738 Chapter 20
during which time it undergoes changes that convert it into a hol-
low structure called a blastocyst ( fig. 20.42 ). The blastocyst con-
sists of two parts: (1) an inner cell mass, which will become the
fetus, and (2) a surrounding chorion, which will become part of
the placenta. The cells that form the chorion are called tropho-
blast cells.
On the sixth day following fertilization the blastocyst
attaches to the uterine wall, with the side containing the inner cell
mass positioned against the endometrium. The trophoblast cells
produce enzymes that allow the blastocyst to penetrate into the
thick endometrium. This begins the process of implantation, or
nidation, and by the seventh to tenth day the blastocyst is com-
pletely buried in the endometrium ( fig. 20.43 ). Approximately
75% of all lost pregnancies are due to a failure of implantation,
and consequently are not recognized as pregnancies.
organism if implanted into a uterus. The nuclei of adult somatic
cells, however, can be reprogrammed to become totipotent if they
are transplanted into egg cell cytoplasm. Through such somatic
cell nuclear transfer, the cloning of an entire adult organism (often
called reproductive cloning ) is possible, and indeed has been accom-
plished in sheep, cattle, cats, and other animals. Somatic cell nuclear
transfer has not worked well in humans, but scientists employing
new techniques recently reported success producing embryonic
stem cells (ES cells, described shortly) from an enucleated human
oocyte and a diploid nucleus from a donor fetal or baby’s skin cell.
The possible use of this technique to clone humans has been widely
condemned by scientists and others for many reasons, including the
low probability of producing healthy children.
Part of the 2012 Nobel Prize in Physiology and Medicine
was awarded to John Gurdon, who first demonstrated (in 1962)
that a somatic cell nucleus from a frog intestinal cell trans-
planted into an enucleated egg cell could develop into a normal
adult frog. This success of somatic cell nuclear transfer dem-
onstrated that (1) genes are not lost during development, and
(2) the effects of aging experienced by the somatic cell nucleus
must be reversed in the egg cell cytoplasm. This latter effect
is also true of normal fertilization; the cellular age of sperm
and eggs, which is the same as the other cells in the parents’
bodies, is reset when fertilization occurs so that the embryo’s
cellular aging clock starts over.
Reproductive cloning differs from the use of somatic cell
nuclear transfer to produce stem cell lines for the treatment
of diseases, a procedure called therapeutic cloning. Once the
embryo has reached the blastocyst stage in vitro, the cells of
the inner cell mass are isolated and cultured. These cells, called
embryonic stem ( ES ) cells, are pluripotent. Pluripotency is
defined as the ability to form the three embryonic germ layers —
ectoderm, mesoderm, and endoderm (see fig. 20.45 a )—and
thereby form all adult tissues. Ectoderm gives rise to epidermis
and neural tissues; mesoderm gives rise to connective tissues
and muscle tissue; and endoderm gives rise to the epithelium
of the lungs, gut, and the gut derivatives. However, pluripotent
ES cells cannot form the trophoblast and so cannot be used for
reproductive cloning.Figure 20.42 Scanning electron micrographs of preembryonic human development. A human ovum fertilized in a
laboratory ( in vitro ) is seen at ( a ) the 4-cell stage. This is followed by ( b ) cleavage at the 16-cell stage and the formation of ( c ) a morula
and ( d ) a blastocyst.
(a) (b) (c) (d)CLINICAL APPLICATION
Mifepristone ( Mifeprex ) is a synthetic steroid that works
as a progesterone receptor antagonist to block proges-
terone action. Through this effect, it promotes abortion in
early pregnancy, and so has been called the “abortion pill.”
First developed in France and called RU-486, it has been
approved for use in many countries, including in the United
States, for pregnancies of 49 days or less. Mifepristone is
used together with a prostaglandin such as misoprostol
( Cytotec ) to cause abortion, but it can be used by itself in
low doses as an emergency contraceptive (probably by pre-
venting ovulation) after unprotected sex. Because of this, it
has also been called the morning after pill. Additonally, mife-
pristone has anti-glucocorticoid as well as anti-progesterone
actions, and its anti-glucocorticoid effect may be used in the
treatment of Cushing syndrome.Embryonic Stem Cells and Cloning
Only the zygote and its early cleavage cells up to the eight-cell stage
are totipotent, a term that refers to their ability to create the entire