Biomedical Engineering Reference
In-Depth Information
Figure 2
. (a) Epithelial theory; (b) mesenchymal theory.
2.
PREVIOUS WORK
There are two major theories of branching morphogenesis, each supposing
that the force is from a different tissue.
1. The
epithelial theory
(7,21,56) hypothesizes a morphogenetic force at the
basal end of the epithelium (Figure 2a), where it is supposed that microfilaments
contract in the cleft region, in response to an external signal. The external signal
is believed to be linked to the observed high rate of turnover of the basal lamina
(6) in the proto-lobules. Other similar epithelial phenomena may be governed by
the epithelial ECM (30).
2. The
mesenchymal theory
(26,41,43,44) hypothesizes a morphogenetic
force in the contractile behavior of fibroblasts in the mesenchyme (47), condens-
ing the mesenchyme near the epithelium (Figure 2b). The stresses created by the
cellular traction forces are believed to align collagen fibrils into thick cords,
which when pulled taut by further traction, push deeply into an epithelial lobule,
creating a cleft.
The strongest evidence in favor of the epithelial theory is that epithelia need
no mechanical link to mesenchyme in order to form tubules (18,20,46,48,59,
