Chapter 28
Pregnancy and Human Development
1089
28
In the most common ART process,
in vitro fertilization
(
IVF
),
harvested oocytes are incubated with sperm in culture dishes
(
in vitro
) for several days to allow fertilization to occur. In cases
where the quality or number of sperm is low, the oocytes are in-
jected with sperm. Embryos reaching the two-cell or blastocyst
stage are then carefully transferred into the woman’s uterus in
the hope that implantation will occur.
In
zygote intrafallopian transfer
(
ZIFT
), oocytes fertilized in
vitro are immediately transferred to the woman’s uterine (fal-
lopian) tubes. ±e goal is to have development to the blastocyst
stage occur followed by normal implantation in the uterus.
In
gamete intrafallopian transfer
(
GIFT
), no in vitro proce-
dures are used. Instead, sperm and harvested oocytes are trans-
ferred together into the woman’s uterine tubes in the hope that
fertilization will take place there.
Although there is a great deal of hubbub in the scientific
community about using cloning as another possible avenue to
produce offspring, humans have proved notoriously difficult
to create and sustain past very early (blastocyst) development.
Cloning entails insertion of a somatic cell nucleus into an oocyte
from which the nucleus is removed and then an incubation pe-
riod to dedifferentiate the inserted nucleus. ±is technique has
proved more successful in creating stem cells for therapeutic
use in treating selected diseases than for reproductive cloning to
produce whole and healthy human offspring. Furthermore, hu-
man reproductive cloning is currently fraught with legal, moral,
ethical, and political roadblocks.
In this chapter, we have focused on changes that occur during
human development in utero. But having a baby is not always
what the interacting partners have in mind, and we humans
have devised a variety of techniques for preventing this outcome
(see
A Closer Look
on p. 1090).
We must admit that the description of embryonic develop-
ment here has fallen short because we have barely touched upon
the phenomenon of differentiation. How does an unspecialized
cell that can become
anything
in the body develop into a specific
something
(a heart cell, for example)? And what paces the devel-
opmental sequence, so that if a particular process fails to occur
at a precise time, it never occurs at all? Scientists are beginning
to believe that there are master switches in the genes. In Chapter
29, the final chapter of this book, we describe a small part of the
“how” as we examine the interaction of genes and other compo-
nents that determine who we finally become.
but those on sound diets usually replace lost bone calcium a²er
weaning the infant.
While prolactin levels are high, the normal hypothalamic-
pituitary controls of the ovarian cycle are damped, probably
because stimulation of the hypothalamus by suckling causes
it to release beta endorphin, a peptide hormone that inhibits
hypothalamic release of GnRH and, consequently, the release
of gonadotropins by the pituitary. Because of this inhibition of
ovarian function, nursing has been called natural birth control.
Nonetheless, there is a good deal of “slippage” in these controls,
and most women begin to ovulate even while continuing to
nurse their infants.
Check Your Understanding
23.
What hormone causes the let-down reflex?
For answers, see Appendix H.
Assisted Reproductive
Technology and
Reproductive Cloning
Describe some techniques of ART including IVF, ZIFT, and
GIFT.
So far we have been describing how babies are made. But if a
couple lacks that capability for some reason, what recourse do
they have? Hormone therapy may increase sperm or egg pro-
duction in cases where that is the problem, and surgery can
open blocked uterine tubes. Recently, an artificial ovary, which
allows eggs to mature outside the body, has been developed that
preserves a woman’s fertility if she needs to undergo treatment
for cancer.
Beyond that are
assisted reproductive technology
(
ART
) pro-
cedures that entail surgically removing oocytes from a woman’s
ovaries following hormone stimulation, fertilizing the oocytes,
and then returning them to the woman’s body. ±ese proce-
dures, now performed worldwide in major medical centers,
have produced thousands of infants, but they are expensive,
emotionally draining, and painful for the oocyte donor. Unused
oocytes, sperm, and embryos can be frozen for later attempts at
accomplishing pregnancy.
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