During the first two weeks of development: 1) The trophoblast differentiates into the cytotrophoblast and syncytiotrophoblast layers, beginning primitive placental circulation. 2) The embryoblast forms two layers, the epiblast and hypoblast, and the amniotic cavity develops over the epiblast. 3) The extraembryonic mesoderm splits into the somatopleuric and splanchnopleuric layers between the trophoblast and embryo. Implantation is usually complete by the end of the second week, though abnormal implantation can occur outside the uterus, risking ectopic pregnancy.

2. SUMMARY OF THE FIRST WEEK
 oogenesis and ovulation
 Spermatogenesis
 After an oocyte is penetrated by a sperm,
completes the second meiotic division
 A mature oocyte and a second polar body are
formed.
 female pronucleus
 male pronucleus
 Fertilization is complete when the male and
female pronuclei unite and the maternal and
paternal chromosomes intermingle

3. Contd…
 cleavage (a series of mitotic cell divisions) into a
number of smaller cells-blastomeres.
 Approximately 3 days after fertilization, morula-enters
the uterus.
 A cavity forms in the morula, converting it into a
blastocyst consisting of the embryoblast, a
blastocystic cavity, and the trophoblast.
 the zona pellucida is shed and the trophoblast
adjacent to the embryoblast attaches to the
endometrial epithelium and implantation begins
(interstitial)

4. TWO, FOUR, EIGHT, AND SIXTEEN CELL
STAGE ZYGOTE

5. EARLY AND LATE
BLASTOCYST

6. STAGES OF IMPLANTATION

7. 2nd week of development
 Implantation of the blastocyst
begins at the end of the 1st week
completes by the end of the 2nd week.
 Normal site of implantation: along the
anterior or posterior wall of the body of the
uterus near the fundus
 Development of embryo from fertilization up
to 2nd week is described as the pre-
organogenesis period as no organs are as
yet recognizable.

8. Day 8
 Partially embedded in endometrial stroma.
 Trophoblast differentiates into
 cytotrophoblast
 syncytiotrophoblast which produces a hormone-
human chorionic gonadotrophin (hCG)

9. Contd…
 By the end of 2nd week enough hCG give a
+pregnancy test
 inner cell mass or embryoblast also
differentiate into two layers forming bilaminar
embryonic disc
hypoblast (cuboidal cells )
epiblast (high columnar cells)
 a small cavity appears within the epiblast. This
cavity enlarges to become the amniotic cavity.
 Epiblast cells adjacent to the cytotrophoblast
are called amnioblasts.

10. Day 9; Lacunar stage
 Deeply embedded in
endometrium
 Vacuoles fuse to form
lacunae, thus called
lacunar stage
 a thin membrane,
exocoelomic (Heuser)
membrane lines the
inner surface
cytotrophoblast
 Now the blasocyst
cavity lined with
Heuser’s membrane is

11. Days 11 and 12
 completely
embedded in the
endometrial stroma
 syncytiotrophoblast
erode the endothelial
lining of the maternal
capillaries
 Lacunae filled with
maternal blood
establishing the
uteroplacental
circulation
 fluid in the lacunar spaces -embryotroph passes to
the embryonic disc by diffusion and provides
nutritive material to the embryo

12. Contd…
 a layer of loose connective tissue appears between the inner
surface of the cytotrophoblast and the outer surface of the
exocoelomic cavity called extra-embryonic mesoderm,
because it lies outside the embryonic disc

13. Contd…
 small cavities develop in the
extraembryonic mesoderm
fuse to form
extraembryonic coelom,
or chorionic cavity
 extraembryonic mesoderm
splits into two layers.
 extraembryonic
somatopleuric
mesoderm
 extraembryonic
splanchnopleuric
mesoderm
 extraembryonic
somatopleuric mesoderm +
the two layers of
trophoblast = chorion

14. Contd…
 connecting stalk which
later develop into
umbilical cord
 As the embryo implants,
cells of the endometrium
swell because of the
accumulation of glycogen
and lipid in their
cytoplasm and they are
known as decidual cells
 These changes, known as
the decidua reaction
 primary function of the
decidual reaction is to
provide nutrition for the
early embryo.

15. Day 13
 Trophoblast characterized
by villous structures.
 cytotrophoblast proliferate
and penetrate into the
syncytiotrophoblast,
forming cellular columns
surrounded by syncytium;
primary villi
 yolk sac becomes much
smaller than before;
secondary yolk sac

16. 2nd week : Week of two
 The Trophoblast differentiates into two layer
 Cytotrophoblast and Syncytiotrophoblast
 The embryoblast differentiates into 2 layers together
forming a bilaminar disc
 Epiblast and Hypoblast
 Formation of two cavity
 amniotic cavity superior to the epiblast layer
 Yolk sac
 The extraembryonic mesoderm splits into two layers,
 Somatopleuric mesoderm
 splanchnopleurc mesoderm

17. Summary of 2nd week
 The Trophoblast differentiates into
 an inner, layer, Cytotrophoblast, and
 an outer layer, Syncytiotrophoblast
 Primitive uteroplacental circulation begins
 The cytotrophoblast, forms cellular columns
surrounded by the syncytium - primary villi
 The Inner cell mass or embryoblast differentiates
into 2 layers together forming a bilaminar disc
 the Epiblast and
 the Hypoblast
 Epiblast cells give rise to amnioblasts that line the
amniotic cavity superior to the epiblast layer.
 Hypoblast cells are continuous with the Heuser’s
membrane, and together they surround the primitive
yolk sac

18. Contd…
 Extraembryonic mesoderm appears between the
inner surface of the cytotrophoblast and the outer
surface of the exocoelomic cavity.
 When vacuoles develop in this tissue, the
extraembryonic coelom or chorionic cavity forms
 The extraembryonic mesoderm splits into two
layers,
 Somatopleuric mesoderm
 splanchnopleurc mesoderm
 Implantation may also occur outside the uterus,
such as in the rectouterine pouch, on the
mesentery, in the uterine tube, or in the ovary -
ectopic pregnancies

19. Abnormal Implantation
 Occasionally, implantation
takes place outside the
uterus, resulting in
extrauterine pregnancy, or
ectopic pregnancy
 Ectopic pregnancies may
occur at any place in the
abdominal cavity, ovary, or
uterine tube
 Tubal pregnancies most
common type about 95%,
and
 Ovarian or abdominal
pregnancies are
exceptionally rare
 In most cases, the embryo
dies about the 2nd month of
gestation, causing severe

20. Contd…
 Occasionally, the
blastocyst implants close to
the internal os (opening),
so that later in
development, the placenta
bridges the opening and
 the condition is called
placenta previa- a
placenta that partially or
completely covers the os
and causes severe, even
life-threatening bleeding
during delivery