Gamete Transport And Fertilization Flashcards
How sperm change in the female system
Part 1: capacitation
Destabilization of sperm plasma membrane:
- required for fertilization
- occurs in uterus (in response to proteolytic enzymes and high ionic strength)
- removes the glycoprotein coat on the spermatids surface- exposing glycostransferases needed to interact with egg
How sperm change in the female system
Part 2: activation/hyper activation
Flagellum beating
- on way to Fallopian tube, and in close proximity to the egg
- ca2+ in follicular fluid increases vigor of sperm tail beating, which is essential for sperm to get through the zona pellucida.
- involves further changes to spermatids plasma membrane: to fuse with egg
- activated sperm have a chemotactic response towards the egg
- activates sperm lose viability rapidly
Importance of Ca2+ channels for fertilization
- nice genetically made without catsper are infertile but have a normal sperm count. Without catsper the sperm do not have hyper activation thus low motility
- Cabot penetrate oocytes with a zona pellucida
- however they can penetrate oocytes when the zona pellucida is removed artificially
Vaginal sperm
- semen deposited in vagina near external cervical os
- within 1 minute: seminal coagulation due to fibrogenase enzyme
- seminal fluid increases pH of vagina to be more alkaline which increases sperm motility
- after seminal coagulation, sperm re-liquify in about 20 minutes due to fibrinolytic enzymes in seminal plasma
- movement largely controlled by female: pressure gradient due to orgasmic platform and contractions in lower end of vagina.
When do sperm reach Fallopian tube
-Sperm can be found in Fallopian tube within 5 minutes (99% lost already)
When can sperm enter the cervix
Only sperm present at the right time will have the possibility of entering the cervix due to blockage by thick mucus at other times
-near ovulation due to high estrogen and low progesterone the mucus is thinner and permits sperm transport, and even nourishes sperm
Sperm may remain in _______ for many hours or even days
Cervical crypts
-fewer than 1M of about 200m even make it this far
The cervix is about _____ long and contains several hundred ______ which are
20-30mm
Cervical crypts
Glands lined by cells that respond to hormone levels
P mucus
The most superior crypts- liquefies L mucus so sperm stored in lower crypts can be released when the time is right
S mucus
Secreted in crypts in superior cervix. Produced string like mucus and present just before and up to three days after ovulation- nourishment for high quality spermatids and channels for their transport
L mucus
Attracts low quality spermatids for elimination or temporary storage
G mucus
Produces thick mucus and closes cervix during most of the cycle
Uterine sperm
- movement largely controlled by female (love and dead spent move at the same rate) for passed through uterus.
- 3mm/minute though spermatids tails are beating and may be involved in part of movements
- fluid currents are set up by uterine muscular contractions
- arrival of sperm in uterus sets up massive invasion of leukocytes to engulf dead/dying sperm (only few thousand make it)
What percent of sperm go to the wrong oviduct
50%
Oviduct sperm
- most sperm that read the Fallopian tube do so in 4-24 hours
- rarely more than 100 sperm at a time here which is controlled by: rate of movement out of cervical crypts, passage through utero-tubal junction, Soren linger in the vicinity of the isthmus
-more friendly environment for sperm: leukocyte attack reduced and sperm move more slowly as well as make connections with epithelial cells for nourishment from mucus
Intermittent sphincter
Uterine-tubal junction
Ensures quality of sperm
Sperm move up the oviduct by…
Swimming and by Cilia in dell recesses that beat towards ovary
-egg is moving down the tube on ciliary ridges beating towards uterus
3 navigation methods of sperm
Rheoraxis- movement against the fluid flow (mucus secreted by follicle)
Thermotaxis- movement in a temperature gradient (warmer in fertilization site
Chemotaxis- movement of a cell in a gradient of chemistrractant (molecules released by oocyte/ follicular fluid)
Where does fertilization occur?
Ampullary-isthmic junction
What structures do spermatids have to move through to penetrate the oocyte from external to internal?
