W4L1 Mon fertilisation Flashcards
Structure of oocytes
§ Oogonium gives rise to 1 haploid egg (n) + 2 polar bodies (2n, n)
§ Zona pellucida: acellular glycoprotein coat which provides structure
§ Cumulus oophorus: when oocyte surrounded by suspended cumulus cells secreting hyaluronic acid, maintain the 3D structure of the complex. help with egg collection
How does sperm store DNA
Head: contains highly condensed DNA (protamine instead of histones), acrosome contains enzymes to help penetrate ovum
§ Midpiece: helical mitochondria wrapped around axoneme (motor of flagellum)
Oocyte arrest
-Oogonia proliferate through mitosis.
-But when they enter into primary oocytes they immediately enter prophase of meiosis I and in many species, including humans, stay in this state for years.
-This is referred to the Germinal Vesicle (GV) Stage
Oocyte formation
§ Embryo: oogonia → 1st meiotic prophase arrest
§ Adult: prophase I arrested oocyte → primary oocyte + 1st polar body → metaphase II arrest
§ Fertilisation: metaphase II arrested oocyte → secondary oocyte → fertilised egg + 2nd polar body
Ø Meiosis not complete until after fertilisation
Growing oocyte acquires raw materials, mitochondria, stable RNA, peptides + proteins to sustain embryo
What is polar body
-asymmetrical meiosis of oocyte waste product, little cytoplasm
1st polar body is diploid (2n), 2nd polar body is haploid (n) + the contained DNA breaks down over time
Egg transport from the ovary
Fimbrium (collects egg after ovulation) → infundibulum → ampulla (fertilisation) → isthmus → uterus
Ø Cumulus oocyte complex is rich in hyaluronic acid which assists with oocyte collection
Sperm formation, maturation and storage
§ Rete testes: passive transport by bulk flow + myoid cell contractions
§ Epididymis: androgen dependent + regionally differentiated, 5-11 day journey
Ø Caput epididymis: fluid resorption + secretions, sperm immotile + incapable of fertilising egg, peristaltic movements along epididymis for maturation
Ø Caudal epididymis: sperm storage, sperm immotile but capable of motility if activated
§ Vas deferens: contractions move sperm up through vas deferens → ampulla/ejaculatory duct → urethra
Ø Seminal fluid released via contractions of smooth muscle in walls of seminal vesicle + prostate
What is in Caput’s secretion and their role
§ Carnitine: powerful antioxidant, protects sperm from ROS (reactive oxygen species) produced by mitochondria
§ Fructose: energy source, contains specific isoenzymes, not utilised by female tract
§ Glycoproteins: coat sperm, stripped by female reproductive tract to allow fertilisation
Semen (sperm and seminal plasma)
*consists of sperm suspended in a fluid that nourishes them and facilitates fertilization
*components of seminal fluid produced in seminal vesicles, prostate gland and bulbourethral glands/Cowper gland
*all components of semen join in the urethra and ejaculated through the penis by muscle contractions
Volume of sperm in the semen
Sperm comprise just 5% of the volume of the ejaculate
Sperm count from 1 to 200 million per ml
Where does seminal plasma come from and its fuction
60% of the volume comes from seminal vesicles – seminal fluid is thick, containing mucus and fibrinogen
~30% of the fluid is from the Prostate gland…..alkaline to neutralise the acid
environment of the vagina.
Contains a clotting enzyme to act on fibrinogen from seminal vessel to convert semen into a coagulum….a gelatinous mass
Sperm transport – male
tract
-smooth muscle in walls of seminal vesicle and prostate contract > seminal fluid
-caudal epididymis and vas deferens > contractions
-sperm transferred to vagina via penile urethra
Sperm transport – female tract
Vagina: acidic pH 5.7, most species carried to uterus by muscular contractions (mouse)
Ø Pigs deposit semen in uterus, some species deposit copulatory plugs to promote sperm transport/prevent leak
§ Cervix: cervical mucus barrier (viscosity varies with cycle), filters out sperm with poor motility/abnormal morphology
§ Uterus: uterine contractions speed sperm transport
§ Utero-tubal junction: gap b/w uterus + fallopian tube, 5000 sperm remain, require certain proteins on head to pass
§ Oviduct/Fallopian tube: sperm storage in crypts/mucosal folds, binding to epithelium preserves sperm fertility, slow release reduces polyspermy (fertilisation by >1 sperm)
What is capacitation
- Stripping of much of the glycoprotein coat on the sperm that was acquired during passage along the epididymis.
-Sperm exhibit a change in the surface membrane properties - This prepares sperm to undergo acrosome reaction
What is hyperactivation
- changed motility à penetration of cumulus & zona
- change in flagellar beating pattern - increased flagellar bend angle
- triggered by influx of Ca ++
- triggered near egg / binding to zona
What is ampulla
sperm meets egg = fertilisation
How does the sperm find the egg
Thermotaxis: temperature gradient
- ampulla 2°C warmer than isthmus (rabbit)
Chemotaxis: chemical gradient
- resact – 14 aa peptide from sea-urchin egg guides sperm
- formyl peptide/odourant receptors at base of flagellum in human, dog and rat
Oocyte requirement prior to fertilisation
cytoplasmic + membrane maturation (accumulates abundant reserves of enzymes/lipids/histones/mRNA/energy stores during meiosis II arrest; produces cortical co in sub-cortical location – below plasma membrane of egg),
nuclear maturation (oocyte specific DNA imprinting, meiosis, entered arrest – MII in humans), zone pellucida formation
Acrosome Reaction
§ Sperm binds to oocyte at zona pellucida (zp 2+3 in humans) + then undergoes acrosome reaction
§ Triggered when sperm contacts cumulus/zona + facilitated by progesterone
→ breakdown of outer acrosomal membrane, fenestration (formation of gaps in outer membrane)
→ release of beta-hexosamine which digests local zp3 receptor (prevents other sperm binding)
→ release of hyaluronidase from acrosome (digests hyaluronic acid that sticks cumulus together)
→ penetrates zona pellucida
→ exposure of inner acrosomal membrane (IAM) → sperm binds + fuses to oocyte via IAM
Sperm-Egg Adhesion
-Involves membrane proteins: integrins, fertilins (ADAM1, ADAM2), ADAM3, CRISP1 (help ensure species specificity)
§ Egg only takes sperm DNA + centriole, any mitochondria from sperm destroyed
Egg Corticol Reaction
- triggered by sperm contact
- calcium waves
- cortical granules exocytosis
- hydrolytic enzymes modify zona; zp2 and zp3 are targetted
- block to polyspermy
Sperm Activating Factor PLC-z
- PLC-z stimulating the release of the second messengers inositol triosphosphate (IP3) and diacylglycerol (DAG)
- IP3 activates calcium release from mitochondria
- DAG activates protein kinase C (PKC), which is essential for the development of the conceptus
Fertilisation Stages
Sperm activated by female reproductive tract → sperm binds to zona pellucida → acrosome reaction → sperm lyses zona
→ cortical granule reaction (via PLC) → sperm + egg membrane fuse (egg activation, completes meiosis)
Calcium in Fertilisation
Sperm provides the Activation Factor…it does not provide
the Calcium…..Calcium is derived from intracellular stores,
e.g. mitochondria, endoplasmic reticulum
