Fertilisation, Implantation & Gestation Flashcards
(54 cards)
How long does the ovum survive in humans and other mammals?
~24 hrs
How long does sperm usually survive in humans and other mammals?
About 48 hours but can survive up to 7 days in female reproductive tract
What is capacitation?
- The process of sperm maturation (or activation) that occurs once the sperm leaves the vagina and enters the uterus
What happens during capacitation?
Mechanical:
- Sperm brushes on ciliates mucosal cells lining uterus + isthmus of fallopian tube
- Rubs away glycoprotein coat that covers sperm head, exposing receptors + making membrane more permeable to calcium (Ca)
Biochemical:
- Destabilisation of acrosomal sperm head membrane allowing greater binding between sperms + oocyte
- Influx of Ca+ that increases sperm mobility (hyperactivation)
How long does capacitation take roughly?
9 hours
What is capacitation aided by?
Secretion of lipoproteins, proteolytic & glcosaminoglycans (enzymes) from uterus
For IVF + IUI, when must sperm capacitation take place?
- In vitro
- Sperm cells collected through ejaculation or from epididymus
- It is induced by adding media similar to composition of electrolytic composition of fallopian tubes of species involved
Media is used to induce capacitation. What does the media contain?
- Energy substrates (e.g. lactate + pyruvate)
- Bovine serum albumin or human serum albumin; used as a cholesterol acceptor to remove cholesterol from sperm cell membrane
- Other chemicals (e.g. calcium + bicarbonate) to induce hyperactivation of sperm motility
What are the several stages to fertilisation?
- Acrosomal membrane of sperm must fuse with egg plasma membrane
- Sperm must penetrate the zona pellucida (ZP) to gain access to perivitelline space
- Sperm must recognise + fuse to egg membrane to create single cell
- Fertilised egg must prevent additional sperm fusing to avoid creating non viable polyploid embryo
How is the acrosomal reaction activated?
- Sperm binds with sperm receptor, zona pellucida glycoprotein 3 (ZP3), on the zona pellucida
In species where fertilisation is external, what occurs to prevent different species from fertilising the egg?
- A protein receptor on the egg, named Bouncer
- Selects sperm from same species
What is SPACA4?
- Sperm Acrosome Associated 4 Receptor
- Required for efficient fertilisation
- Similar to ‘Bouncer’, however not required for interaction of sperm/egg membrane
What happens once the sperm has penetrated the zona pellucida?
- Head of sperm fuses with oocyte vitelline membrane
- Allows sperm nucleus to enter oocyte
- Sperm pronucleus formed once in oocyte
- Ovum undergoes Meiosis II - forms egg pronucleus
- Sperm nucleus fuses with ovum, enabling fusion of genetic material -> zygote
- Fertilisation causes increase in Ca2+ levels in cytoplasm, triggering cortical reaction
- This reaction inactivates ZP3 + hardens vitelline membrane, blocking further sperm from entering + preventing polyspermy.
How does the block to polyspermy occur?
- Second meiotic division of egg
- Fertilisation-induced Ca2+ release occurs
- Triggers exocytosis of enzyme-filled ‘cortical’ granules in oocyte
- Enzymes released into zona pellucida, where they inactivate ZP3
- Prevents further sperm binding + entry
After fertilisation, what does the ova differentiate into?
A morula - a solid ball of cells
Where does the morula remain initially?
In ampulla (upper part of fallopian tube), due to constriction between ampulla + lower part of oviduct
What hormone prepares uterus for implantation?
Progesterone secretion from corpus luteum
What does the morula develop into?
A blastocyst
What is a blastocyst?
A single layer, hollow ball of ~50 cells with a fluid-filled cavity + dense mass of cells to one side (inner cell mass)
What does the inner cell mass become?
The embryo
What does the outer layer of cells become?
The trophoblast - will become the foetal portion of placenta
What 3 stags are involved in implantation?
- Apposition
- Adhesion
- Embedding (invasion)
What happens during apposition?
- Can only take place during ‘implantation window’, when endometrium is at a particular stage of maturity
What happens during the implantation window?
- Changes take place in endometrial cells that bring blastocyst nearer to endometrium + immobilise it
- Secretion of cytokines that attract blastocyst to a specific region of uterus
- Inner cell mass of blastocyst aligns closest to decidua
- Growth factors increase endometrial vascularisation through angiogenesis at site3 of blastocyst apposition
- Uterine glands grow + produce secretions
