Biology Reference
In-Depth Information
FIGURE 20.9
Features of the tips of the branching ureteric bud epithelium of the kidney. This schematic is
assembled using information from several sources
77,81
and I know of no real image that shows all features simultan-
eously; the laminin-rich spikes remain controversial. It should therefore be interpreted with appropriate caution.
S
WITCHING BETWEEN MODES OF BRANCHING
In some branching epithelia, mesenchyme-derived signals can switch the epithelium
between different modes of morphogenesis. This is demonstrated clearly by mammary
epithelium, which is capable of developing either a long spindly branched system of milk
ducts, which is what happens early in development, or of developing rounded secretory
alveoli that actually produce the milk. During early development, the mesenchyme that
surrounds the growing ducts produces copious quantities of HGF but during early preg-
nancy it produces neuregulin (which signals via ErbB instead). Mammary glands will
grow well in organ culture. Adding HGF to their medium promotes the production of
a branched ductal tree. Neuregulin, on the other hand, induces the same epithelium to
produce numerous alveoli, rather than further branches, from the ducts
59
(
Figure 20.10
).
FIGURE 20.10
The different types of morphogenesis elicited by HGF and by neuregulin treatment of cultured
mammary glands. The drawings are based on original micrographs in Yang et al.
107
