Biology Reference
In-Depth Information
the circumstances. An interesting paper on primary cultures of mammary epithelial cells
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shows that they make cyst-like organoids in collagen gels and, when these are treated
with ramogens, they make thin, spiky, single cell-led, motile structure-rich branching
systems. If they are cultured instead in gels that are rich in laminin, they make similar orga-
noids and, when treated with the same ramogens, they make much smoother branching
systems with smooth, multicellular tips and evidence of actin at the apical ends of the cells.
This observation strongly suggests that the type of branching depends not on innate differ-
ences in the abilities of cells, but rather on the signals coming from, and possibly also the
mechanical properties of, their environment. Different shapes and behaviours therefore do
not have to imply fundamental differences between what the cells can do.
BRANCHING BY CLEFTING
Even if all examples of branching by sprouting are considered to be variations on one
mechanistic theme, there is still a question about whether branching by clefting is also
another variation, or truly distinct. Epithelia that branch by clefting are characterized by
having enlarged tips, usually called 'ampullae' because of the way that their shape resembles
their namesake Romano-Greek perfume bottles. The first morphological sign of branching
occurs when clefts develop in an ampulla so that its leading edge can progress forwards
only between the clefts, and the old ampulla has therefore effectively divided into two or
more new ones (
Figure 20.11
).
Branching by clefting has been studied most closely in the submandibular salivary glands
of the mouse. These glands form from the epithelium of the oral cavity, which thickens and
pushes down into the mesenchyme of the first branchial arch of the embryo. The epithelium
forms a solid ampullary structure that undergoes branching morphogenesis and later
FIGURE 20.11
Clefting in the salivary gland. The diagram shows only the epithelium, but in reality it is sur-
rounded by mesenchymal cells.
