879
CHAPTER TWENTY-THREE
Reproductive System
are called
blastomeres.
The ball of cells that results from
these initial cell divisions is also called a cleavage embryo.
From Science to Technology 23.2 describes genetic tests of
blastomeres.
The tiny mass of cells moves through the uterine tube to
the uterine cavity, aided by the beating of cilia of the tubular
epithelium and by weak peristaltic contractions of smooth
muscles in the tubular wall. Secretions from the epithelial
lining bring nutrients to the developing organism.
The trip to the uterus takes about three days, and by then,
the structure consists of a solid ball, called a
morula
(mor
u-
lah), of about sixteen cells
(f g. 23.5)
. The morula remains free
within the uterine cavity for about three days. Cell division
continues, and the solid ball of cells gradually hollows out.
During this stage, the zona pellucida of the original secondary
oocyte degenerates, and the structure, now hollow and called
a
blastocyst
(blas
to-sist), drops into one of the tubules in the
endometrium. Figure 23.5 diagrams this process.
the egg donor’s (or another woman’s) uterus. If all
goes well, a pregnancy begins.
A woman might undergo IVF if her ovaries
and uterus function but her uterine tubes are
blocked. To begin, she takes a hormone that has-
tens maturity of several secondary oocytes. Using
a laparoscope to view the ovaries and uterine
tubes, a physician removes a few of the largest
egg cells and transfers them to a dish, then adds
chemicals similar to those in the female repro-
ductive tract, and sperm cells.
If a sperm cell cannot penetrate the egg cell
in vitro,
it may be sucked up into a tiny syringe and
injected using a tiny needle into the female cell (±
g.
23A). This variant of IVF, called intracytoplasmic
sperm injection (ICSI), has a 68% fertilization rate.
It can help men with very low sperm counts, high
numbers of abnormal sperm, or injuries or illnesses
that prevent ejaculation. However, if ICSI is done to
help a man become a father who has an inherited
form of infertility, he may be passing the problem
on to a son. In ICSI, minor surgery removes testicu-
lar tissue, from which viable sperm are isolated and
injected into eggs. A day or so later, a physician
transfers some of the resulting balls of eight or
sixteen cells to the woman’s uterus. The birth rate
following IVF for a woman implanted with her own
egg is about 26%, compared with 31% for natural
conceptions.
Gamete Intrafallopian Transfer
One reason that IVF rarely works is the arti±
cial
fertilization environment. A procedure called
GIFT, which stands for gamete intrafallopian
transfer, moves fertilization to the woman’s
body. A woman takes a superovulation drug
for a week and then has several of her largest
eggs removed. A man donates a sperm sample,
and a physician separates the most active cells.
The collected eggs and sperm are deposited
together in the woman’s uterine tube, at a site
past any obstruction so that implantation can
occur. GIFT is 22% successful.
In another variation called zygote intrafallo-
pian transfer (ZIFT), a physician places an
in vitro
fertilized ovum in a woman’s uterine tube.
Pipette
Secondary
oocyte
Sperm in pipette
FIGURE 23A
Intracytoplasmic sperm injection (ICSI) enables some infertile men and men
with spinal cord injuries and other illnesses to become fathers. A single sperm cell is injected
into the cytoplasm of an egg.
23.3
PRENATAL PERIOD
The prenatal period of development usually lasts for thirty-
eight weeks from conception. It can be divided into a period
of cleavage, an embryonic stage, and a fetal stage.
Period of Cleavage
Conception occurs when the pronuclei of sperm cell and sec-
ondary oocyte merge, forming a zygote. Thirty hours later,
the zygote undergoes mitosis, giving rise to two new cells.
These cells, in turn, divide to form four cells, which then
divide into eight cells, and so forth. The divisions occur rap-
idly with little time for the cells to grow
(f
g. 23.4)
. Thus,
with each subsequent early division, the resulting cells are
smaller and smaller. This rapid cell division and distribution
of the zygote’s cytoplasm into progressively smaller cells is
called
cleavage
(kle
¯v
ij), and the cells produced in this way
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