Fertilisation and Luteal phase (repro) Flashcards
Ejaculated sperm
- ejaculated sperm is coagulated
- prostatic and seminal vesicle secretions comprise seminal fluid which coagulates
- this prevents loss and later liquifies
- movement through cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris
- sperm then passes into cervix
- cervical mucus is less viscous in the absence of progesterone, allowing sperm to pass
- sperm can inhabit cervical crypts which may form a reservoir
- some evidence of thermotaxis, but mechanism not yet elucidated
- fertilisation typically within 24-48 hours but sperm have been recovered alive after 5 days
Journey to the Egg
- passage through uterus well understood
- currents set up by uterine or tubal cilia may have a role
- chemo-attractants released from oocyte cumulus complex may attract the sperm
- sperm become hyper-activated
- forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels
Sperm capacitation and Acrosome reaction
Capacitation:
- partly achieved by removing sperm from seminal fluid
- uterine or tubal fluid may contain factors which promote capacitation
- biochemical rearrangement of surface glycoprotein and changes in membrane composition must occur before acrosome reaction can occur
Acrosome reaction:
- occurs in contact with the zone-cumulus complex
- ayrosomal membrane on sperm head fuses releasing enzymes that cut through complex
- acrosin bound to the inner ayrosomal membrane digests the zone pellucid so sperm can enter
Ovulation
- LH spike causes resumption of meiosis and ovulation
- converts primary oocyte plus 1st polar body
- basement membrane breaks so blood pours into middle
- oocyte cumulus complex extruded out and caught by fimbrae of uterine tube
- theca and granulosa become mixed and empty follicle is known as corpus luteum
- produces progesterone in luteal phase
Progesterone and oestrogen
Progesterone:
- makes endometrium secretory and receptive to implantation
- surpresses cilia in uterine tubes once oocyte has passed
- makes cervical mucus viscous again to prevent further sperm penetration
Oestrogen:
- helps to maintain endometrium in luteal phase
- causes proliferation is follicular phase
Demise of the Corpus Luteum
- if fertilisation does not occur, CL has an inbuilt finite lifespan of 14 days
- regression of CL is essential to initiate new cycle
- fall in CL-derived steroids causes inter-cycle rise in FSH
- cell death occurs, vasculature breakdown, CL shrinks
- overtime it becomes a corpus albicans
- CL reduced in pregnancy by hCG from embryo binding to its LH receptors
- CL continues to produce progesterone and maintain endometrium
Menstrual cycle outcomes
- selection of a single follicle and oocyte
- regular spontaneous ovulation
- correct haploid number of chromosomes in oocyte by completing meiosis l and beginning meiosis ll
- cyclical changes in cervix and uterine tubes to enable egg transport and sperm access
- preparation of the endometrium of uterus to receive fertilised egg
- support of the implanting embryo and endometrium by corpus luteum progesterone
- initiating a new cycle if fertilisation does not occur
Oocyte at ovulation
Cumulus oophorus - protect egg
- derived from granulosa cells
Corona oophorus - innermost layer of cumulus cells in contact with zone pellucid
- formed by granulosa cells adhering to oocyte before it leaves ovarian follicle
Zona pellucida
- secreted by egg
- becomes impenetrable after fertilisation
1st polar body
- 1st meiotic division completed
Sperm binding and penetration
1) Acrosome reaction occurs in contact with zona-cumulus complex. Sperm penetrate cumulus and bind to ZP
2) Sperm enzymes cut through ZP and sperm fuses with plasma membrane
3) sperm taken in by phagocytosis. Phospholipase Zeta activated by basal Ca2+ inside egg. PIP2 to DAG + IP3. Causes release of intracellular Ca2+ leading to large Ca2+ spike
4) Cortical reaction as wave of Ca2+ sweeps around egg. release of proteases, peroxides and hyaline prevents polyspermy
Syngamy
- After meiosis l, oocyte has 23X chromosomes, but 2 copies of each chromosome arranged as sister chromatids
- entry of sperm causes an increase in Ca2+via phospholipase zeta from sperm
- Ca2+ causes completion of meiosis ll expelling the second polar body and cortical reaction
- sperm nuclear membrane breaks down
- chromatin decondeses and chromosomes separate
- 4-7 hours after sperm penetration, the 2 sets of haploid chromosomes become surrounded by distinct membranes forming two pronuclei
- these haploid structures synthesise DNA in preparation for first mitotic division
- pronuclei fuse and mitotic metaphase spindle forms with chromosomes assuming their position as its equator
- mitosis is completed and one cell zygote becomes a 2 cell embryo
Fertilisation summary
Sperm enters oocyte causing Ca2+ waves:
- cortical granules fuse with zona pellucida to block polyspermy
- nucleus of sperm transformation to male pronucleus
- completion of Mll expulsion of 2nd polar body and female pronucleus forms
- these 2 pronucleii come together, membranes break down and alignment of chromosomes on spindle (mitosis)
Early embryo development
- fertilised egg has 2 pronuclei
- this is first sign of fertilisation
- developing embryo contains 6-8 cells, 3 days after fertilisation
- five days after fertilisation it os called a blastocyst and differentiates into inner cell mass, blastocoel and trophoblast
Glossary
- Capacitation – molecular changes in sperm to enable the acrosome reaction
- Acrosome reaction – Reaction in sperm as approaching the egg releasing enzymes
- Luteal phase – latter phase of the menstrual cycle after ovulation
- Corpus luteum – a hormone-secreting structure that develops from a follicle after ovulation
- Cortical reaction – release of cortical granules from the egg at fertilisation, preventing polyspermy
- Syngamy – the fusion of two cells, or of their nuclei, in reproduction
- Pronuclei – gametic nucleus after meiosis but before fusion that forms of the nucleus of the zygote
Blastocyst – 5 day old embryo with fluid filled cavity
