Chapter 28
Pregnancy and Human Development
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28
(skle
9
ro-tōm; “hard piece”) migrate medially, gather around the
notochord and neural tube, and produce the vertebra and rib at
the associated level.
Dermatome
(“skin piece”) cells help form
the dermis of the skin in the dorsal part of the body. Te
myo-
tome
(mi
9
o-tōm; “muscle piece”) cells develop in conjunction
with the vertebrae. Tey form the skeletal muscles of the neck,
body trunk, and, via their
limb buds
, the muscles of the limbs.
Cells of the
intermediate mesoderm
form the gonads and
kidneys. Te
lateral plate mesoderm
consists of paired meso-
dermal plates: the
somatic mesoderm
and the more inferior
splanchnic mesoderm
. Cells of the superior plates help to form
(1) the skin dermis; (2) the parietal serosa that lines the ventral
body cavity; and (3) most tissues of the limbs. Te more infe-
rior plates provide the mesenchymal cells that form the heart
and blood vessels and most connective tissues of the body, as
well as nearly the entire wall of the digestive and respiratory
organs. Te lateral mesodermal layers cooperate to form the
serosae of the
coelom
(se
9
lom), or ventral body cavity. Te
mesodermal derivatives are summarized in
Figure 28.13
.
By the end of the embryonic period, the bones have begun to
ossify and the skeletal muscles are well formed and contracting
spontaneously. Metanephric kidneys are developing, gonads are
formed, and the lungs and digestive organs are attaining their
final shape and body position. Blood delivery to and from the
placenta via the umbilical vessels is constant and efficient. Te
heart and the liver are competing for space and form a conspic-
uous bulge on the ventral surface of the embryo’s body. All this
by the end of eight weeks in an embryo about 2.5 cm (1 inch)
long from crown to rump!
Specialization of the Endoderm
Te tube of endoderm formed by the folding process described
is called the
primitive gut
. It becomes the epithelial lining
(mucosa) of the gastrointestinal tract
(Figure 28.11)
. Te or-
gans of the GI tract (pharynx, esophagus, etc.) quickly become
apparent, and then the oral and anal openings perforate. Te
mucosal lining of the respiratory tract forms as an outpocketing
from the
foregut
(pharyngeal endoderm), and glands arise as
endodermal outpocketings at various points further along the
tract. For example, the epithelium of the thyroid, parathyroids,
and thymus forms from the pharyngeal endoderm.
Specialization of the Ectoderm
Te first major event in organogenesis is
neurulation
, the dif-
ferentiation of ectoderm that produces the brain and spinal cord
(Figure 28.12)
. Tis process is
induced
(stimulated to happen)
by chemical signals from the
notochord
, the rod of mesoderm that
defines the body axis, mentioned earlier. Te ectoderm overlying
the notochord thickens, forming the
neural plate
(Figure 28.12a).
Ten the ectoderm starts to fold inward as a
neural groove
. As
the neural groove deepens it forms prominent
neural folds
(Figure 28.12b). By day 22, the superior margins of the neu-
ral folds fuse, forming a
neural tube
, which soon pinches off
from the ectodermal layer and becomes covered by surface
ectoderm (Figure 28.12c).
As we described in Chapter 12, the anterior end of the neural
tube becomes the brain and the rest becomes the spinal cord.
Te associated
neural crest cells
(Figure 28.12c) migrate widely
and give rise to the cranial, spinal, and sympathetic ganglia (and
associated nerves), to the chromaffin cells of the adrenal me-
dulla, to pigment cells of the skin, and contribute to some con-
nective tissues.
By the end of the first month of development, the three pri-
mary brain vesicles (fore-, mid-, and hindbrain) are apparent.
By the end of the second month, all brain flexures are evident,
the cerebral hemispheres cover the top of the brain stem (see
Figure 12.2), and brain waves can be recorded. Most of the re-
maining ectoderm forming the surface layer of the embryonic
body differentiates into the epidermis of the skin. Other ecto-
dermal derivatives are indicated in Figure 28.13.
Specialization of the Mesoderm
Te first evidence of mesodermal differentiation is the appear-
ance of the notochord in the embryonic disc (see Figure 28.12a).
Te notochord is eventually replaced by the vertebral column,
but its remnants persist in the springy
nucleus pulposus
of the
intervertebral discs. Shortly therea±er, three mesodermal aggre-
gates appear on either side of the notochord (Figure 28.12b, c).
Te largest of these, the
somites
(so
9
mīts), are paired mesoder-
mal blocks that hug the notochord on either side. All 40 pairs of
somites are present by the end of week 4. Flanking the somites lat-
erally are small clusters of segmented mesoderm called
interme-
diate mesoderm
and then double sheets of
lateral plate mesoderm
.
Each
somite
has three functional parts—
sclerotome, der-
matome
, and
myotome
(Figure 28.12d). Cells of the
sclerotome
Esophagus
Pharynx
Parathyroid
glands and
thymus
Thyroid
gland
Trachea
Right and
left lungs
Stomach
Liver
Pancreas
Gallbladder
Small intestine
Large intestine
Umbilical
cord
Connection
to yolk sac
Allantois
5-week embryo
Figure 28.11
Endodermal differentiation.
Endoderm forms
the epithelial linings of the digestive and respiratory tracts and
associated glands.
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