Fertilisation and the Luteal Phase

Question 1

What is meant when ejaculated sperm is referred to as ‘coagulated’?

Answer 1

• prostatic and seminal vesicle secretions comprise seminal fluid which coagulates
• this prevents loss but later liquefies

Question 2

What does the cervical mucus do to ejaculated sperm?

Answer 2

removes the following

• seminal fluid
• abnormally morphological sperm
• cellular debris

Question 3

What is the texture of the cervical mucus like when the sperm are passing?

Answer 3

• less viscous
• due to absence of progresterone
• allowing sperm to pass

Question 4

The sperm don’t only simply pass through the cervix - where else around here can they inhabit?

Answer 4

• Sperm can inhabit cervical crypts which form a reservoir
• Some evidence of thermotaxis
• Mechanism not yet elucidated

Question 5

How long can sperm survive in the female?

Answer 5

24-48 hours usually

(but can live up to 5 days in some)

Question 6

Describe the sperm’s journey to the egg

Answer 6

• passage through uterus not well understood
• currents set up by uterine/tubal cilia
• chemo-attractants released from oocyte cumulus complex
• sperm become hyperactivated
• forceful tail beats w/ inc freq + amplitude mediated by ca²⁺ influx
• via CatSper channels (on sperm tail)

Question 7

Fresh ejaculate is not capable of fertilising an egg. What is capacitation?

Answer 7

• capacitation partly achieved by removing sperm from seminal fluid
• also uterine or tubal fluid may contain factors which promote capacitation
• biochemical rearrangement of surface glycoprotein
• changes in membrane composition
• must occur before acrosome rxn can occur

Question 8

What is the acrosome reaction?

Answer 8

• occurs in contact with the cells surrounding the egg
• the acrosomal membrane on sperm head fuses
• releasing enzymes that cut through outer layers of cumulus surrounding egg
• acrosin bound to the inner acrosomal membrane binds + digests the zona pellucida so the sperm can enter

Question 9

What happens at ovulation?

Answer 9

• LH spike causes resumption of meiosis + ovulation
• converts primary oocyte -> secondary oocyte + 1^st polar body
• basement membrane breaks -> blood pours into middle
• oocyte cumulus complex extruded out + caught by fimbrae of uterine tube
• theca + granulosa become mixed

Question 10

What is the role of progesterone in the luteal/secretory phase?

Answer 10

• makes endometrium secretory + receptive to implantation
• suppresses cilia in uterine tubes once oocyte has already passed
• makes cervical mucus viscous again to prevent further sperm penetration

Question 11

What is the role of oestradiol in the lutea/secretory phase?

Answer 11

• helps to maintain endometrium in luteal phase
• causes proliferation of endometrium in follicular phase (where it’s more important)

Question 12

What is the corpus albicans and how does it form?

Answer 12

• if fertilisation does not occur, corpus luteum has an inbuilt finite lifespan (14days)
• regression of CL essential to initiate new cycle
• fall in CL-derived steroids (progesterone) -> inter-cycle rise in FSH
• cell death occurs, vasculature breakdown, CL shrinks
• over time it becomes a corpus albicans

Question 13

What hormone will prevent the corpus luteum from degenerating?

Answer 13

• hCG - binds to LH receptors on CL
• prevents from degenerating into corpus albicans
• keeps prog high, maintains endometrium + CL
• huge rise of hCG in first few weeks of pregnancy
• hCG is what is detected by weeing on pregnancy test stick

Question 14

What does the menstrual cycle achieve?

Answer 14

• selection of a single follicle + oocyte
• regular spontaneous ovulation
• correct haploid number of chromosomes in oocyte
• cyclical changes in cervix + uterine tubes, to enable egg transport + sperm access
• prep of endometrium of uterus to receive fertilised egg
• support of implanting embryo + endometrium by CL progesterone
• initiating a new cycle if fertilisation does not occur

Question 15

Describe the structure of the oocyte at ovulation

Answer 15

• cumulus cells:
  
  • cumulus oophorus (was granulosa cells)
  • corona radiata (secretion of EC matrix - hylauronic acid)
  • protect egg, secrete mucus matrix projections into plasma membrane
• zona pellucida (secreted by egg around projections) - sperm binds to specific proteins expressed in zona pellucida (ZP3), species specific too
• periviteline space - under ZP
• 1^st polar body - suggests 1st meiotic divison resumed + completed

Question 16

What happens to allow sperm binding and penetration?

Answer 16

• sperm release enzymes to cut through cumulus + bind to ZP (eg. ZP3) triggering acrosome rxn
• acrosome rxn, sperm enzymes cut through ZP + sperm fuses w/ plasma membrane
• sperm taken in by phagocytosis, tail + mitochondria left behind
• movement of cortical granules in egg + release of contents hardens the oocyte coat + prevents polyspermy

the sperm has enzymes which degrade the hylauronic acid in the corona radiate as well

Question 17

Describe syngamy (part 1)

Answer 17

Question 18

Describe syngamy (part 2/2)

Answer 18

Question 19

Summarise fertilisation following the sperm entering the oocyte resulting in Ca²⁺ waves

Answer 19

Question 20

Describe early embryo development

• first sign of fertilisation?
• when is it called a blastocyst?

Answer 20

• the fertilised egg has 2 pronuclei - first sign of fertilisation
• developing embryo contains 6-8 cells 3 days after fertilisation
• five days after fertilisation it is called a blastocyst + has approx 100 cells

Question 21

Briefly describe the timing of oocyte to embryo

Answer 21

• ovulated secondary oocyte
• fertilisation
• zygote
• day 2 - 2 cells
• day 3 - 4 cells
• day 4 - morula
• day 5 - blastocyst implants
• embryo