1076
UNIT 5
Continuity
28
Tail
Amnion
Head
Yolk sac
Lateral
fold
Future gut
(digestive
tube)
Somites
(seen
through
ectoderm)
Primitive
gut
Foregut
Hindgut
Neural
tube
Notochord
Yolk
sac
Yolk sac
Ectoderm
Mesoderm
Endoderm
Trilaminar
embryonic
disc
Head
fold
Tail
fold
(a)
(b)
(c)
(d)
Figure 28.10
Folding of the embryonic body, lateral views.
(a)
Model of the flat three-layered embryo as three sheets of paper.
(b, c)
Folding begins with lateral folds, then head and tail folds
appear.
(d)
A 24-day embryo in sagittal section. Notice the primitive
gut, which derives from the yolk sac, and the notochord and neural
tube dorsally.
(16-day embryo). Gastrulation begins when a groove with
raised edges called the
primitive streak
appears on the dorsal
surface of the embryonic disc and establishes the longitudinal
axis of the embryo (Figure 28.9e). Surface (epiblast) cells of the
embryonic disc then migrate medially across other cells and
enter the primitive streak. Te first cells to enter the groove dis-
place the hypoblast cells of the yolk sac and form the most infe-
rior germ layer, the
endoderm
(Figure 28.9f). Tose that follow
push laterally between the cells at the upper and lower surfaces,
forming the
mesoderm
(Figure 28.9g). As soon as the meso-
derm is formed, the mesodermal cells immediately beneath the
early primitive streak aggregate, forming a rod of mesodermal
cells called the
notochord
(no
9
to-kord), the first axial support
of the embryo (see Figure 28.12a). Te cells that remain on the
embryo’s dorsal surface are the
ectoderm
. At this point, the em-
bryo is about 2 mm long.
Te three primary germ layers serve as the
primitive tissues
from which all body organs derive. Ectoderm (“outer skin”) fash-
ions structures of the nervous system and the skin epidermis.
Endoderm (“inner skin”) forms the epithelial linings of the diges-
tive, respiratory, and urogenital systems, and associated glands.
Mesoderm (“middle skin”) forms virtually everything else.
Both ectoderm and endoderm consist mostly of cells that
are securely joined to each other and are
epithelia
. Mesoderm,
by contrast, is a
mesenchyme
[literally, “poured into the middle
(of the embryo)”], an embryonic tissue with star-shaped cells
that are free to migrate widely within the embryo. Figure 28.13
(p. 1079) lists the germ layer derivatives. Some of the details of
the differentiation processes are described next.
Organogenesis: Differentiation
of the Germ Layers
Define organogenesis and indicate the important roles of
the three primary germ layers in this process.
Describe unique features of the fetal circulation.
Gastrulation lays down the basic structural framework of the
embryo. It also sets the stage for the rearrangements that occur
during
organogenesis
(or
0
gah-no-jen
9
ĕ-sis), formation of body
organs and organ systems. By the end of the embryonic period
at 8 weeks, when the embryo is about 22 mm (slightly less than 1
inch) long from head to buttocks (referred to as the
crown-rump
measurement
), all the adult organ systems are recognizable. It is
truly amazing how much organogenesis occurs in such a short
time in such a small amount of living matter.
As
Figure 28.10
shows, the embryo starts off as a flat plate,
but as it grows, it folds to achieve a cylindrical body shape which
li±s off the yolk sac and protrudes into the amniotic cavity. In
the simplest sense, this process resembles three stacked sheets of
paper folding laterally into a tube (Figure 28.10a, b). At the same
time, the folding occurs from both ends (the head and tail re-
gions) and progresses toward the central part of the embryonic
body, where the yolk sac and umbilical vessels protrude. As the
endoderm undercuts and its edges come together and fuse, it
encloses part of the yolk sac (Figure 28.10d).
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