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

1145
Development of
Multicellular Organisms
chapter
21
An animal or plant starts its life as a single cell—a fertilized egg, or zygote. During
development, this cell divides repeatedly to produce many different kinds of cells,
arranged in a final pattern of spectacular complexity and precision. The goal of
developmental cell biology is to understand the cellular and molecular mechanisms
that direct this amazing transformation (Movie 21.1).
Plants and animals have very different ways of life, and they use different
developmental strategies; in this chapter, we focus mainly on animals. Four processes
are fundamental to animal development: (1) cell proliferation, which produces
many cells from one; (2) cell–cell interactions, which coordinate the behavior
of each cell with that of its neighbors; (3) cell specialization, or differentiation,
which creates cells with different characteristics at different positions; and (4) cell
movement, which rearranges the cells to form structured tissues and organs (Figure
21–1). It is on the fourth point that plant development differs radically: plant
cells are unable to migrate or move independently through the embryo because
each one is contained within a cell wall, through which it is cemented to its neighbors,
as discussed in Chapter 19.
In a developing animal embryo, the four fundamental processes are happening
in a kaleidoscopic variety of ways, as they give rise to different parts of the
organism. Like the members of an orchestra, the cells in the embryo have to play
their individual parts in a highly coordinated manner. In the embryo, however,
there is no conductor—no central authority—to direct the performance. Instead,
development is a self-assembly process in which the cells, as they grow and proliferate,
organize themselves into increasingly complex structures. Each of the millions
of cells has to choose for itself how to behave, selectively utilizing the genetic
instructions in its chromosomes.
At each stage in its development, the cell is presented with a limited set of
options, so that its developmental pathway branches repeatedly, reflecting a large
set of sequential choices. Like the decisions we make in our own lives, the choices
made by the cell are based on its internal state—which largely reflects its history—and
on current influences from other cells, especially its close neighbors.
To understand development, we need to know how each choice is controlled and
how it depends on previous choices. Beyond that, we need to understand how
the choices, once made, influence the cell’s chemistry and behavior, and how cell
behaviors act synergistically to determine the structure and function of the body.
In This Chapter
OVERVIEW OF DEVELOPMENT
MECHANISMS OF patterN
FORMatION
DEVELOPMENtaL TIMING
MORPHOGENESIS
GROWTH
NEURAL DEVELOPMENT
CELL PROLIFERATION CELL SPECIALIZATION CELL INTERACTION CELL MOVEMENT
Figure 21–1 The four essential cell processes that allow a multicellular organism to be made.