1070
UNIT 5
Continuity
28
Te trophoblast cells overlying the inner cell mass adhere
to the endometrium
(Figure 28.5a, b)
and secrete digestive
enzymes and growth factors against the endometrial surface.
Te endometrium quickly thickens at the point of contact and
takes on characteristics of an acute inflammatory response—the
uterine blood vessels become more permeable and leaky, and
inflammatory cells including lymphocytes, natural killer cells,
and macrophages invade the area.
Te trophoblast then proliferates and forms two distinct lay-
ers (Figure 28.5c). Te cells in the inner layer, collectively called
the
cytotrophoblast
(si
0
to-tro
9
fo-blast) or
cellular tropho-
blast
, retain their cell boundaries. Te cells in the outer layer
lose their plasma membranes and form a multinuclear cytoplas-
mic mass called the
syncytiotrophoblast
(sin-sit
0
e-o-tro
9
fo-
blast;
syn
5
together,
cyt
5
cell) or
syncytial trophoblast
. Tis
syncytial region sends out long protrusions that invade the en-
dometrium and rapidly digest the uterine cells it contacts. As
the endometrium is eroded, the blastocyst burrows into this
thick, velvety lining and is surrounded by a pool of blood leaked
from degraded endometrial blood vessels. Shortly, proliferation
of the endometrial cells covers and seals off the implanted blas-
tocyst from the uterine cavity (Figure 28.5d).
In cases where implantation fails to occur, a receptive uterus
becomes nonreceptive once again. It is estimated that a mini-
mum of two-thirds of all zygotes formed fail to implant by the
end of the first week or spontaneously abort. Moreover, an es-
timated 30% of implanted embryos later miscarry due to ge-
netic defects of the embryo, uterine malformation, or unknown
problems.
When successful, implantation takes about five days and is
usually completed by the 12th day a±er ovulation—just before
the endometrium normally begins to slough off. Menstrua-
tion would flush away the embryo as well and must be pre-
vented if the pregnancy is to continue. Viability of the corpus
luteum is maintained by an LH-like hormone called
human
chorionic gonadotropin (hCG)
(ko
0
re-on
9
ik go-nad
0
o-
trōp
9
in) secreted by the trophoblast cells. hCG bypasses
hypothalamic-pituitary-ovarian controls at this critical time
and prompts the corpus luteum to continue secreting proges-
terone and estrogen. Te
chorion
, the extraembryonic mem-
brane that develops from the trophoblast a±er implantation,
continues this hormonal stimulus. In this way, the develop-
ing conceptus takes over the hormonal control of the uterus
during this early phase of development. Besides rescuing the
corpus luteum, hCG has protease activity and is an autocrine
growth factor that promotes placental development. hCG lev-
els are particularly high in areas where the trophoblast faces
the endometrium.
Usually detectable in the mother’s blood one week a±er fer-
tilization, blood levels of hCG continue to rise until the end
1
After the sperm
penetrates the
secondary oocyte, the
oocyte completes
meiosis II, forming the
ovum and second polar
body.
2
Sperm and ovum
nuclei swell, forming
pronuclei.
3
Pronuclei approach
each other and mitotic
spindle forms between
them.
4
Chromosomes of
the pronuclei intermix.
Fertilization is
accomplished. Then,
the DNA replicates in
preparation for the first
cleavage division.
Polar bodies
Female pro-
nucleus (swollen
ovum nucleus)
Male pro-
nucleus
Zygote
Male and female
pronuclei
Polar bodies
Male
pronucleus
Mitotic spindle
Centriole
Female
pronucleus
Sperm nucleus
Extracellular
space
Second meiotic
division of oocyte
Second meiotic
division of first
polar body
(b)
(a)
Corona
radiata
Zona
pellucida
Figure 28.3
Events of fertilization. (a)
Events from sperm pen-
etration to zygote formation.
(b)
Micrograph of an oocyte in which
the male and female pronuclei are beginning to fuse to accomplish
fertilization and form the zygote. Occurs in time between steps
3
and
4
of (a).
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