W e e k s 4 - 8 o f d e v e l o p m e n t .
During the fourth week
after fertilization, the embryo undergoes dramatic changes
in shape and size, nearly tripling its size. All major organs
appear from weeks 4 through 8 of embryonic development.
By 5 weeks of development, the embryo has expressed
neither male nor female reproductive structures. Instead,
it has precursors to both systems. Some of these precur-
sor structures develop exclusively into male organs; some
develop only into female organs; and others can become
either male or female, depending on the hormonal influ-
ence. If the embryo is male (that is, has a Y chromosome),
it will begin to secrete testosterone and form testes and
male reproductive organs and external genitalia by 7 to
8 weeks; the female precursor organs degenerate. In the
absence of testosterone (if the embryo has only X chromo-
somes) the high level of estrogens from the mother’s body
will stimulate the male precursor organs to degenerate
and the female organs to develop.
By the end of the eighth week, all major body systems
have begun to develop, although their functions are mini-
mal. The embryo now has clearly human characteristics,
with ears, eyes, arms and legs, even fingers and toes. The
embryo also has a four-chambered heart, and the external
genitals begin to differentiate. The embryonic period now
ends, and the fetal period begins.
The Fetal Period Is Devoted to Growth
and Refinement of Body Structures
During the fetal period, which lasts from week 9 un-
til birth, tissues and organs that developed during the
embryonic period grow and differentiate. Very few new
structures appear during the fetal period, but the rate of
body growth is remarkable. For example, during the last
two-and-one-half months of the fetal period, half of the
full-term weight is added. At the beginning of the fetal
period, the head is half the length of the body. By the end
of the fetal period, however, the head size is one-quarter
the length of the body. During the same period, the fetal
limbs increase in size from one-eighth to one-half the fe-
tal length. By birth, both the external genitals and inter-
nal reproductive organs have developed. The fetus is also
less vulnerable to the damaging effects of drugs, radia-
tion, and microbes than it was as an embryo.
The fetus obtains its nourishment and oxygen and elim-
inates wastes and carbon dioxide via the placenta rather
than through the exchange with the external environment.
The lungs, kidneys, and gastrointestinal organs do not be-
gin to function until after birth. Therefore, circulation of
blood through the fetal vessels (Figure 16.14) is a bit dif-
ferent than in an individual after birth.
A significant portion of the fetal blood flows through
vessels to and from the chorionic portion of the placenta.
Minimal amounts of blood are sent through the pulmonary
circulation as blood is routed away from the lungs through
special vascular pathways: the foramen ovale and ductus ar-
teriosus. This is because the lungs are not currently needed
for ventilation (breathing) and external respiration.
The foramen ovale (fo-RA-men o-val-e) exists in the
septum between the right and left atria. About one-third
of the blood that enters the right atrium passes through
the foramen ovale into the left atrium and joins the sys-
temic circulation. The blood that does pass into the right
ventricle is pumped into the pulmonary trunk, but little of
this blood reaches the nonfunctioning fetal lungs. Instead,
most is sent through the ductus arteriosus (ar-te-re-O-
sus), a vessel that connects the pulmonary trunk with the
aorta, so that most blood blood bypasses the fetal lungs.
The blood in the aorta is carried to all fetal tissues through
the systemic circulation. When the common iliac arteries
branch into the external and internal iliacs, part of the
blood flows into the internal iliacs, into the umbilical ar-
teries, and back to the placenta for another exchange of
materials.
At the time of birth, the umbilical cord is tied off and
blood no longer flows through the umbilical arteries; they
fill with connective tissue. The placental vessels seal off,
along with the unusual vascular pathways that were in
place to help bypass the pulmonary circulation. When an
infant takes his or her first breath, the lungs expand and
blood flow to the lungs increases. Blood returning from the
lungs to the heart increases pressure in the left atrium.
This closes the foramen ovale, and permanent closure oc-
curs in about a year. Within a week after birth, the typical
circulation pattern is usually established.
Many fetuses born prematurely at 26 to 29 weeks of
development survive if given intensive care because their
lungs can provide adequate ventilation and the central
nervous system is developed enough to control breathing
and body temperature. Fetuses 33 weeks and older usually
survive if born prematurely. Even after birth, however, an
infant is not completely developed. An additional year is
required, especially for complete development of the ner-
vous system.
Figures 16.15a and 16.15b on the following pages
show some photos from the embryonic and fetal periods
and detail the changes taking place during these remark-
able stages.
492 CHAPTER 16
The Reproductive Systems
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