Gamete Transport & Fertilization

Where does fertilization occur?

The ampulla (oviduct - middle piece)

How many days after ovulation does the embryo arrive in the uterus?

3-5 days post ovulation

What is retrograde flow?

When sperm flows out of the female after ejaculation, different species have different tactics to overcome this issue

What is the main barrier to sperm transport?

The cervix

Why is the cervix the main barrier to sperm transport?

It has mucus that serves as a selective medium, forms “privileged” channels or micropathways that allow only motile, morphologically normal sperm to easily pass

What barrier does the uterotubal junction present to sperm transport?

It selects against abnormal sperm, sperm with low motility, and pathogens, right at entrance to oviducts

What are some barriers to sperm transport?

Cervix and oviduct (mucus, crypts, folds, etc.), UTJ, phagocytosis by neutrophils

How do sperm know where to go?

A combination between chemical signals and physical forces

What are the bidirectional forces facilitating the transportation of sperm (3x)?

Fluid movement (ejaculation), oviduct contraction, and ciliated cells

What are the unidirectional forces facilitating the transportation of sperm (3x)?

Chemotactic factors (oviduct, follicular fluid, oocyte)

What are sperm cells drawn towards?

High levels of estrogen

What are the 2 phases of sperm transport?

Rapid transport and sustained transport

What does the rapid transport phase of sperm entail?

Sperm cells that reach the oviduct in just a few minutes after ejaculation, utilize motility of the female tract

What does the sustained transport phase of sperm entail?

Sperm reservoirs in the cervix and oviduct allow for sperm docking (attachment to the epithelium)

How long can sperm last when docked vs when not?

Sperm can last for several days when docked vs they will die within 6-10 hrs after ejaculation without docking

What is the capacitation process?

Final sperm maturation to promote fertility and increase in motility (without process, sperm cannot fertilize oocyte), removal of seminal plasma and other molecules from the sperm cell

Where does capacitation occur?

In the cervix, uterus, and oviduct

What are the 6 steps for the oocyte-spermatozoa interaction?

1. Attachment - spermatozoa contacts the zona pellucida

2. Acrosome reaction - enzymes in the acrosome digest the zona pellucida to allow entrance of sperm nucleus

3. Penetration - zona pellucida penetration and interaction with perivitelline space

4. Fusion - plasma membrane of the oocyte and sperm cell fuse

5. Fertilization - sperm nucleus penetrates the oocyte plasma membrane

6. ZP Block - cortical granules fuse with the egg plasma membrane to block other sperm from entering (polyspermy)

What is the importance of cortical granules fuse with the oocyte plasma membrane?

If more than one sperm cells penetrates the zona pellucida, there would be too many chromosomes and therefore genetic material for the embryo to survive and grow correctly

What is syngamy?

The fusion of the male and female pronucleus

What is zygote genomic activation?

When the embryo’s own genes begin to transcribe after fertilization, switching from oocyte (maternal) to zygotic control

How do initial embryonic cells communicate?

Via gap junctions

Why are monozygotic twins identical?

They are derived from the same inner cell mass, which was formed from the same genetic material, the inner cell mass divides evenly to create two embryos

What is the difference between dizygotic and monozygotic twins?

Dizygotic twins arise from double ovulation whereas monozygotic twins arise from the ICM splitting into 2

What regulates the first few mitotic divisions of the embryo?

Maternal gene control (oocyte)

How does the cervix and oviduct’s capacity as sperm reservoirs help boost fertility?

They can help improve fertility potential by extending the sperm cell's lifespan so that there is more time for them to reach the oocyte, and by filtering abnormal or low quality sperm which increases the chances that a high-quality sperm cell will be the one to fertilize the oocyte

What are decapacitating factors?

Factors that are produced by accessory sex glands to stabilize the plasma membrane and limit fertility and motility (help reserve energy), must be removed in the female tract prior to fertilization