of evolutionary history (one could imagine an
alternative way of building a kidney, in which nephrons developed as branches from the
urinary-collecting ducts, in the way that mammary alveoli develop as branches from milk
ducts in the mammary gland).
There is an important distinction between the epithelial fusions described in this chapter
and the hole closures described in Chapter 16. In hole closures, there is a break in the epithe-
lium and cells that meet across it already have the correct apico-basal polarization for the
final tissue and meet lateral edge to lateral edge: all that needs to happen is that these edges
adhere ( Figure 19.2 a). In fusions, each interacting epithelium is intact and cells therefore meet
basal surface to basal surface, as when tubules join, or apex to apex as when palatal shelves
join. To fuse into a single epithelium, cells have to change polarity, neighbour relationships or
both ( Figure 19.2 b). Fusions are particularly prone to failure in humans, with cleft palate
(failure of fusion of palatal shelves in the mouth) and spina bifida (failure of fusion of the
crests of the invaginated neural tube) being among the most common congenital abnormal-
ities detected in children.

TRACHEAL FUSION IN DROSOPHILA MELANOGASTER

The tracheal system of Drosophila melanogaster, the branching of which will be discussed in
Chapter 20, develops as a series of epithelial tubules that come in from the outside of the
body and ramify through the tissues. The tubes are small so that their tips consist of just
one cell which is specialized for navigation and, where appropriate, can be specialized for
fusion. The tip cells approach and connect with adjacent tracheal systems at stereotyped