1. Fertilization involves the fusion of an egg and sperm, forming a zygote with a full set of chromosomes. The zygote then undergoes cleavage, forming a morula by day 3 and a blastocyst by day 5 as it travels down the fallopian tube. 2. The blastocyst implants in the uterine wall around 1 week after fertilization. The trophoblast cells of the blastocyst initiate implantation and will later form the placenta, while the inner cell mass gives rise to the embryo. 3. During the first week, the zygote develops from a single cell through cleavage, compaction, blastulation and begins implantation in the uterus.

1. PROF. DR. AISHA QAMAR

2. • Describe the
phases of
fertilization
• Describe the
events of first
week of
development

3.  At the end of this session, students should be
able to:
Describe the first week of human
development with reference to:
1. Fertilization process
2. Cleavage of the zygote
3. Blastogenesis
4. Initiation of implantation

4. • Q-1. A newborn baby girl showed upward
slanting eyes, depressed nasal bridge and
simian crease in hand, but her muscle tone
was normal. Karyotyping revealed that some
cells had an extra copy of chromosome 21,
while other cells were normal

5. • Q-2. A 35-year-old primigravida presented to the
emergency room with profuse bleeding per
vagina after 2 months of secondary amenorrhea.
The karyotyping of the aborted conceptus was
done which showed triploidy

6. • Fertilization
• Cleavage
• Blastocyst
formation
• Initiation of
implantation

7. • Process by which
male and female
germ cells fuse
• Site: ampullary
region of
fallopian/
uterine tube

8. Capacitation: period of
conditioning (7 hours)
Acrosome reaction:
occurs after binding to
zona pellucida
Induced by zona proteins
Release of enzymes like
trypsin and acrosin-like
substances

9. • Phases
1. Penetration of
corona radiata
2. Penetration of
zona pellucida
3. Fusion of oocyte
and sperm cell
membranes
Takes
approximately 24
hours

10. Capacitated sperm
passes freely through
corona radiata
Results mainly from
action of enzyme
hyaluronidase released
from acrosome of
sperm
Tubal mucosal enzymes
Movement of sperm tail

11. • Facilitated by
release of
acrosomal
enzymes like
acrosin,
esterase, and
neuriamidase
• Zona reaction:
prevents
polyspermy

12. • Results in:
• completion of 2nd
meiotic division
of oocyte and
• formation of
female
pronucleus
• Replicates its
DNA

13. • Nucleus of sperm
enlarges to form
male pronucleus
• Tail degenerates
• Both pronuclei
identical
• Replicate their DNA
• Oocyte containing
two haploid
pronuclei is known
as ootid

14. • Pronuclei fuse
into a single
diploid
aggregation of
chromosomes,
ootid becomes
zygote
• Chromosomes
become arranged
on a cleavage
spindle

15. • Completes second
meiotic division
• Restores diploid number
of chromosomes
• Results in variation of
human species
• Determines
chromosomal sex of
embryo
• Causes metabolic
activation of zygote

16. Repeated mitotic
divisions of zygote
(blastomeres)
Cells become
smaller with each
division
Zygote lies within
zona pellucida
Division of zygote
into blastomeres
begins 30 hrs after
fertilization

17. • Compaction: After
nine-cell stage,
blastomeres
tightly align
themselves to form
a compact ball of
cells
• 12 to 32-
blastomere stage is
called morula,
formed on 3rd day
after fertilization

18. Fluid filled cavity
(blastocoele)
appears as it enters
uterus
Divides blastomeres
into 2 parts;
1. Outer cell mass/
Trophoblast:
placenta
2. Inner cell mass/
Embryoblast:
embryo

20. Early pregnancy
factor
(immunosupressant
protein) secreted by
trophoblast 24-48
hours after
fertilization
Forms basis of
pregnancy test
zona pellucida
degenerates

21. • As blastocyst
attaches to uterine
endometrium,
trophoblast
differentiates into
• Cytotrophoblast
and
• Syncytiotrophoblast

22. • Starts on appox day
6 (day 20 of a 28
day cycle)
• Trophoblast
proliferates rapidly
• By end first week,
blastocyst
superficially
implanted in
compact layer of
endometrium

23. • A layer of cells,
hypoblast, appears
at day 7 on surface
of embryoblast
facing blastocele
• Arises by
delamination of
blastomeres from
embryoblast

24. • Triploid conception
• Sex ratio
• IVF and embryo
transfer
• Mosaicism
• Early spontaneous
abortions

26. Q-1. A newborn baby girl showed upward slanting
eyes, depressed nasal bridge and simian crease in
hand, but her muscle tone was normal. Karyotyping
revealed that some cells of her body had an extra
copy of chromosome 21, while other cells were
normal
• a. Which condition was most likely present in this
neonate?
• b. Exlain the reasons for having two types of
chromosome constitution on karyotyping

27. Q-2. A 35-year-old primigravida presented to ER
with profuse bleeding per vagina after 2 months of
secondary amenorrhea. The karyotyping of the
aborted conceptus was done which showed triploidy.
• a. What is the reason for above condition?
• b. Which step in fertilization prevents above
mentioned condition?

28. • Langman’s Medical Embryology: Saddler
TW; 12th Edition, 2012
• The developing human. Moore et al.,
11th Edition; 2019