Implantation

Hatching

~ day 5 – 6

Glandular epithelium present (swollen) – hormone dependent changes

increased vascularization

Blastocyst reaches uterine cavity and loses ("hatches") zona pellucida (enzymatically digests hole in it and squeezes out)

Attachment

Day 6 – 7

Microvilli on surface of trophoblasts interdigitate with microvilli on surface of endometrium

Penetration of the Uterine Epithelium

Occurs at the embryonic pole

Trophoblasts overlying the ICM produce syncytiotrophoblast (invader cells)

Syncytiotrophoblasts work their way between cells of the endothelium and surround the embryo

Trophoblast consists of 2 layers

Trophoblast secretes HCG (human chorionic gonadotropin)

The Lacunar Stage

As syncytiotrophoblasts invade the endometrium, fluid filled spaces develop and coalesce to form large lacunae

The lacunae become filled with maternal blood and bring maternal circulation into contact with the surface of the placental villi

The lacunae will eventually become the intervillus spaces of the mature placenta

10 – 12 days: embryo is in endometrium and endometrial epithelium regrow over it

At this time:

The Villus Stage

Day 13

Cytotrophoblasts form extensions that grow into syncytiotrophoblast

Placental Circulation

Arteries of endometrium empty into lacunae or intervillus spaces

Placental villi are bathed in maternal blood

Embryo can obtain nutrients and eliminate wastes

Ectopic Pregnancy

Implantation in tubes, ovary, near cervix, intestine, peritoneal cavity

Causes pseudo-decidual reaction