Fertilisation, Implantation & Gestation Flashcards
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
What are cytokines?
Signalling proteins
What is the decidua?
The mucosal lining of uterus
What happens during the adhesion stage?
- A stronger attachment of blastocyst takes place
- Trophoblastic cels release protein-digesting enzymes, allowing finger-like cords of trophoblastic cells to penetrate into endometrium
- As these cells penetrate decidua, they differentiate to become a new type of cells - syncytiotrophoblasts
- Cell adhesion proteins (integrins) ensure adhesive nests between embryo + endometrium
- During this stage, blastocyst can still be eliminated + pregnancy aborted by flushing it out of uterus
What happens during the embedding (invasion) stage?
- Embedding continues with synctiotrophoblasts, reaching basal membrane beneath decidual cells, penetrating it + further invading uterine tissue
- Endometrial tissue at contact site undergoes dramatic changes due to chemical messengers released from blastocyst
- Endometria cells secrete prostaglandins that increase local vascularisation + nutrient storage
- Blastocyst embeds in nutrient rich decidua + a layer of endometrial cells then covers surface
- Syncytiotrophoblasts come into contact with maternal blood + form chorionic villi
This is the beginning of formation of placenta
What is placentation?
Formation of placenta
Formed from endometrium layer of uterus + embryonic membranes
What is the function of the placenta?
- Transfer of nutrients from maternal blood to foetus
- Removal of waste products
Name the 3 extra-embryonic membranes
- Chorion (outside, attaches to uterus)
- Amnion (inside, nearest embryo)
- Allantois (middle, fluid-filled bag)
What is collected in the allantois?
Foetal urine produced during pregnancy
What is amniocentesis?
- A procedure that involves removing + testing a small sample of cells from amniotic fluid
- Used primarily in prenatal diagnosis of chromosomal abnormalities (if suspected) as well as form sex determination
- Carries risks of miscarriage + infection
Name a newer technique used in replacement to amniocentesis
- Cell-free DNA (cfDNA)
- Non-invasive prenatal diagnosis for assessing potential problems
- Cell-free foetal DNA is foetal DNA that enters maternal blood via placenta + circulates freely in maternal blood
How can cell-free foetal DNA be sampled?
Simple venipuncture
When does the chorion form?
By day 12 when the trophoblastic layer is 2 cells layers thick
Trophoblast expands + forms network of cavities in decidua
Maternal blood leaks into these cavities
Finger-like projections (chorionic villi) extend into pools of maternal blood + receive nutrients
When does the placenta become functional (not fully developed)?
By 5 weeks
What is a diffuse placenta?
Almost the entire surface of foetal membranes involved in formation of placenta
What is a cotyledonary placenta?
Multiple, discrete areas of attachment called cotyledons are formed by interaction of patches of allanto-chorion with endometrium
What is a zonary placenta?
Placenta forms a complete or incomplete band of tissue surrounding foetus
What is a discoidal placenta?
A single placenta formed is discoid in shape
How are the different types of placenta classified?
By how many of the potential layers of maternal tissue separate maternal blood from foetal blood
What does epitheliochorial mean?
All maternal tissue layers are present
E.g. pig, horse
What does endotheliochorial mean?
Two maternal tissue layers present
E.g. dog, cat
What does haemochorial mean?
No maternal tissues separate the two
Nutrients + O2 simply diffuse into foetal cells from maternal blood
What hormone breaks down progesterone?
Prostaglandin
Why must progesterone levels be maintained at sufficiently high levels?
- Implantation of developing embryo in endometrium of uterus can take place
- Endometrium is maintained in state of conducive to embryonic survival
What does maternal recognition rely on?
Signal from embryo that prevents literal regression, allowing corpus luteum to persist + continue to secrete progesterone
What happens if a signal is not delivered to the mother?
Progesterone conc will decline + pregnancy terminated
What does hCG stand for?
Human Chorionic Gonadotrophin
- Hormone
What does hCG do?
- Stimulates continued secretion of progesterone by the corpus luteum + may also block signals in corpus luteum that causes it to regress
- This maintains endometrium + survival of blastocyst whilst placenta develops
How long does the corpus luteum continue to produce progesterone + oestrogen before its function declines?
~8 weeks
What hormone do dogs depend on during pregnancy?
Progesterone
What is gestation?
The period between fertilisation and parturition
