Fertilisation and Implantation

Question 1

What are the two tissue types of the Cervix?

Answer 1

Endocervix: ​
Columnar epithelium (cont w endometrium)
Glands, crypts (storage sites for sperm),
Fibrous stroma, a few smooth muscle cells

• *Ectocervix**
• Stratified squamous (flattened) epithelium*

These join in the space on 1 cell!

Question 2

What is the cervical smears and cervical immunisations for? Who should get them?

Answer 2

1. Sexually active women from 20-70yrs should have a cervical smear every 3 years. Having regular smears can decrease a womens risk of cervical cancer by 90%!
2. Vaccine called Gardasil is an immunisation given to males and females: but only against specific serotypes, so still reccomend cervical screening even with vaccine!

HPV → carcinoma in situ at cervix → cervical cancer

Question 3

How does the cervical mucous change every cycle?

Answer 3

You will see changes due to a change in hormonal profile of E₂ and P₄ in:

1. Volume
2. Viscosity
3. Threadability (spinnbarkeit): in the fertile window (ovulation, this occurs

Question 4

How does E₂ and P₄ change the cervical mucous each cycle?

Answer 4

E2: increases volume, decreases viscosity (clear, watery mucous), and inceases threadability. Makes mucous receptive to sperm, and contains channels for sperm access.

P4: Stimulates a highly viscous and cross-linked mucous that is a barrier to sperm penetration.

Test putting mucous in a capillary tube, and testing how far through sperm can get. Shows lots of movement only with high estrogen (ovulation!!)

Question 5

Stage 1) Meiosis in a female

Answer 5

Following LH surge, meiosis is resumed (also causes oocyte released) and the first reductive division is complete. (2n → n + polar body)

Process stops again at second metaphase.

Thus at ovulation, the egg consists of the oocyte and a small polar body (not used by oocyte) surrounded by the Zona Pellucida.
(Polar body retained in the ZP)

Meiosis is resumed for the second divison, at ovulation

Question 6

What happens to the sperm once ejaculated?

Answer 6

Leaves male reproductive tract bathed in seminal plasma

• As the prostatic and seminal vesicle fluids mix for the 1^st time (in the vagina), a coagulum is formed that:
  
  1. Holds the ejaculate together
  2. Adheres the ejaculate to the female reproductive tract during/after coitus
• Within 2-15min after vaginal insemination; “liquefaction” occurs and the sperm can begin their quest for an egg

Question 7

What is sperm liqufaction?

Answer 7

Breakdown of coagulated ejaculate 2-15min post insemination, dispersing the coagulum and therefore allow sperm to find an egg

Question 8

How does the pH of the vagina affect sperm?

Answer 8

Vaginal pH: ~4.7 (3.5-7.4)
This pH is inhibitory to sperm motility and survival

Seminal plasma buffers the vaginal pH to around 7.2, allowing sperm to be motile.

Question 9

Describe the path sperm must take and the barriers they overcome to get to the uterus

Answer 9

From vagina → uterus → fallopian tubes

1. Migrate from vagina to cervix, the 1^st barrier being the cervix
2. The cervix has a short window of time (around ovulation) where it’s cervical mucous is not hostile to sperm transport
3. During this time of ‘periovulation’ the cervical mucous is thin and produced in large amounts. it aligns it’s fibres allowing sperm passage

Question 10

How fast do sperm move through the cervix?

Answer 10

~1.2-3mm / minute

Question 11

How does cervical mucous ‘pick the best sperm’?

Answer 11

The tails of normal sperm cause cervical mucous fibres the vibrate in rhythm, ‘in cadence’ allowing them to break through.
(think of movement on a bridge)

Failure of normal sperm to induce this vibration → stops progress.

Abnormal sperm can’t get through!

Question 12

How long do sperm survive, and how does that effect transport?

Answer 12

Survival Times:

• Vagina: 2.5h (due to acidic environment)
• Cervix: 48h (stored/released from crypts)
• Uterus: 24h
• Oviduct: 48h

Overal: 48+ hours post coitus. After this, sperm will usually die

Note: clinical advice to people AVOIDING getting pregnant say ~5days (just in case), ask them to work back from menstruation, but as this hasn’t happened yet its very hard!

Question 13

How long is transport time of sperm from vagina → fallopian tubes.

Answer 13

~2-7hours

Swimming speed= ~55micrometres/sec

= 20cm/hr

Speed is related to fertility
Speed decreases with time
Speed decreases with freezing and thawing

Question 14

How does sperms swimming style indicate fertility?

Answer 14

Thrashing motion helps them get around barriers.

Question 15

Upto _____ million sperm may be contained in a human ejaculate, but of these only ____ million will enter the uterus.

Once past the cervix, sperm may move to the utero-tubal junction in about ____ minutes.