Cumulus oophorus, zona pellucida, perivitelline space, oocyte
Binding to zona pellucida and acrosomal reaction part 1
- sperm passage through the cumulus oophorus by acrosomal hyrunonidase to dissolve hyaluronic acid
- Soren passage through the zona pellucida: sperm plasma membrane bind to receptors on the zona pellucida glycoproteins ZP3 and ZP4 which will increase intracellular Ca2+, cAMP and pH and causes exocytosis
Binding to zona pellucida and acrosomal reaction part 2
Triggers acrosome reaction in which spermatids plasma membrane fuses with outer acrosomal membrane
-breakdown of outer acrosomal membrane and exocytosis of contents such as acrosin
Binding to zona pellucida and acrosomal reaction part 3
Degrade zona pellucida near spermatids head to create a tunnel for sperm
-Soren inner acromsomal membrane now exposed, and binds to receptors on the zona pellucida glycoprotein ZP2.
Throughout the whole process of acrosomal reaction the sperm are still
Swimming
Binding to zona pellucida and acrosomal reaction part 4
Sperm fuse to egg
- once through zona pellucida, Soren approaches egg sideways and the back part of the head attaches to microvili of the egg cell membrane
- as soon as fusion occurs, Soren stops moving
From start of movement through cumulous oophorus to membrane fusion it takes…
10-20 minutes
Binding to zona pellucida and acrosomal reaction part 5 (after fusion)
The Soren nucleus, mid piece and most of the tail sink into the egg cytoplasm= egg activation
Egg activation step 1
- first step of egg activistion is the cortical reaction: to prevent polyspermy
- In Response to fusion with sperm, Ca+ released from endoplasmic reticulum in egg
- cortical granules located beneath egg plasma membrane respond to the increased Ca2+ and release enzymes to perivitelline space
- this alters ZP glycoproteins and inhibits acrosin so no more sperm could gain access
- the cell membrane of the egg becomes non-fusible due to changes in ion flow across
Egg activation step 2
- occurs about 2-3 hours after spermatids and egg members be fuse (egg finally resumes and completes meiosis 2)
- also initiated in response to high calcium
- results in release of second polar body
- final formation of the haploid ovum
- haploid nucleus of the newly formed ovum is the female pro nucleus
Creation of male pro nucleus
- once inside the egg, the sperm nucleus undergoes a series of changes that result in the formation of the male pro nucleus
- de-condensation of chromatin
- breakdown of original nuclear membrane
- swelling of the nucleus and formation of a new membrane around the pro nucleus
- completed in 4-7 hours lost fusion
What happens to the other parts of the sperm during pro nucleus
- tail and mitochondria are destroyed and recycled by the egg because they are labelled in spermatogenesis with ubiquitous to make them for destruction
- we inherit all our mitochondrial DNA from our mother
- centriole becomes part of the zygote
Syngamy
The fusion of two haploid gametes to form a a zygote
- 12hours from beginning of egg activation
- haploid male and female pro nucleus move toward the Center of the cell and fuse
- the new zygote is formed and the end of fertilization process and the beginning of the new diploid individual
Zygote
Fertilized egg
Divides by mitosis called cleave divisions= because nuclear replication and cytokinesis occur without any cellular growth
All material required for the zygote to develop come form the
Egg (RNA, cytoplasm, new membrane materials, microtubules, energy etc
-Sperm contribute their haploid chromosomes and centrosome
First cleavage division complete=
A pre embryo consisting of 2 blastomeres
Completed about 18-21 hours after initial fusion of egg and sperm
After fertilization
- conceptus remains in Fallopian tube for 3-4 days
- divides by mitosis a number of times and slowly moves towards the uterus
- blastomeres becomes smaller as the entire structure remains the same size and is still surrounded by the zona pellucida
- at 16 cell stage a compact sphere called a morula is formed
- cells in morula increase metabolic activity
- embryo continues to cleave, producing a hollow so where of about 64 cells by day 4= blastocyst
- blastocyst reaches uterus and is ready for implantation
