1) The document discusses the key stages in the first week of human development including fertilization, cleavage, formation of the morula and blastocyst, and implantation. 2) Fertilization involves the fusion of an ovum and spermatozoa to form a zygote, which occurs in the fallopian tube. The zygote then undergoes cleavage divisions as it moves through the uterine tube. 3) By day 5-6, the blastocyst has formed with an inner cell mass and outer trophoblast layer. The blastocyst implants in the endometrium around day 7, initiating formation of the placenta and decidua. Abnormal implantation can result in

1. I week of development
Dr. G. Prabavathy
Assistant Professor
Department of Anatomy

2. Objectives
• Gametogenesis:
– Spermatogenesis
– Oogenesis
• Changes in the first week:
– Fertilisation- stages, effects of it
– Cleavage
– Morula
– Blastocyst
– Implantation

4. Fertilization
• Fertilization means the fusion of two mature
germ cells, an ovum and a spermatozoa, to
form a single cell, the Zygote.
• It occurs in ampulla of the
uterine tube.

5. Fertilization

6. Interstitial part

7. Stages of fertilization
a) Approximation of the gametes – involves
transport of sperms and ova – Prostaglandins,
Oxytocin
b) Contact and fusion of the gametes
c) Effects of fertilization

8. How the sperms reach
the site of fertilization?
1. By movement of their
tail.
2. By the movement of
uterine cilia.
3. By chemo attraction.
How the oocyte reaches
the site of fertilization?
1. By the movement
fimbria of the uterine
tube.
2. By peristaltic
movement of the
uterine tube.

9. Secondary oocyte
• Secondary
oocyte(metaphase of 2nd
meiotic division) with
corona radiata enters the
fallopian tube.
Vitelline
membrane

10. Contact and Fusion of the Gametes
• Out of 200 – 300 million
sperms-300-500 sperms
reach ovum; only one
unites with oocyte.
• 3 barriers protect female
gamete
– Corona radiata
– Zona pellucida
– Vitelline membrane
Vitelline
membrane

12. Before the sperm fertilizes the oocyte it must
undergo 2 processes
Capacitation:
• Final step of maturation of the spermatozoa in female
genital tract.
• Requires 7 hrs of time.
• Glycoprotein &seminal proteins are removed from
the surface of the sperm.
Acrosome Reaction:
• It occurs after capacitation where proteolytic
enzymes as acrosin, esterases, hyaluronidase, and
neuraminidase are liberated in order to penetrate the
zona pellucida.

13. • First barrier disintegration-
Hyaluronidase
• Second barrier – Acrosin-zona
reaction
• Third barrier – disintegrin
peptides

14. – Completion of second meiotic
division, formation of mature
ovum & extrusion of second
polar body.
Male pronucleus – sperm
head with its nuclear envelope
swollen, tail degenerates.
Female pronucleus – nucleus
of mature ovum.
Female
pronucleus

16. Effects of fertilization
a) Completion of second meiotic division – mature
ovum & 2nd polar body.
b) Restoration of diploid number of chromosomes in
zygote.
c) Determination of chromosomal sex.
d) Initiation of cleavage division of the zygote.
e) Determination of polarity of the embryo.

17. Assisted Reproductive Techniques
In vitro Fertilization (IVF)
Two or three of the resulting
zygotes are placed back in
the uterus (embryo
transfer ) after 3 days of
fertilization.

18. Intracytoplasmic sperm injection
It is the method of choice in cases of infertility due to the male
factor (severe oligospermia). It implies the same process as
IVF except that the best sperm is selected and injected into
the ovum cytoplasm.

19. Stages of Embryology
1. Germinal period : 1st to 3rd week
- cleavage division, blastocyst & its implantation,
differentiation of trophoblast & chorion, Bilaminar &
Trilaminar germ disc.
2. Embryonic period : 4th to 8th week
- shape & external appearance of embryo.
3. Foetal period : 3rd month upto birth
- rapid growth of fetus, complete placental development.

20. Cleavage
Definition: It is the repeated mitotic divisions of the
zygote, resulting into a rapid increase in
the number of cells that are called blastomeres.
Site: The uterine tube medial to the ampula.

22. Cleavage
• 2 cell stage – 30hrs after
fertilization
• 4 cell stage – 40-50 hrs after ftz
• 12 cell stage – 72hrs after ftz
• 16 cell stage – 96 hrs after ftz.