Answer 15

Upto 200 million sperm may be contained in a human ejaculate, but of these only 1 million will enter the uterus.

Once past the cervix, sperm may move to the utero-tubal junction in about 30 minutes.

*some sperm may stay in the cervical crypts for a bit then move to uterus late

Question 16

What happens if sperm get to the fallopian tubes prior to ovulation?

Answer 16

Their rate of tail beating slows and they wait in the narrow isthmus of the tube.

At ovulation sperm move up to the egg and stored sperm may be released from cervical crypts

Question 17

Where does fertilisation usually occur?

Answer 17

Near the isthmus/ampulla junction.

Question 18

If only one sperm fertilises an egg, what happes to the rest of the sperm?

Answer 18

Sperm in the uterus induces an innate immune response.

You get an infilitration of leucocytes (leucocytosis), which phagocytose dead/dying sperm.

There’s an idea this could be an adaptation where mum can see what dad looks like?

Question 19

What is capacitation?

Answer 19

A series of events, occuring in the female reproductive tract, to allow sperm to be mobile

Freshly ejaculated sperm are not capable of fertilising an ovum!
Only capacitated sperm can undergo the acrosome reaction

Involves:

• Removal of capacitation inhibitory substances from sperm
• Loss of cholesterol from sperm membrane.
• Induces hyperactivtion of sperm.

Question 20

Capacitated sperm are said to be ‘hyperactive’. WHat does this mean and what can they now do?

Answer 20

• Tail movement changes with increased bending of tail and swinging of the head “ thrashing”
• Facilitates movement of sperm through tortuous folds of the fallopian tube!! (instead of get stuck

Question 21

What’s the acrosome?

Answer 21

Organelle over anterior head of sperm. A membranous pouch containing proteinases and other enzymes such as hyalluronidase.

Question 22

What’s the acrosome reaction for?

Answer 22

Fusion of the sperm heads acrosomal and plasma membranes.

This allows release of the acrosomal contents (help breakdown of 2^o oocyte exterior) to the environment around the sperm.

This triggers: an influx of calcium into the cell.

• Ca²⁺ influx is triggered by several factors, eg; progesterone or ZP₃

Location: the acrosomal reaction must occur in close proximity to the oocyte for natural fertilisation. Usually around 10% spontaneously undergo reaction distally → infertile sperm

Question 23

Exposed acrosomal membrane post A.R contains?….

Answer 23

Receptors for zona pellucida (ZP₃) molecules and oocyte membrane.

Question 24

Steps of fertilsation?

Answer 24

1. Oocyte meets sperm in ampulla (oocyte survives <24hrs if unfert.)
2. In vivo, the sperm penetrates the layer of cumulus cells surround the egg.
   
   • This is assisted by hyaluronidase secreted from the acrosome→ digests the basement membrane, allowing sperm to squeeze past c.cells
   • This secretion of hyaluronidase may be only partial release of acrosomal products, stimulated by progesterone contained in the fluid surround the oocyte
3. Sperm digests its way past the corona radiata (layer of tightly packed follicle cells)
4. Gets to Zona pellucida (full Acrosomal reaction induced, binds to ZP₃ molecules in ZP)
5. Aided by enzymes it breaks down ZP, gets to perivitelline space surrounding 2^o oocyte
6. Sperm attaches to microvilli on the oolemmal membrane, and the 2 membranes fuse → sperm nucleus can enter the ovum

Question 25

What’s the Zona Pellucida?

Answer 25

Protein coat surrounding ovum (like egg but not calcified)

ZP₁ , ZP₂ and ZP₃ main proteins.

ZP₃ : primary sperm receptor. Contact between ZP₃ and the sperm ligan for ZP₃ → induces complete acrosomal reaction

Ligandfor ZP₃ is unknown

Question 26

What’s the Cortical Reaction?

Answer 26

Happens post-fertilisation to allow only one sperm to fertilise the egg.

• intracellular Calcium released post sperm entering the egg.
• Followed by regular spikes of Calcium in the oocyte.
• THIS is what resumes meiosis!!
• Cortical granules released into perivitelline space → release of enzymes that attack Zona pellucida , hardening it. (eg proteases and beta-hexosaminidase)
  
  • Cleave ZP₂, digest ZP₃ and cross link the zona (involves ZP₁)
• BLOCKS POLYSPERMY

Question 27

Meiotic divisons in the follicle are unequal.

Answer 27

2nd meotic division is induced by fertilisation

• Ca rise
• Destabilises a protein complex that held chromosome in metaphase
• Loss of complex allows chromosome to migrate to poles and for meiosis to resume

and involves a second polar body. Still kept in the zona pellucida!

Question 28

What happens to a polyspermic fertilisation?

Answer 28

Will not survive. triploidy not liveable!