Fertilisation and embryo development Flashcards
What are the steps in fertilisation in mammals?
- Sperm capacitation
- Sperm binding
- Acrosome reaction
- Sperm membrane fusion
Describe sperm capacitation
- Removal of glycoprotein coat and alteration of sperm mitochondria
- Hyperacitve motility occurs
What receptor is required is required for sperm binding?
ZP3 on the zona pellucida
Describe the acrosome reaction
- Loss of acrosome contents (enzymes)
- Digests ZP
- Exposed sperm surface proteins bind to ZP3
- Once bound, sperm and egg can fuse
Describe sperm membrane fusion
- Causes depolarisation of egg membrane
- Sperm completely invaginated into egg
- Stimulates cortical reaction (release of cortical granules)
- Blocks polyspermy
- Membrane depolarised
- Oocyte activation
- Formation of ootid
- Tail of sperm lost
How does sperm membrane fusion depolarise the oocyte membrane?
- Granules induce calcium influx into egg from extracelular space
- Induces more calcium release from endoplasmic reticulum
- Wave of depolarising calcium
What is the function of membrane depolarisation in sperm membrane fusion?
- Blocks polyspermy
- Once depolarised, ZP impermeable to other spem
What occurs in oocyte activation following sperm membrane fusion?
- Completion of meiosis
- Extrusion of hte second polar body
Where does membrane fusion begine?
The equatorial segment of the sperm
Describe the chromosomal events that occur during sperm membrane fusion
- Immediately after fusion, chromatin of sperm completely decondensed (nucleus of sperm loses shape)
- Nucleus appears larger and fluffier
- 2 pro-nuclei now present in cell = ootid
What is syngamy?
Fusion of the 2 pro-nuclei to form a zygote
Where does fertilisation take place?
In the oviduct
What is the zygote?
Single celled embryo formed following syngamy
What is formed from the first cleavage division of the zygote?
- 2 cell embryo
- Each cell within is called blastomere
- ZP still present
Describe cleavage of the zygote
- No cytoplasm synthesis, only DNA replication
- So individual blastomere volume increases
- Eary stages, each blastomere has potential to develop into separate offspring
What happens to the blastomeres following the first cleavage division of the zygote?
- Further divisons resulting in 4, 8 and tehn 16 cell embryos
What is the morula?
Last point in the embryo where all cells are identical (16 cells)
(Is the last point at which a cell can be used for genetic analysis)
Define totipotent
Having the ability to form all cell necessary for normal development
In what stages of development are all cells in the embryo totipotent?
Up to and including the morula, not demonstrated beyond 16 cell stage
Describe the formation of the blastocyst
- Outer cells within morula more squashed than inner
- Outer cells form cell-cell adhesions (tight junctions, expression of proteins)
- Inner cells have gap-junctions
- Outer cells pump sodium into morula
- Increases ionic concentration, water in through ZP = fluid accummulation causing increased pressure
Define blastocyst
The embryological stage where there is a distinct cavity formed
What do the outer cells of the blastocyst become?
Trophoblasts
What do the inner cells of the blastocyst become?
Inner cell mass
What do the trophoblasts give rise to?
Chorion and placenta
What does the inner cell mass give rise to?
The embryo
Describe the hatching of the blastocyst
- Continues to undergo mitosis and fluid accumulation
- Proteolytic enzymes produced by trophoblast
- ZP weakens (increased pressure, enzymes)
- ZP splits, blastocyst emerges (hatches)
What happens to the blastocyst following hatching?
Ready to implant (following formation of germ layers), does not occur in same way in all species, some fast, some slow
Describe the formation of the 3 germ layers
- Cells of ICM flatten to form embryonic disc
- Outer ectoderm and inner endoderm formed
- Mesoderm produced
What are the nutritional requirements of the pre-implantation embryo?
- Carbohydrate, proteins and lipids
- Supplied by endometrial glands via uterine milk/histiotroph
- Required for massive growth during this period
What are the different types of embryo progression?
- Slow, continuous
- Discontinuous
- Fertilised
What species have slow, continuous embryo progression?
Rabbit and sow
What species have discontinuous embryo progression?
Cow, ewe, human
Describe discontinuous embryo progression
- Prolonged stay in uterine tube
- Anti-peristaltic contractions maitain egg for 2-3 days
- Stronger, peristaltic contractions then transport egg onwards
- Regardless of egg being fertilised or not
Why do ruminants use discontinuous progression of the embryo?
- Foetus needs to grow to large size
- More time needed for embryo to develop before implantation
What species use fertilised embryo pregression?
Horses, rats, hamsters
Describe fertilised embryo progression in the mare
Only fertilsied eggs pass into uterus
- Embryo secretes progstaglandins E and F
- These promote embryo migration through oviduct as well as uterus
Describe fertilsied embryo progression in rats and hamsters
- Ferrtilised eggs pas more rapidly and synchronously
- May secrete motility regulatory factors
Outline the process of ovulation
- Spike in plasma LH
- Spontaneous or induced
- Follicular wall bursts, oocyte and granulsa cells released from grnaulosa cell anchor point
- Oocyte carried with follicular fluid to peritoneal cavity, captured by fimbriae of infundibulum
What is the fertilisation period?
The period following ovulation during which the oocytes can be fertilised
- In most species is immediately after ovulation for up to a few hours (days in bitch)
What is the fertile period?
- The period of time in which mating can result in a pregnancy
- Longer than fertilisation period due to sperm storage in female following mating
At what stage in oocyte development are the oocytes ovulated in most species?
Metaphase of meiosis II
Describe the oocytes ovulated in bitches
- Metaphase of meiosis I
- Completion of meiosis II and formation of the second polar body occurs after fertilisation
- Immature oocytes ovulated that can be penetrated by not fertilised
When is the fertile and fertilisation period in the bitch?