23. • The zygote gradually migrates from uterine tube,
reaches the uterine cavity at 12 to 16 cell stage – 72
hrs after ftz.

24. Morula
• 12 cell to 16 cell stage.
• It has mulberry appearance.
• Inner cell mass – embryo proper.
• Outer cell mass – protective &
nutritive coverings of the
embryo.
• Presence of Zona pellucida.
Zona pellucida

25. Blastocyst
• Shortly after the morula enters
the uterus, uterine fluid passes
through the ZP to form a fluid
filled cavity – Blastocele.
• The morula after the formation
of blastocele is called
blastocyst
• Outer cell mass – trophoblast
(future placenta)
• Inner cell mass – embryoblast
(future embryo)

26. • Blastocyst is formed
within ZP on 4th or 5th day
of fertilization .
• Zona pellucida disappears
– end of 5th day of
development
• Implantation of blastocyst
– 6th or 7th day after
fertilization.
Blastocele.
trophoblast.
embryoblast

27. Implantation

28. Implantation
Definition: Penetration of the blastocyst into the
superficial (compact) layer of the endometrium.
Time: Implantation occurs at the 6th day after
fertilization and is completed about the 11th day.

29. • Site: The normal site is the endometrium of the
posterior wall of the fundus of the uterus in or near
the middle line. The endometrium after implantation
is called decidua.

30. Types (parts) of decidua
1. Decidua basalis (part of
endometrium between the
blastocyst & the
myometrium ).
2. Decidua paritalis
(endometrium covers the
wall of the uterus).
3. Deciduas capsularis
(endometrium between the
blastocyst & uterine cavity).
4. Decidua marginalis
(endometrium surrounding
the site of implantation).
Definition: It is the compact layer of the endometrium
after implantation of the blastocyst.

31. • By the 5th day the Zona pellucida degenerates.
• Trophoblast cells penetrate the epithelium of the
endometrium.
Stages of Implantation

32. By 6th day the blastocyst adheres to the endometrium
By 7th day, Trophoblast differentiated into 2 layers
• Cytotrophblast, inner layer,
mitotically active.
• Syncytiotrophoblast (outer
multinucleated mass, with
indistinct cell boundary.
By 8th day the blastocyst is
superficially embedded in
the compact layer of the
endometrium

34. Implantation - embedding of blastocyst into
uterine lining begins at day 7
Blastocyst - with blastocoele cavity
Trophoblast - outer layer of cells
Inner cell mass - will form embryo
Lacunae and primary villi formed by
trophoblast
All of these form placental tissues
Fig 28-3
Trophoblast forms syncytial trophoblast- erodes
into endometrium
Cellular trophoblast - carries nutrients to inner
cell mass

35. Implantation
can be detected by:
1- Ultrasonography.
2- hCG (human
chorionic gonadotrophin
which is secreted by the
Syncytiotrophoblast)
about the end of 2nd week

37. Abnormal sites of implantation
1. Uterine ectopic pregnancy:
a. At the cornu of the uterus
(the site of attachment of
the uterine tube) leading
to early abortion.
b. At the lower uterine
segment leading to
placenta previa.
c. Cervix, leading to cervical
pregnancy and early
abortion.
mesentry
tube
ovary
Internal os
It is also called ectopic
pregnancy. It might result in
death of the embryo or early
abortion with severe internal
hemorrhage.
1,A
2,C
1,C
1,B
2,A

38. Extra uterine ectopic pregnancy:
a. The commonest site is in the Fallopian
tube leading to tubal pregnancy that
leads to early abortion or tubal rupture
with severe internal haemorrhage.
b. Ovary (ovarian pregnancy), broad
ligament or omentum (rare).
c. Peritoneum of Douglas pouch
(abdominal pregnancy).

39. Ectopic Pregnancy

40. Tubal Ectopic Pregnancy

41. PLACENTA PREVIA
Definition:
• Implantation of the blastocyst
in the lower segment of the
uterus close to the internal
os.
• Thus the placenta will
precede the fetus at delivery
(normally, the fetus is the
lowermost and fills the lower
segment). Previa = at the
front of the fetus.

42. Important questions
• Fertilisation
• Cleavage
• Morula
• Blastocyst
• Implantation
• Abnormal sites of implantation

43. Thank You