402
8 Morphogenesis
morphogenesis. But the existence of redundant mechanisms makes experimental
testing of mathematical models, as well as determining specific mechanisms,
particularly challenging. Notably, although the final shape of an organ is generally
consistent between individuals of a given species, intermediate morphologies can
differ considerably during development.
Nevertheless, models are extremely useful for examining the physical plausibility
of proposed mechanisms. The inherent complexity of problems in morphogenesis
sometimes can defy physical intuition, leading investigators to suggest mechanisms
that are inconsistent with fundamental biophysical principles. Models based on
the laws of mechanics can reveal whether a proposed mechanism can produce the
ultimate shape of a structure, as well as how it evolves during development.
Models also can be used to suggest new experiments. For example, a model
may show that a given process should lead to tension or compression in certain
regions, and how the magnitudes of these stresses change during development. Local
dissections could be used to characterize these stresses, potentially supporting or
refuting a potential mechanism. In many cases, however, cell-level observations and
measurements may be needed to sort out the various possibilities.
This chapter focuses on morphogenesis in the developing embryo, which undergoes relatively rapid and dramatic changes in form. As mentioned in Chap. 1, the
embryo contains two types of cells: mesenchymal and epithelial. While both play
important roles in development, the bulk of this chapter is devoted to epithelial morphogenesis, which can be simulated using the theories for growth and contraction
presented earlier. A theory for mesenchymal morphogenesis is considered near the
end of the chapter.
8.1 Cellular Mechanisms of Epithelial Morphogenesis
Epithelia (cell sheets) undergo extensive and often complex changes in geometry
during embryogenesis. Numerous organs, including the heart, brain, and lungs,
begin as relatively simple tubular structures composed primarily of epithelia.
Understanding how tubes develop into organs is crucial in determining the causes
of congenital defects (Taber 2014b). Before analyzing events at the tissue level, we
discuss what happens at the cell level.
In the following, we divide epithelial morphogenesis into two types: stretching
and bending. During stretching morphogenesis, epithelia elongate or shorten with or
without changes in surface area, 1 while bending morphogenesis causes deformation
out of the plane. Both types can be caused by external loads, intrinsically generated
loads, or some combination of the two.
1 Technically, stretching involves deformation caused by stress. For convenience, as used here, the
term also includes dimensional changes that do not involve true deformation, such as growth that
does not generate stress.
8 Morphogenesis
morphogenesis. But the existence of redundant mechanisms makes experimental
testing of mathematical models, as well as determining specific mechanisms,
particularly challenging. Notably, although the final shape of an organ is generally
consistent between individuals of a given species, intermediate morphologies can
differ considerably during development.
Nevertheless, models are extremely useful for examining the physical plausibility
of proposed mechanisms. The inherent complexity of problems in morphogenesis
sometimes can defy physical intuition, leading investigators to suggest mechanisms
that are inconsistent with fundamental biophysical principles. Models based on
the laws of mechanics can reveal whether a proposed mechanism can produce the
ultimate shape of a structure, as well as how it evolves during development.
Models also can be used to suggest new experiments. For example, a model
may show that a given process should lead to tension or compression in certain
regions, and how the magnitudes of these stresses change during development. Local
dissections could be used to characterize these stresses, potentially supporting or
refuting a potential mechanism. In many cases, however, cell-level observations and
measurements may be needed to sort out the various possibilities.
This chapter focuses on morphogenesis in the developing embryo, which undergoes relatively rapid and dramatic changes in form. As mentioned in Chap. 1, the
embryo contains two types of cells: mesenchymal and epithelial. While both play
important roles in development, the bulk of this chapter is devoted to epithelial morphogenesis, which can be simulated using the theories for growth and contraction
presented earlier. A theory for mesenchymal morphogenesis is considered near the
end of the chapter.
8.1 Cellular Mechanisms of Epithelial Morphogenesis
Epithelia (cell sheets) undergo extensive and often complex changes in geometry
during embryogenesis. Numerous organs, including the heart, brain, and lungs,
begin as relatively simple tubular structures composed primarily of epithelia.
Understanding how tubes develop into organs is crucial in determining the causes
of congenital defects (Taber 2014b). Before analyzing events at the tissue level, we
discuss what happens at the cell level.
In the following, we divide epithelial morphogenesis into two types: stretching
and bending. During stretching morphogenesis, epithelia elongate or shorten with or
without changes in surface area, 1 while bending morphogenesis causes deformation
out of the plane. Both types can be caused by external loads, intrinsically generated
loads, or some combination of the two.
1 Technically, stretching involves deformation caused by stress. For convenience, as used here, the
term also includes dimensional changes that do not involve true deformation, such as growth that
does not generate stress.