This document discusses various stages of human embryology including oocyte meiosis, fertilization, early embryonic development, and implantation. It notes that oocyte meiosis is arrested until ovulation. Fertilization occurs in the fallopian tube before the second polar body is released. Implantation begins around 6 days after ovulation as the blastocyst attaches and begins burrowing into the endometrium over subsequent days.

1. Embryology
Lectures on Gynecology
Dr Magda Helmi

4. Ovulation

5. Mature follicle

6. oocyte meiosis
 only three pairs of
chromosomes are depicted. 1-
4, Prophase stages of the first
meiotic division, which occur in
most mammals during fetal life.
The meiotic process is arrested
at the diplotene stage (“Hrst
meiotic arrest”), and the oocyte
enters the dictyate stages (5-6
When meiosis is resumed, the
first maturation division is
completed (7-11 Ovulation
occurs usually at the
metaphase II stage (11), and
the second meiotic division (12-
14) takes place in the oviduct
only after sperm penetration.

7. Penetration

9. Fresh, unmounted human eggs and
embryos
 Early maturing oocyte

10. Fresh, unmounted human eggs and
embryos
 Mature oocyte surrounded by granulousal cells,
zona pellucida visible

11. Fresh, unmounted human eggs and
embryos
Fertilized oocyte demonstrating male and female
pronuclei and both polar bodies

12. Fresh, unmounted human eggs and
embryos
Two-cell zygote. E,

13. Fresh, unmounted human eggs and
embryos
Fourcell embryo

14. Fresh, unmounted human eggs and
embryos
 Eight-cell embryo

15. follicle growth, ovulation

16. Implantation of the plastocyst

18. Event of implantation
Event Days after
Ovulation
Zona pellucida disappears
4-5
Blastocyst attaches to epithelial surface of endometrium
6
Trophoblast erodes into endometrialstroma
7
Trophoblast differentiates into cytotrophoblastic and 7-
8
syncytial trophoblastic layers
Lacunae appear around trophoblast
8-9
Blastocyst burrows beneath endometrial surface
9-10
Lacunar network forms
10-11
Trophoblast invades endometrial sinusoids, establishing an
11-12
uteroplacental circulation

19. the
urinary
system in
the male
and
female

20. male and female
reproductive systems

21. Development
of the external
genitalia in
different
stages

22. Differences
in
developme
nt in
gonads of
each sex

23. •Oocyte meiosis is arrested at prophase I from the fetal period until the time of
ovulation.
•Fertilization occurs in the ampulla of the fallopian tube before the second polar body
is cast off.
•After fertilization, first cell division leading to the two-cell embryo takes 20 hours.
•The human embryo enters the uterus somewhere between 3 and 4 days after
conception. At this point it will be between the 32-cell and blastocyst stages of
development.
•Implantation occurs when trophoblastic cells contact endometrium and burrow
beneath the surface by enzymatic action. This generally takes place 3 days after the
embryo enters the uterus.
•Twinning may occur at any time until the formation of the blastula, after which time
each cell is no longer multipotential.
•The earliest fetal epithelium to develop is the ectoderm; the second is the endoderm;
and the third is the mesoderm.
•Chorionic gonadotrophin is secreted by the syncytiotrophoblast at about the time of
implantation. It doubles in quantity every 1.2 to 2 days until 7 to 9 weeks of gestation.
•Angiogenesis is seen by day 15 or 16. Embryonic heart function begins in the third
week of gestation.