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

OVERVIEW OF DEVELOPMENT
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OVERVIEW OF DEVELOPMENT
Animals live by eating other organisms. Thus, despite their remarkable diversity,
animals as different as worms, mollusks, insects, and vertebrates share anatomical
features that are fundamental to this way of life. Epidermal cells form a protective
outer layer; gut cells absorb nutrients from ingested food; muscle cells
allow movement; and neurons and sensory cells control behavior. These diverse
cell types are organized into tissues and organs, forming a sheet of skin covering
the exterior, a mouth for feeding, and an internal gut tube to digest food—with
muscles, nerves, and other tissues arranged in the space between the skin and
the gut tube. Many animals have clearly defined axes—an anteroposterior axis,
with mouth and brain anterior and anus posterior; a dorsoventral axis, with back
dorsal and belly ventral; and a left-right axis. In this section, we discuss some fundamental
mechanisms underlying animal development, beginning with how the
basic animal body plan is established.
Conserved Mechanisms Establish the Basic Animal Body Plan
The shared anatomical features of animals develop through conserved mechanisms.
After fertilization, the zygote usually divides rapidly, or cleaves, to form
many smaller cells; during this cleavage, the embryo, which cannot yet feed, does
not grow. This phase of development is initially driven and controlled entirely by
the material deposited in the egg by the mother. The embryonic genome remains
inactive until a point is reached when maternal mRNAs and proteins rather
abruptly begin to be degraded. The embryo’s genome is activated, and the cells
cohere to form a blastula—typically a solid or a hollow fluid-filled ball of cells.
Complex cell rearrangements called gastrulation (from the Greek “gaster,” meaning
“belly”) then transform the blastula into a multilayered structure containing
a rudimentary internal gut (Figure 21–3). Some cells of the blastula remain external,
constituting the ectoderm, which will give rise to the epidermis and the nervous
system; other cells invaginate, forming the endoderm, which will give rise to
the gut tube and its appendages, such as lung, pancreas, and liver. Another group
of cells moves into the space between ectoderm and endoderm and forms the
mesoderm, which will give rise to muscles, connective tissues, blood, kidney, and
various other components. Further cell movements and accompanying cell differentiations
create and refine the embryo’s architecture.
fertilized egg blastula gastrula
(A)
right
anterior
dorsal
ventral
(B)
CLEAVAGE
posterior
left
neural tube (ectoderm)
notochord (mesoderm)
head
eye
ectoderm
GASTRULATION
epidermis (ectoderm)
mesoderm
endoderm
somite (mesoderm)
gut cavity (lined
by endoderm)
tail bud
lateral plate mesoderm
endoderm and yolk
Figure 21–3 The early stages of
development, as exemplified by a frog.
(A) A fertilized egg divides to produce a
blastula—a sheet of epithelial cells often
surrounding a cavity. During gastrulation,
some of the cells tuck into the interior to
form the mesoderm (green) and endoderm
(yellow). Ectodermal cells (blue) remain on
the outside. (B) A cross section through
the trunk of an amphibian embryo shows
the basic animal body plan, with a sheet
of ectoderm on the outside, a tube of
endoderm on the inside, and mesoderm
sandwiched between them. The endoderm
forms the epithelial lining of the gut, from
the mouth to the anus. It gives rise not only
to the pharynx, esophagus, stomach, and
intestines, but also to many associated
structures. The salivary glands, liver,
pancreas, trachea, and lungs, for example,
all develop from the wall of the digestive
tract and grow to become systems of
branching tubes that open into the gut or
pharynx. The endoderm forms only the
epithelial components of these structures—
the lining of the gut and the secretory
cells of the pancreas, for example. The
supporting muscular and fibrous elements
arise from the mesoderm.
The mesoderm gives rise to the
connective tissues—at first, to the loose
mesh of cells in the embryo known as
mesenchyme, and ultimately to cartilage,
bone, and fibrous tissue, including the
dermis (the inner layer of the skin). The
mesoderm also forms the muscles, the
entire vascular system—including the heart,
blood vessels, and blood cells—and the
tubules, ducts, and supporting tissues of
the kidneys and gonads. The notochord
forms from the mesoderm and serves
as the core of the future backbone and
the source of signals that coordinate the
development of surrounding tissues.
The ectoderm will form the epidermis
(the outer, epithelial layer of the skin) and
epidermal appendages such as hair,
sweat glands, and mammary glands. It will
also give rise to the whole of the nervous
system, central and peripheral, including
not only neurons and glia but also the
sensory cells of the nose, the ear, the eye,
and other sense organs. (B, after T. Mohun
et al., Cell 22:9–15, 1980. With permission
from Elsevier.)