Thus, the 16- to 32-cell conceptus that reaches the uterus is
essentially the same size as the original fertilized egg.
Each of these cells is
—that is, has the capac-
ity to develop into an entire individual. Therefore, identical
(monozygotic) twins result when, at some point during cleav-
age, the dividing cells become completely separated into two
independently growing cell masses. In contrast, fraternal (dizy-
gotic) twins result when two eggs are ovulated and fertilized.
After reaching the uterus, the conceptus fl oats free in
the intrauterine fl uid, from which it receives nutrients, for
approximately three days, all the while undergoing further
cell divisions. Soon the conceptus reaches the stage known as
by which point the cells have lost their totipoten-
tiality and have begun to differentiate. The blastocyst consists
of an outer layer of cells called the
inner cell
and a central fl
uid-fi lled cavity (
Figure 17–25
). During
subsequent development, the inner cell mass will give rise to
the developing human—called an
during the fi rst two
months and a
after that—and some of the membranes
associated with it. The trophoblast will surround the embryo
and fetus throughout development and be involved in its nutri-
tion as well as in the secretion of several important hormones.
The period during which the zygote develops into a blas-
tocyst corresponds with days 14 to 21 of the typical menstrual
cycle. During this period, the uterine lining is being prepared
by progesterone (secreted by the corpus luteum) to receive
the blastocyst. By approximately the twenty-fi rst day of the
cycle (that is, seven days after ovulation),
embedding of the blastocyst into the endometrium—begins
(see Figure 17–25). The trophoblast cells are quite sticky, par-
ticularly in the region overlying the inner cell mass, and it is
this portion of the blastocyst that adheres to the endometrium
and initiates implantation.
The initial contact between blastocyst and endome-
trium induces rapid proliferation of the trophoblast, the cells
of which penetrate between endometrial cells. Proteolytic
enzymes secreted by the trophoblast allow the blastocyst to
bury itself in the endometrial layer. The endometrium, too,
is undergoing changes at the site of contact. Implantation
requires communication—via several paracrine agents—
between the blastocyst and the cells of the endometrium.
Implantation is soon completed (
Figure 17–26
), and the
nutrient-rich endometrial cells provide the metabolic fuel and
raw materials required for early growth of the embryo.
Inner cell
Inner cell mass
Figure 17–25
Contact (a) and implantation (b) of the blastocyst into the uterine wall at about 6–7 days after the previous LH peak. The trophoblast cells
secrete hGC into the maternal circulation, which rescues the corpus luteum and maintains pregnancy. The trophoblast eventually develops into
a component of the placenta. (c) Monkey blastocyst in contact with uterine wall (150
(c) Source: Section through a macaque blastocyst, after Heuser and Streeter. Reproduced with permission from R. O’Rahilly and F. Müller,
Human Embryology and Teratology,
Wiley-Liss, New York, 3rd
Edition, 2001.
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