Embryology

Embryology

Lectures on Gynecology

Dr Magda Helmi

Ovulation

Mature follicle

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 (“Hrstmeiotic arrest”), and the oocyteenters 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.

Penetration

Fresh, unmounted human eggs and

embryos

Early maturing oocyte

Fresh, unmounted human eggs and

embryos

Mature oocyte surrounded by granulousal cells,

zona pellucida visible

Fresh, unmounted human eggs and

embryos

Fertilized oocyte demonstrating male and female

pronuclei and both polar bodies

Fresh, unmounted human eggs and

embryos

Two-cell zygote. E,

Fresh, unmounted human eggs and

embryos

Fourcell embryo

Fresh, unmounted human eggs and

embryos

Eight-cell embryo

follicle growth, ovulation

Implantation of the plastocyst

Event Days after Ovulation

Zona pellucida disappears 4-5

Blastocyst attaches to epithelial surface of endometrium6

Trophoblast erodes into endometrialstroma7

Trophoblast differentiates into cytotrophoblastic and 7-8

syncytial trophoblastic layers

Lacunae appear around trophoblast8-9

Blastocyst burrows beneath endometrial surface 9-10

Lacunar network forms 10-11

Trophoblast invades endometrial sinusoids, establishing an 11-12

uteroplacental circulation

Event of implantation

the

urinary

system in

the male

and

female

male and female reproductive systems

Development

of the external

genitalia in

different

stages

Differences

in

developme

nt in

gonads of

each sex

•Oocyte meiosis is arrested at prophase I from the fetal period until the time ofovulation.•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 afterconception. At this point it will be between the 32-cell and blastocyst stages ofdevelopment.•Implantation occurs when trophoblastic cells contact endometrium and burrowbeneath 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 ofimplantation. 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.