Implantation

Question 1

Outline the steps taken after fertilization to implantation.

Answer 1

Day 1: 2-cell stage in isthmus

DAy 3: morula in isthmus/corona radiata lost

Day 4: blastocyst in uterus

Day 5: loss of zona pellucida

Day 6: beginning of implantation

Question 2

What is the process of shedding the zona pellucida called?

Answer 2

-blastocyst hatching

Question 3

What are the stages of implantation?

Answer 3

1. Attachment of expanded blastocyst to the endometrial epithelium
2. Penetration of the uterine epithelium
3. Invasion into the tissues underlying the epithelium
4. Erosion of the maternal vascular supply

Question 4

What occurs during the attachment of expanded blastocyst to the endometrial epithelium?

Answer 4

• initial contact is between endometrial surface and the embryonic pole of the blastocyst
• adhesion molecules expressed by the endometrium and additional adhesion molecules expressed by the trophoblast cells facilitate attachment
• leukemia-inhibiting factor expressed by the endometrium and respective receptors on the trophoblast may be important

Question 5

What occurs during the penetration of the endometrium?

Answer 5

• trophoblast separates into a cellular cytotrophoblast and multinucleated syncytiotrophoblast
• syncytiotrophoblast projections penetrate the endometrial basal lamina and begging invading the endometrial stroma
• trophoblastic lacuna appear
• eroded endometrial blood vessels fill the trophoblastic lacunae with maternal blood
• trophoblast is less invasive at this point

Question 6

What is the decidual reaction? Why is this important?

Answer 6

• the response of the endometrial stroma cells to the invading blastocyst
• primary function is to provide an immunologically privileged site to protect the developing embryo from being rejected

-without this reaction, the trophoblast could potentially erode all the way through the endometrial wall

Question 7

What are the decidual cells derived from?

Answer 7

-endometrial stroma cells that accumulate glycogen and lipid droplets

Question 8

How does the decidual reaction work?

Answer 8

-forms a massive cellular matrix that surrounds the embryo and fills most of the endometrium

• infiltrating leukocytes in the endometrium secrete interleukin-2
• interleukin-2 prevents the mother’s immune system from recognizing the embryo as a foreign body

Question 9

What is an ectopic pregnancy?

Answer 9

-implantation of the blastocyst somewhere other than the uterus

Question 10

Where do ectopic pregnancies typically occur?

Answer 10

• 54% ampullary
• 25% isthmic
• 17% fimbrial
• 2% interstitial
• 0.5% ovarian
• 0.3% cervical
• 1% rectouterine pouch

Question 11

When and where does fertilization typically occur?

Answer 11

-in the upper ampulla within 12 hours after ovulation