Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1154 Chapter 21: Development of Multicellular Organisms
A
C is induced
by signal from
B acting on A
A
C
D and E are
induced by signal
from C acting
on A and B,
respectively
A
D
C
E
Figure 21–13 Patterning by sequential
induction. A series of inductive interactions
can generate many types of cells, starting
from only a few.
B
B
B
back to the other two cell types nearby, generating a fourth and a fifth cell type,
and so on (Figure 21–13).
This strategy for generating a progressively more complicated pattern is called
sequential induction. It is chiefly through sequential inductions that the body
plan of a developing animal, after being first roughed out in miniature, becomes
MBoC6 m22.16/22.13
elaborated with finer and finer details as development proceeds.
Developmental Biology Provides Insights into Disease and Tissue
Maintenance
The rapid progress in understanding animal development has been one of the
great success stories in biology over the last few decades, and it has important
practical implications. Some 2 to 5% of all human babies are born with anatomical
abnormalities, such as heart malformations, truncated limbs, cleft palate, or spina
bifida. Advances in developmental biology help us understand how these defects
arise, even if we cannot yet prevent or cure most of them.
Less obvious, but even more important from a practical point of view, is that
developmental biology provides insights into the workings of cells and tissues
in the adult body. Developmental processes do not halt at birth; they continue
throughout life, as tissues are maintained and repaired. The fundamental mechanisms
of cell growth and division, cell–cell signaling, cell memory, cell adhesion,
and cell movement are involved in adult tissue maintenance and repair—just as
they are in embryo development.
Embryos are simpler than adults, and they allow us to analyze such basic processes
more easily. Studies of the early Drosophila embryo, for example, were crucial
to the discovery of several conserved signaling pathways, including the Wnt,
Hedgehog, and Notch pathways. They also provided the key to understanding the
central role of these pathways in the maintenance of normal adult human tissues
and in diseases such as cancer.
In Chapter 22, we shall consider how other developmental mechanisms operate
in the normal adult body, especially in tissues that are continually renewed by
means of stem cells—including the gut, skin, and the hematopoietic system. But
now, we must look more closely at the way in which an early embryo generates its
spatial pattern of specialized cells, beginning with the transformations that create
the adult body plan.
Summary
Animal development is a self-assembly process, in which the cells of the embryo
become different from one another and organize themselves into increasingly complex
structures. The process begins with a single large cell—the fertilized egg. This
cell cleaves to form many smaller cells, producing a blastula. The blastula undergoes
gastrulation to generate the three germ layers of the embryo—ectoderm, mesoderm,
and endoderm—consisting of cells determined for different fates. As development
continues, the cells become more and more narrowly specialized according
to their locations and their interactions with one another. Through cell memory,
these cell–cell interactions, even though transient, can have lasting effects on each