Reproductive System
Preimplantation Genetic Diagnosis
ix-year-old Molly Nash would probably have
died within a year or two of Fanconi anemia
had she not received a very special gift from
her baby brother Adam—his umbilical cord stem
cells. Adam was not only free of the gene that
causes the anemia, but his cell surfaces matched
those of his sister, making a transplant likely to
succeed. But the parents didn’t have to wait until
Adam’s birth in August 2000 to know that his cells
could save Molly—they knew when he was a mere
eight-celled cleavage embryo (±
g. 23B).
When the Nashs could not ±
nd a compatible
bone marrow donor for Molly, they turned to pre-
implantation genetic diagnosis (PGD). Following
in vitro
fertilization, described in From Science to
Technology 23.1, researchers at the Reproductive
Genetics Institute at Illinois Masonic Medical
Center removed a single cell from each of several
eight-celled cleavage embryos and probed the
cells to detect those free of the disease-causing
mutation. They also scrutinized the HLA genes,
which control rejection of a transplanted organ,
and chose the ball of cells that would be Adam.
The cleavage embryo divided in the labora-
tory until it was about 120 cells, and then it was
implanted into Lisa Nash’s uterus. Adam was
born, and a month later physicians infused the
umbilical cord stem cells into his sister. Today,
Molly is healthy.
PGD works because of a feature of many ani-
mal species called indeterminate cleavage. Up until
a certain point in early development, a cell or two
can be removed, yet the remain-
der of the embryo can continue to
develop normally if implanted into
a uterus. PGD, invented in 1989, at
first helped a few families avoid
devastating inherited illnesses in
their sons. Then, in 1992, Chloe
O’Brien was born free of the cys-
tic ±
brosis that made her brother
very ill, thanks to PGD. In 1994
came another milestone, when a
girl was conceived and selected to
provide umbilical cord stem cells
that cured her teenage sister’s leu-
kemia. This case became the basis
of a popular novel.
Thousands of children have
been born worldwide following
PGD, free of the disorders that
run in their families. In addition to
enabling families to circumvent
many inherited conditions, PGD
enables couples who repeatedly
lose early embryos due to chro-
mosome abnormalities—they suf-
fer repeat miscarriages—to select
chromosomally normal embryos.
PGD is becoming routine with
IVF, because it ensures that only
embryos with normal-appearing
chromosomes or lacking a family’s
mutation are implanted.
terone. Thus, the uterine wall continues to grow and develop
(f g. 23.8)
. At the same time, release of FSH and LH from the
anterior pituitary gland is inhibited, so normal reproductive
cycles cease.
Secretion of hCG continues at a high level for about
two months, then declines to a low level by the end of four
months. Although the corpus luteum persists throughout
pregnancy, its function as a source of hormones becomes
less important after the ± rst three months (± rst trimester),
when the placenta secretes suf± cient estrogens and proges-
g. 23.9)
Hormonal Changes During Pregnancy
During a typical reproductive cycle, the corpus luteum
degenerates about two weeks after ovulation. Consequently,
concentrations of estrogens and progesterone decline rapidly,
the uterine lining is no longer maintained, and the endome-
trium sloughs off as menstrual fl
ow. If this occurs following
implantation, the embryo is lost in a spontaneous abortion.
The hormone hCG normally helps prevent spontaneous
abortion. It functions similarly to LH, and it maintains the cor-
pus luteum, which continues secreting estrogens and proges-
If genetically healthy,
cleavage embryo is
implanted in woman
and develops into
a baby (7 cells can
complete normal
If the gene for the
disease is present,
cleavage embryo is
not implanted into
1 cell removed for
genetic analysis
DNA probes
genetic diagnosis probes disease-
causing genes in an eight-celled
cleavage embryo.
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