Fertilisation Flashcards
What is the normal volume of semen?
2-4ml
What is the normal sperm count in semen?
20-200 x 106 per ml, or >40 x 106 per ejaculate
What % of sperm is swimming forward vigorously in normal semen?
60%
What % of sperm has abnormal morphology in normal semen?
<30%
How long does liquefaction take to occur in normal sperm?
Within 1 hour
What is classified as oligozoospermia?
<20 x 106 sperm/ml
Draw sperm with abnormal morphology
Where do the glandular components of semen come from?
- Bulbourethral glands (Cowper’s glands)
- Seminal vesicles
- Prostate
What % of semen volume comes from Cowper’s glands?
5%
What do Cowper’s gland secrete?
Alkaline fluid
What is the function of Cowper’s glands secretions?
Mucous lubricates the tip of the penis and urethral lining
What % of semen volume comes from seminal vesicles?
60%
What does secretions from seminal vesicles consist of?
- Alkaline fluid
- Fructose
- Prostaglandins
- Clotting factors
What is the function of the alkaline fluid secreted by seminal vesicles?
Neutralises the acid in the male urethra and female reproductive tract
What is the function of fructose from seminal vesicles?
ATP production
What is the function of prostaglandins from the seminal vesicles?
- Increase sperm motility
- Increase female genital smooth muscle contraction
What clotting factor in particular is produced by seminal vesicles?
Semenogelin
What % of semen volume is from the prostate?
25%
Describe prostate secretion
Milky, slightly acidic fluid
What do prostatic secretions consist of?
- Proteolytic enzymes
- Citric acid
- Acid phosphatase
What proteolytic enzymes are found in prostate secretions?
- PSA
- Pepsinogen
What does pepsinogen in semen do?
Breaks down clotting factors from seminal vesicles
How long does pepsinogen take to re-liqeufy semen?
10-20minutes
What is the purpose of citric acid in prostate secretions?
For ATP production
When does endometrial proliferation typically occur in the ovarian cycle?
Days 7-14
What does endometrial proliferation occur under the influence of?
Oestrogen
What effect does oestrogen alone have on cervical mucus?
It makes an abundant, clear, and non-viscous
What is the result of the clear, non-viscous cervical mucus during endometrial proliferation?
Easy for sperm to get in
When in the ovarian cycle does ovulation occur?
Day 14
What happens on days 14-28 of the ovarian cycle?
Uterine secretory phase
What happens during the uterine secretory phase?
Oestrogen and progesterone from the corpus luteum produce hospitable environment for fertilisation and implantation
What is produced in the uterine secretory phase by oestrogen and progesterone?
Thick, sticky, mucus plug
What does oxytocin do?
Stimulates uterine contraction, which helps the sperm get to the ovum
What are the components of oocyte maturation?
- Nuclear maturation
- Cytoplasmic maturation
What happens in the nuclear maturation stage of oocyte maturation?
- Oocyte undergoes meiosis I
- Nuclear membrane of oocyte disappears
- First polar body seperates and enters perivitelline space
- Second meiotic division takes place and stops in metaphase II
What happens in cytoplasmic maturation stage of nuclear maturation?
- Mitochondria are dispersed through the cytoplasm
- Endoplasmic reticulum undergoes changes
- Lipid droplet provide energy
- Cytoskeleton is formed by microfilaments migrating towards the oocyte cortex
What happens to the endoplasmic reticulum during cytoplasmic maturation stage of oocyte maturation?
- Accumulates in the oocyte cortex
- Protein and lipid synthesis produce cortical granules
- Become cortical during maturation
What happens to cortical granules in the immature oocyte?
They are displaced throughout the cytoplasm
What is the energy produced from lipid droplets in the cytoplasmic maturation stage of oocyte maturation needed for?
- Meiosis
- Maturation
- Fertilisation
- Early embryo development
What is the average number of sperm per ejaculation?
200-300 million
How many sperm reach the fertilisation site?
300
How many sperm are needed for fertilisation?
1
What happens to the first 299+ sperm that reach the fertilisation site?
They are sacrified to disperse the zona pellucida
How long can sperm survive in the female genital tract?
Up to 5 days
How long can an oocyte survive
Up to 5 days, then gets phagocytosed
What is the fertile period for sperm deposition?
Up to 3 days prior to ovulation, or on day of ovulation
How long does it take for the oocyte to travel from the ovary to the body of the uterus?
3-4 days
How is the oocyte transported from the ovary to the body of the uterus?
By cilia and Fallopian tube peristalsis
What does sperm need to penetrate?
The corona radiata (follicular cells) and zona pellucida (glycoprotein membrane)
What allows the sperm to penetrate the corona radiata and zona pellucida?
It undergoes further maturation in the female reproductive tract, called capacitation
What happens in capacitation?
Sperm cell membrane changes to allow fusion with oocyte cell surface
What changes to the sperm cell membrane allows fusion with oocyte cell surface?
Removal of protein coat of sperm
What happens when the protein coat of sperm is removed?
Acrosomal enzymes are exposed
What is true of the acrosome when sperm contacts the corona radiate?
It has in intact acrosome
What happens when sperm contacts the corona radiata?
It pushes through the granulosa cells of the corona radiata
What happens once sperm has pushed through the granulosa cells on the corona radiata?
Proteins on sperm head bind to ZP3 proteins on zona pellucida
What does the binding of proteins on the sperm head to ZP3 proteins on zona pellucida trigger?
