8.6 Neural Development
455
(see Sect. 1.3), these results suggest that the proposed mechanisms for c-looping are
physically plausible.
After stage HH12, the process of s-looping brings the ends of the HT closer
together (Fig. 8.33c). Like the torsional part of c-looping, s-looping involves
bending and twisting driven primarily by external loads. For the most part, these
loads are supplied by the SPL and the developing neural tube, which also bends and
twists (Ramasubramanian et al. 2013, 2019).
As discussed in Sect. 1.4.2, the heart valves develop from regional wall thickenings called endocardial cushions, which consist of cardiac jelly enclosed by the
endocardium. Experiments have shown that blood flow plays an important role in
valve development (Hogers et al. 1997), and Buskohl et al. (2012) developed a
finite-element model for atrioventricular valve formation that includes the effects
of hemodynamic loads. Using a stress-based growth law, these authors showed that
the combined effects of blood pressure and fluid shear stress can produce realistic
morphology as the cushions evolve into valves.
8.6 Neural Development
Next, we turn to the major structures of the central nervous system. This section
first considers formation of the neural tube and how it develops into the eyes
and primitive brain tube. Folding of the cerebral cortex is studied next. Before
tackling this last problem, however, we will need to introduce the fundamentals
of mechanical stability and buckling.
8.6.1 Neural Tube Formation
The process of neurulation creates the neural tube (NT), which is a closed fluidfilled tube derived from ectoderm. Depending on anatomic location and species,
the NT is created in one of two ways (Lowery and Sive 2004). During primary
neurulation, a region of ectoderm thickens and forms the neural plate, which is
composed of columnar neuroepithelial cells. A long groove, or furrow, then forms
in the plate and closes into a tube (see Fig. 1.4). In contrast, secondary invagination
involves the formation of a solid rod of mesenchymal cells that hollows out into a
tube. In vertebrates, primary and secondary neurulation generally create the cranial
and caudal parts of the NT, respectively.
Here, we consider primary neurulation, which has been studied extensively for
many years. Failure of the NT to close completely leads to congenital disorders,
including spina bifida, which causes paraplegia, and anencephaly (missing parts of
the brain).
455
(see Sect. 1.3), these results suggest that the proposed mechanisms for c-looping are
physically plausible.
After stage HH12, the process of s-looping brings the ends of the HT closer
together (Fig. 8.33c). Like the torsional part of c-looping, s-looping involves
bending and twisting driven primarily by external loads. For the most part, these
loads are supplied by the SPL and the developing neural tube, which also bends and
twists (Ramasubramanian et al. 2013, 2019).
As discussed in Sect. 1.4.2, the heart valves develop from regional wall thickenings called endocardial cushions, which consist of cardiac jelly enclosed by the
endocardium. Experiments have shown that blood flow plays an important role in
valve development (Hogers et al. 1997), and Buskohl et al. (2012) developed a
finite-element model for atrioventricular valve formation that includes the effects
of hemodynamic loads. Using a stress-based growth law, these authors showed that
the combined effects of blood pressure and fluid shear stress can produce realistic
morphology as the cushions evolve into valves.
8.6 Neural Development
Next, we turn to the major structures of the central nervous system. This section
first considers formation of the neural tube and how it develops into the eyes
and primitive brain tube. Folding of the cerebral cortex is studied next. Before
tackling this last problem, however, we will need to introduce the fundamentals
of mechanical stability and buckling.
8.6.1 Neural Tube Formation
The process of neurulation creates the neural tube (NT), which is a closed fluidfilled tube derived from ectoderm. Depending on anatomic location and species,
the NT is created in one of two ways (Lowery and Sive 2004). During primary
neurulation, a region of ectoderm thickens and forms the neural plate, which is
composed of columnar neuroepithelial cells. A long groove, or furrow, then forms
in the plate and closes into a tube (see Fig. 1.4). In contrast, secondary invagination
involves the formation of a solid rod of mesenchymal cells that hollows out into a
tube. In vertebrates, primary and secondary neurulation generally create the cranial
and caudal parts of the NT, respectively.
Here, we consider primary neurulation, which has been studied extensively for
many years. Failure of the NT to close completely leads to congenital disorders,
including spina bifida, which causes paraplegia, and anencephaly (missing parts of
the brain).