Fertilization and implantation
Dr. Esraa AL – Qassab
FICOG\CABOG
Fertilization
The union of egg and sperm at fertilization represents one of the most important processes in biology. Ovulation
frees the secondary oocyte and adherent cells of the cumulus-oocyte complex from the ovary. Although technically
this mass of cells is released into the peritoneal cavity, the oocyte is quickly engulfed by the infundibulum of the
fallopian tube.
Further transport through the oviduct is accomplished by directional movement of cilia and tubal peristalsis.
So Fertilization is fusion of male and female gamates, normally occurs in the oviduct, and it is generally agreed that it
must take place within a few hours, and no more than a day after ovulation.
Because of this narrow window of opportunity, spermatozoa must be present in the tube at the time of oocyte
arrival. Almost all pregnancies result when intercourse occurs during the 2 days preceding or on the day of ovulation.
Spermatozoa are not able to fertilize the oocyte immediately upon arrival in the female genital tract but must
undergo (a) Capacitation and (b) acrosomal reaction.
1-Capacitation , during which time a glycoprotein coat and seminal plasma proteins are removed from the
spermatozoon head
2.Acrosomal reaction , during which acrosin – and trypsin-like substances are released to penetrate the zona
pellucida .
Steps involved with fertilization are highly complex. Molecular mechanisms allow passage of spermatozoa between
follicular cells, through the zona pellucida, and into the oocyte cytoplasm leading to the formation of the zygote.
Steps involved with fertilization are highly complex. Molecular mechanisms allow passage of spermatozoa between
follicular cells, through the zona pellucida, and into the oocyte cytoplasm leading to the formation of the zygote.
After fertilization in the fallopian tube, the mature ovum becomes a zygote—a diploid cell with 46 chromosomes—
that then undergoes cleavage into blastomeres. The blastomeres surrounded by a thick zona pellucida. The zygote
undergoes slow cleavage for 3 days while still within the fallopian tube.
As the blastomeres continue to divide, a solid mulberry-like ball of cells—the morula—is produced. The morula
enters the uterine cavity about 3 days after fertilization.
Gradual accumulation of fluid between the cells of the morula results in the formation of the early blastocyst.
As early as 4 to 5 days after fertilization, the blastocyst differentiates the inner cell mass, and outer cells, called the
trophoblast, can be distinguished from the inner cell mass that forms the embryo. blastocyst is released from the
zona pellucida as a result of secretion of specific proteases from the secretory-phase endometrial glands
Implantation:-
Implantation of the embryo into the uterine wall takes place 6 or 7 days after fertilization. This process can be
divided into three phases: (1) apposition—initial adhesion of the blastocyst to the uterine wall; (2) adhesion—
increased physical contact between the blastocyst and uterine epithelium; and (3) invasion—penetration and
invasion of syncytiotrophoblast and cytotrophoblast into the endometrium, inner third of the myometrium, and
uterine vasculature.
Successful implantation requires receptive endometrium
appropriately primed with estrogen and progesterone. uterine receptivity is limited to days 20 to 24 of the cycle .
Adherence to epithelium is mediated by cell-surface receptors at the implantation site that interact with receptors
on the blastocyst
Development of a receptive epithelium results from the postovulatory production of estrogen and progesterone by
the corpus luteum. If the blastocyst approaches the endometrium after cycle day 24, the potential for adhesion is
diminished because synthesis of anti adhesive glycoproteins prevents receptor interactions .
The blastocyst loosely adheres to the endometrial epithelium by apposition. This most commonly occurs on the
upper posterior uterine wall.
Successful endometrial blastocyst adhesion involves expression The integrins—one of four families of CAMs—are
cell-surface receptors that mediate adhesion of cells .
Trophoblast Differentiation
By the eighth day postfertilization, after initial implantation, the trophoblast has
differentiated into an outer multinucleated syncytiotrophoblast, and an inner layer
of mononuclear cells—cytotrophoblast.
The decidua is a specialized, highly modified endometrium of pregnancy .
Decidualization—transformation of secretory endometrium to decidua—is
dependent on estrogen and progesterone and factors secreted by the implanting
blastocyst.
The decidua is classified into three parts.
Decidua directly beneath blastocyst implantation
is modified by trophoblast invasion and becomes
the decidua basalis. The decidua capsularis
overlies the enlarging blastocyst, and initially
separates it from the rest of the uterine cavity.
This portion is most prominent during the second month of pregnancy, consisting of decidual cells covered by a
single layer of flattened epithelial cells. Internally, it contacts the avascular, extraembryonic fetal membrane—the
chorion laeve.
The remainder of the uterus is lined by decidua parietalis—sometimes called decidua vera when decidua capsularis
and parietalis are joined.
During early pregnancy, there is a space between the decidua capsularis and parietalis because the gestational sac
does not fill the entire uterine cavity. By 14 to 16 weeks, the expanding sac has enlarged to completely fill the
uterine cavity. With fusion of the decidua capsularis and parietalis, the uterine cavity is functionally obliterated.
Embryonic period: starts with generation of embryonic disk during 2
nd
week post fertilization(4wk after LMP),
ends on last day of 8
th
week post fertilization(10
th
week post fertilization. At this time all organ systems are
formed, but are not necessarily mature.
Sama Adeeb