Biology Reference
In-Depth Information
CHAPTER
19
Epithelial Fusion
Most of the epithelial tissues of the body connect to at least one other epithelial tissue to
become part of a system. Frequently, these tissues form by growth and differentiation of
regions of a continuous embryonic epithelium so that they are automatically connected in
the right way. For example, the Bowman's capsule, proximal convoluted tubule, loop of
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and distal convoluted tubule of a nephron in the human kidney form by differentiation
of successive zones of a common progenitor tube so that no additional morphogenetic tricks
are needed to ensure that one leads into the other. Similarly, the respiratory alveoli, bronchi-
oles, bronchi and trachea of the human lung form by differentiation of zones of a branching
epithelium, and are therefore connected automatically. Not all connections can be made this
way, however, and some connections are made by the fusion of epithelial components that
formed separately from one another.
In many cases, the need to form an epithelial structure by the fusion of existing epithelial
structures is forced on the embryo by the laws of mathematics. The simple epithelial sphere
of a typical early embryo is an object of topological genus 0, meaning that it is not pierced by
any holes. Body plans such as those of cnidaria (jellyfish, hydroids, and so on) that have
a blind-ended gut and no other openings are also of topological genus 0: it would be possible,
in principle, to model them from a hollow sphere of flexible clay without having to introduce
tears or joins (
Figure 19.1
a). The metazoan body plan, in which the gut has two openings to
FIGURE 19.1
Formation of a single-ended gut from a hollow sphere is topologically possible without cutting
and fusing, but forming a double-ended gut from a hollow sphere is not. The bottom line of text indicates topo-
logical, rather than biological, genera.
