12
1 Introduction
Fig. 1.6 Human brain development. Cortical folding occurs during the third trimester. Adapted
from Berger (2012)
Fig. 1.7 Development of intestinal villi. Modified and reprinted from Shyer et al. (2013), with
permission from AAAS
area of the cortex and is important for normal brain function (Zilles et al. 2013;
Bayly et al. 2014).
Gut and Lungs The foregut is an open-ended tube that is created, along with the
heart tube, by the folding and fusion of epithelia. The hindgut forms similarly
at the opposite end of the embryo, and the midgut, initially open along its
length, gradually closes to complete the primitive gastrointestinal tract. The foregut
eventually becomes the esophagus and stomach, while the midgut and hindgut
develop into the intestines.
As the intestines grow longer, they loop to fit into the body cavity. The
morphology inside the smooth-walled gut tube also changes, as longitudinal ridges
form along the inner surface, become “zigzagged,” and transform into fingerlike
projections called villi (Coulombre and Coulombre 1958; Shyer et al. 2013)
(Fig. 1.7).
The lungs develop from buds that branch off the foregut. Further branching
produces the bronchial trees (Fig. 1.8).
Bone and Muscle Bone and muscle derive from mesenchymal cells of the mesoderm. Presumptive bone cells differentiate along one of two paths (Bouvier 1989).
The primordia of cancellous (spongy) bone and flat bones, such as the skull,
form a fibrous matrix that transforms directly into bone through a process called
intramembranous ossification. In contrast, long bones, vertebrae, and ribs are
created by endochondral ossification, whereby the mesenchymal cells first
differentiate into cartilage, which is then converted into bone.
In long bones, endochondral ossification begins near midshaft and moves toward
the ends (Fig. 1.9). However, relatively thin layers of cartilage called growth plates
1 Introduction
Fig. 1.6 Human brain development. Cortical folding occurs during the third trimester. Adapted
from Berger (2012)
Fig. 1.7 Development of intestinal villi. Modified and reprinted from Shyer et al. (2013), with
permission from AAAS
area of the cortex and is important for normal brain function (Zilles et al. 2013;
Bayly et al. 2014).
Gut and Lungs The foregut is an open-ended tube that is created, along with the
heart tube, by the folding and fusion of epithelia. The hindgut forms similarly
at the opposite end of the embryo, and the midgut, initially open along its
length, gradually closes to complete the primitive gastrointestinal tract. The foregut
eventually becomes the esophagus and stomach, while the midgut and hindgut
develop into the intestines.
As the intestines grow longer, they loop to fit into the body cavity. The
morphology inside the smooth-walled gut tube also changes, as longitudinal ridges
form along the inner surface, become “zigzagged,” and transform into fingerlike
projections called villi (Coulombre and Coulombre 1958; Shyer et al. 2013)
(Fig. 1.7).
The lungs develop from buds that branch off the foregut. Further branching
produces the bronchial trees (Fig. 1.8).
Bone and Muscle Bone and muscle derive from mesenchymal cells of the mesoderm. Presumptive bone cells differentiate along one of two paths (Bouvier 1989).
The primordia of cancellous (spongy) bone and flat bones, such as the skull,
form a fibrous matrix that transforms directly into bone through a process called
intramembranous ossification. In contrast, long bones, vertebrae, and ribs are
created by endochondral ossification, whereby the mesenchymal cells first
differentiate into cartilage, which is then converted into bone.
In long bones, endochondral ossification begins near midshaft and moves toward
the ends (Fig. 1.9). However, relatively thin layers of cartilage called growth plates