8.7 Intestinal Development
485
the authors found that looping did not occur when these tissues were separated in
the embryo prior to the time this process normally begins.
To examine the physical plausibility of their hypothesis, the investigators created
two models for the gut tube and attached mesentery. One is a physical model
consisting of a rubber membrane that was stretched, attached to a straight rubber
tube, and released. The other model is a computer vertex model used to simulate the
tissues as they undergo differential growth. The deformed shapes of both models
agree well with the shape of the looped gut tube in the chick embryo (Fig. 8.66).
8.7.2 Villus Formation
The formation of intestinal villi is a fascinating problem in the mechanics of
development. The process involves simultaneous remodeling of the wall of the gut
tube and morphogenesis of its luminal surface. As reported by Shyer et al. (2013),
the changes in morphology depend on the remodeling that takes place. 18
Prior to E7 in chick, the wall of the gut tube consists of mesenchyme surrounding
a smooth inner epithelium. Remodeling involves the sequential differentiation of
mesenchymal cells into layers of relatively stiff smooth muscle, which undergo
rhythmic peristaltic contractions after birth to propel food through the small
intestine. As described by Shyer et al. (2013), villus development involves the
following three phases (Fig. 8.67a):
Phase 1: Longitudinal ridges appear on the inner surface of the wall as a layer of
circumferentially oriented smooth muscle forms (E8–E12).
Phase 2: The ridges become zigzagged as a layer of longitudinal muscle forms
just outside the circumferential muscle layer (E13–E15).
Phase 3: The zigzags transform into fingerlike villi as a second layer of longitudinal muscle forms just inside the circumferential muscle layer (E16
onward).
These observations suggest a causal relation between remodeling (smooth muscle
formation) and morphogenesis (villus formation).
To explore this link, the authors first conducted a series of experiments. They
found that when the muscle layer was surgically separated from the inner layers
(epithelial cells and undifferentiated mesenchyme) during Phase 1, the inner layers
expanded and the ridges disappeared (Fig. 8.67b). Moreover, the ridges did not
form when smooth muscle differentiation was blocked using drugs, but they did
form when the diameter of the gut tube without muscle was constrained artificially.
Similar experiments during Phases 2 and 3 revealed that development of the zigzag
pattern and villi required the longitudinal muscle layers.
18 The study of Shyer et al. (2013) represents an outstanding example of how clever experiments,
computational models, and physical models can be integrated to solve a challenging problem in
the mechanics of morphogenesis. It is highly recommended to all readers of this book.
485
the authors found that looping did not occur when these tissues were separated in
the embryo prior to the time this process normally begins.
To examine the physical plausibility of their hypothesis, the investigators created
two models for the gut tube and attached mesentery. One is a physical model
consisting of a rubber membrane that was stretched, attached to a straight rubber
tube, and released. The other model is a computer vertex model used to simulate the
tissues as they undergo differential growth. The deformed shapes of both models
agree well with the shape of the looped gut tube in the chick embryo (Fig. 8.66).
8.7.2 Villus Formation
The formation of intestinal villi is a fascinating problem in the mechanics of
development. The process involves simultaneous remodeling of the wall of the gut
tube and morphogenesis of its luminal surface. As reported by Shyer et al. (2013),
the changes in morphology depend on the remodeling that takes place. 18
Prior to E7 in chick, the wall of the gut tube consists of mesenchyme surrounding
a smooth inner epithelium. Remodeling involves the sequential differentiation of
mesenchymal cells into layers of relatively stiff smooth muscle, which undergo
rhythmic peristaltic contractions after birth to propel food through the small
intestine. As described by Shyer et al. (2013), villus development involves the
following three phases (Fig. 8.67a):
Phase 1: Longitudinal ridges appear on the inner surface of the wall as a layer of
circumferentially oriented smooth muscle forms (E8–E12).
Phase 2: The ridges become zigzagged as a layer of longitudinal muscle forms
just outside the circumferential muscle layer (E13–E15).
Phase 3: The zigzags transform into fingerlike villi as a second layer of longitudinal muscle forms just inside the circumferential muscle layer (E16
onward).
These observations suggest a causal relation between remodeling (smooth muscle
formation) and morphogenesis (villus formation).
To explore this link, the authors first conducted a series of experiments. They
found that when the muscle layer was surgically separated from the inner layers
(epithelial cells and undifferentiated mesenchyme) during Phase 1, the inner layers
expanded and the ridges disappeared (Fig. 8.67b). Moreover, the ridges did not
form when smooth muscle differentiation was blocked using drugs, but they did
form when the diameter of the gut tube without muscle was constrained artificially.
Similar experiments during Phases 2 and 3 revealed that development of the zigzag
pattern and villi required the longitudinal muscle layers.
18 The study of Shyer et al. (2013) represents an outstanding example of how clever experiments,
computational models, and physical models can be integrated to solve a challenging problem in
the mechanics of morphogenesis. It is highly recommended to all readers of this book.