Reproduction Lectures 2021 - FERTILIZATION & SEX DETERMINATION Flashcards
Conception can only take place in a…
limited window
Conception
union of sperm & egg
Conception usually takes place in the…
distal portion of the fallopian tube (fav. envir. for dev. –> small area, so nutrients can be transported quite quickly & easily during early dev.)
Sperm can reach the oocyte within…
30min following intercourse, but can also survive for up to 5 days in the female reproductive tract
What happens if the egg is not fertilized?
it will disintegrate & absorb through phagocytic mech’s
Why is 10’s of millions of sperm ejaculated?
to promote the possibility of fertilization
- & b/c you lose ~97% in the ejaculate already & end up with only 0.001% in the fertilization site
Where is the fertilization site?
upper third of oviduct
Sperm will 1st appear within the…
cervical canal - within 1-3 mins after ejaculation
Female repro. tract also aids in the union of sperm & egg:
- Sperm complete capacitation in the female repro. tract
- become fertile when released into the female repro. tract - Sperm movement from cervical canal –> fallopian tube, aided by a variety of aspects within female repro. tract
Sperm travel aided by:
Estrogens “thinning” cervical mucous, estrogens stimulating cervical and oviduct contraction - oxytocin, chemotaxis, thermotaxis
Describe how sperm travel aided by estrogens “thinning” cervical mucous
Shortly after ovulation, estrogens will target the mucus within the female repro. tract & they thin that mucus such that it aligns with the cervical & the cervical fibres to form channels - through which sperm will swim
The channels formed within the cervical canal are…
an identical dimension that allow sperm to swim & also increase the forward motion of that sperm movement
- additionally, the fibres will resinate at similar frequencies to the beating of the sperm flagella, promoting movement of sperm up through the repro. tract
What happens to the channels if the flagella is beating irregular?
those fibres won’t resinate & therefore irregular swimming is inhibited/movement of sperm that display a regular swimming is inhibited
- but still can make it to the fallopian tube (dead sperm can make it to the fallopian tube at same time as live sperm) –> enhances/solidifies the notion that there is mech’s within the female repro. tract that promotes sperm movement through female repro. tract
Oxytocin
released in males & females during the repro. act
Chemotaxis
cells will follow a chemical gradient
- research on frogs - where the egg releases allurin (allures the sperm to the egg)
- sperm will follow this gradient (highest concen. is at the egg - site of fertilization)
- notion that egg in females might release a chemical - that there is a gradient that sperm will ultimately follow (may only apply to fallopian tube - all depends on timing of fertilization, post ovulation)
Thermotaxis
- temp. gradient b/t cervical canal, vagina & up to the fallopian tube
- small gradient that may facilitate the movement of sperm towards the egg
Acrosomal reaction
- once sperm get to egg, they have to complete the acrosomal rxn to fertilize the egg
- proteins involved are extrem. species specific!
Describe the 4 steps of acrosomal reaction
• Acrosomal enzymes allow sperm to “drill” through the corona radiata and zona pellucida (2 layers on ovulated egg)
• Fertilin on the sperm membrane binds to integrin on the oocyte surface
- highly species specific!
- Induces a change in the oocytes membrane which blocks polyspermy
- On entry into the cell the sperm release NO which induces a release of stored Ca2+ and this is believed to initiate the final meiotic division in the oocyte
Polyspermy
multiple sperm releasing their contents into a single egg
- don’t want this to happen
- won’t fully dev.
Release of acrosomal enzymes that allows for…
the degradation of the cell membranes
Egg-binding protein
facilitate the binding & movement of sperm head through those cell layers
Fertilin & Integrin…
species specificity of fertilizing the egg
What does it mean for a sperm cell to be capacitated?
means it is fully fertile & req’s full capacitation before it is capable of fertilizing the egg
Is the haploid genome the only thing that the sperm delivers to the egg?
Absolutely NOT
Also may deliver:
- prostaglandins
- nitric oxide
- calcium
- sperm will also deliver RNA into the egg that will insist in early stages of dev.
- mitochondrial DNA –> debated (if in paternal too - most say no, strong evidence suggest that paternal mitochondrial DNA is tagged by ubiquinone & degraded, so it plays no role in further dev. of that organism (mech’s are still not explained)), vast maj. of mitochondrial DNA that is expressed in the dev. oocyte embryo & subsequent offspring is maternal
Zygote formation - syngamy
syngamy - merging of 2 haploid genomes
once that sperm has entered the egg & deposited that DNA, we get the zygote formation
Where does the zygote formation - syngamy process normally take place?
in fallopian tube & the zygote will remain in the fallopian tube (restricted area - small, nutrient rich environment so cells can divide & the blastocyst is formed typically within the fallopian tube - then is transported down the fallopian tube into the uterus)
Twins
- Occur in about 1 of every 80-90 pregnancies
- Dizygotic or fraternal twins are the result of fertilisation of two oocytes
- Monozygotic twins or maternal twins are the result of the early embryo dividing in two
- Conjoined (surgically separated later in dev.) or Siamese twins (1st identified in the formal siam)
Dizygotic or fraternal twins are the result of…
fertilisation of two oocytes
Monozygotic twins or maternal twins are the result of…
the early embryo dividing in two
Morula to blastocyst facts
- After about the third day of fertilisation – approximately 32 cells - morula
- Inner cell mass will develop into the embryo
- balstocyst supports the cells dividing at the pole during intrauterine life
Morula to blastocyst process (5 steps)
- Ovulation
- Day 1: Fertilization
- Days 2-4: Cell division takes place
- Day 4-5: Blastocyst reaches uterus
- Days 5-9: Blastocyst implants
Occasionally we’ll get ectopic pregnancies, where…
the egg is fertilized within the abdominal cavity (v. v. rare)
- won’t go to term b/c architecture isn’t there to support that but it can dev. & could be fetal to the female
Ectopic tubule pregnancy
within fallopian tube
- zygote starts to dev. in the blastocyst & doesn’t get transported down into the uterus, but rather it gets stuck in the fallopian tube
- can result in fatality if not arrested & will typically not result in a successful pregnancy
Trophoblast
protects the inner cell mass as they are dividing & that’s ready for implantation (surface layer of cells of the blastocyst)
- typically within 5-9 days of fertilization this is ready for that implantation
Inner cell mass
- will continue to divide & multiple into an embryo & ultimately fetus & ultimately birth of an individual (ultimately forms the embryo & subsequent further dev.)
- filled with embryonic stem cells - peripone/peritoneal? cells that will differentiate into any manner of diff. cells
- part of that necessary part of dev. process