Acrosome reaction
What happens in the acrosome reaction?
- Key signalling mechanisms involves intracellular Ca2+
- Acrosomal enzymes digest path through ZP
- One sperm penetrates, and there is fusion of the plasma membranes
What is the oocyte plasma membrane divided into?
Two major regions;
- The part of the mmebrane that directly overlies the metaphase chromosomes
- The remainder of the oocyte, whcih is rich in microvillar protrusions
Describe the surface of the part of the oocyte plasma membrane that directly overlies the metaphase chromosomes
Has smooth surface, devoid of microvilli
What is the region of the oocyte where the sperm and egg fuse?
The portion of oocyte that is rich in microvillar protrusions
What happens once the sperm and oocyte plasma membranes have fused?
The sperm moves into the cytoplasm, and the oocyte and sperm form a zygote
How is polyspermy blocked?
By the cortical reaction
What are the components of the cortical reaction?
- Fast block
- Slow block
What happens in the fast block cortical reaction?
- Electrical change in oocyte membrane
- Sodium channels open
What is the resting potential of the oocyte?
-75mV
What is the fertilisation potential of the oocyte?
+20mV
How does the wave of depolarisation spread in the fast block cortical reaction?
Starts at the site of entry of sperm, and then propagates across the cytoplasm
What happens in the slow block cortical reaction?
- Ca2+ is released from ER, which induces local exocytosis of cortical granules.
- Granules release enzymes to stimulate adjacent cortical granules to undergo exocytosis.
- A wave of exocytosis occurs around oocyte in 3 dimensions from original site of sperm entry
What happens to the tail movement of the sperm in the female genital tract?
Changes from a beat to a whip-like action
How fast do sperm move in the female genital tract?
3mm/hour
What is responsible for most of the movement of sperm in the female genital tract?
Contraction of the female genital organ
What happens in syngamy?
- The oocyte completes meiosis 1, and expels the polar body
- Male and female pronuclei migrate towards each other, and the union of male and female pronuclei forms a diploid zygote
What is a polyploid embryo?
One containing three or more pronuclei
What can polyploid embryos occur due to?
- Entry of more than one sperm
- Failure of extrusion of the second polar body
What happens in embryonic cleavage?
A series of rapid mitotic divisons and metabolic changes for upcoming cell division and embryogenesis takes place.
There is an increased number of cells (16-32 blastomeres) of decreasing size, without increase in size of fertilised ovum
What is true of cells produced by cleavage?
They are totipotent
What is meant by cells being totipotent?
Each cell has the capacity to develop into entire individual
When do monozygotic twins occur?
When, in cleavage, totipotent cells become divided into 2 independant cell masses
What % of twins follow seperation after the first cleavage?
25-35%
When does dizygotic/non-identical twins occur?
When 2 eggs ovulate, and 2 eggs are fertalised
Why is cleavage important?
Because it generates a large number of cells that can undergo differentiation and gastrulation to form organs
What does cleavage result in?
An increase in the nuclear/cytoplasmic ratio
Why is it important that there is an increase in nuclear/cytoplasmic ratio in the embryo?
Because one nucleus cannot transcribe sufficient RNA to support the enormous cytoplasm of the zygote
Why does cleavage result in an increase in the nuclear/cytoplasm ratio?
No G1 (duplication of organelles and cytosol) or G2 (synthesis or enzymes and proteins) stages in cell cycle during cleavage, so ratio increases with each division
is cleavage synchronous?
No, not all blastomeres divide at the same time
When does compaction occur?
At the 8 cell stage
What happens in compaction?
Blastomeres undergo polarisation and form tight junctions to create ‘inner embryo environment’
When does the morula form?
At the 16 blastomere stage
How long after fertilisation does morula formation occur?
3-4 days
What happens once the morula has formed?
The embryo passes from oviduct into uterus
What happens if there is a failure of transport of morula into uterus?
Ectopic pregnancy occurs
Where does the morula implant in an ectopic pregnancy?
Fallopian tube, ovary, or peritoneal cavity
What is the problem with an ectopic pregnancy?
- Risk of materal haemorrhage
- Embryo is non-viable
Why is the embryo non-viable in ectopic pregnancy?
It has been implanted into a hostile environment
When does the blastocyst stage occur?
5 days after fertilisation
What is the blastocyst?
A fluid filled cavity that develops into the morula
What happens to the cells in the blastocyst stage?
Loss of totipotency
What is the inner cell mass of the blastocyst called?
The embryoblast
What is the outer cell mass of the embryo called?
The trophoblast
What is the function of the trophoblast?
- Contributes to the formation of the placenta
- Produces hCG
What happens in hatching?
Local digestion of the zona pellucida by enzyme produced by trophoblast cells
Where does hatching occur?
Opposite the inner cell mass
Why does hatching occur opposite the inner cell mass?
Minimises risk of enzymatic damage to embryo
What happens after the embryo has escaped from the ZP?
It begins the process of implantation
When does the zygote to blastocyst transformation occur?
Between days 14 and 21 of the uterine cycle
What primes the endometrium for implantation?
Progesterone
How does the conceptus recieve its nutrition for the first 2-3 days?
Nourished in intrauterine fluid, so is floating in the uterus
How does implantation take place?
The trophoblast overlying inner cell mass is ‘sticky’, so adheres to epithelium, and implantation commences
When does implantation occur?
6 days after ovulation