- Fertilisation period: day 2 to day 5 after ovulation
- Fertile period: between 5 days before and 5 days after ovulation
Describe gastrulation
- Begins at posterior end of embryo where node arises
- Cells from epiblast migrate thorugh primitive streak, differentiate into ecto, endo and mesoderm
- During migration, position and become 3 germ layers
In general terms, what features are formed from the ectederm?
Head and spine (nervous system, skin, hair)
In general terms, what features are formed from the mesoderm?
Appendages (muscle, skeleton, cardiovascular, reproductive, kidney, liver)
In general terms, what features are formed from the endoderm?
Intestines (digestive system, lungs, endocrine system)
What mediates the differentiation into the germ layers?
Cytokines and growth factors
What does the ICM give rise to before gastrulation?
Epiblast and hypoblast (the cells that form the embryo)
What do the epiblast cells form?
The amionic ectoderm (amnion)
- Epiblast cells form from cells in the middle being “squashed”
What do the hypoblast cells form?
Extraembryonic or primitive endoderm (primitive endoderm will go on to form the yolk sac)
What is formed from the primitive endoderm?
- Lining of blastocoel
- When complete results in cavity called yolk sac
- This is following implantation but before formaiton of placenta and will provide nutrients for developing embryo
How are the extraembryonic membranes formed?
Embryonic mesoderm forms sac around yolk sac
How are the amniotic folds formed?
- Extra-embryonic mesoderm folds dorsally around the embryo
- Amniotic folds form the amnion (ectoderm is the amnion)
- Mesoderm closes around the amnion to form the placenta
How is the chorioallantois formed?
- Outpouching of hindgut is allantois
- Makes contact with chorion
What is the function of the chorioallantois?
Analogous to the waste bag of the foetus
Describe the chorion?
- Membranes between developing foetus and mother
- Formed by extra-embryonic mesoderm and 2 layers of trophoblast
- Surrounds the embryo and other membranes
Define maternal recognition of pregnancy
- Adaptation of the maternal endocrine system to the presence of the conceptus
- I.e. maintian CL and progesterone production
Where does the signal for maternal recognition of pregnancy come from?
Must come from embryo before luteolysis
Define pseudopregnancy
Being in the same endocrinological state as would be in pregnancy without actually being pregnant
In what species does pseudopregnancy commonly occur?
- Monooestrus
- E.g. rodents, rabbits, dogs
Other than normal physiological processes, what may cause pseudopregnancy to occur?
- Sterile mating
- Induced ovulation
What are the 2 types of pseudopregnancy found in dogs?
- Overt (show signs)
- Covert (no signs)
Outline pseudopregnancy in the bitch
- Increased prolactin
- Develop mammary gland, lactate, build nests, mothering behaviour
- Will occur with every cycle due to the extended luteal phase
What is diapause?
Embryological state of dormancy, where the embryo does not immediately implant
What species commonly have embryonic diapause?
Bagers, roe dear, marsupials
What are the 2 types of embryonic diapause?
- Facultative
- Obligate
Describe facultative embryonic diapause
- Occurs in times of metabolic stress e.g. lactation
- Commonly in marsupials, some insectivores
Describe facultative diapause in marsupials specifically
- Forms embryo soon after parturition
- While lactating embryo remains dormant
- Once suckling has finished, embryo implants
- Therefore marsupails are always lactating or pregnant at all times
Describe obligate embryonic diapause
- Mustelids, roe deer, armadillo
- Time birth for favourable conditions
List the maternal recognition hormones in common domestic animals
- Early conception factor
- interferon tau (ruminants)
- Oestradiol (pigs)
- Chorionic gonadotrophin (primates)
- Prolactin and placental lactogen in rodents
Describe luteolysis in non-pregnant animals
- occurs in absence of embryo
- Induced by decline in progesterone and increase in PGF2a from uterus
- CL regression occurs allowing return to oestrus
How is PGF2a produced?
- From arachidonic acid via conversion by cyclo-oxygenase 1/2
- Uterine derived
What is the function of PGE2?
Luteotrophic, supports the CL
Describe the role of the endometrial oxytocin receptors in luteolysis in ruminants
- OXTR absent in luteal phase, suppressed by high P4
- Reappear before P4 falls, stimulated by decreasing P4 levels
- Oxytocin produced by CL binds to OCTR
- Stimulates pulsatile PGF2a release
- Increases oestradiol as well which increases OXTR expression (positive feedback)
Describe the countercurrent system used for PGF2a
- PGF2a released from endometrium
- Into uterine vein
- From vein to ovarian artery to go to ovary and stimulate oestradiol production
Describe the oxytocin-PGF2a postivie feedback loop
- PGF2a acts on CL to reduce progesterone and increase oxytocin
- Acts on uterus to make more PGF2a
Describe the early conception factor in maternal recognition of pregnancy
- Maternal response within hours of fertilisation
- Produces early conception factor
- protexts embryo by binding to T-lymphocytes
- Presence does not indicate whether the pregnancy will be estbalished or maintained, only that conception has taken place
In what species is the early conception factor present?
- Mice
- Sheep
- Cattle
- Pigs
- Human
What is the function of progesterone during pregnancy?
- Maintains uterine gland secretory function
- Suppresses myometrial contractions and luteolytic mechanisms
- Suppresses FSH and LH secretion (reduces follicular development and suppresses ovulation directly)
- Inadequate leads to embryo mortality
What is the maternal recognition factor in ruminants?
Interferon tau (cytokines, typically cantivirals)
Describe the expression of interferon tau
- Initiated during trophoblast elongation
- Expressed in trophoectoderm
- Stimulated by uterine growth factors
- Tight window of expression (days 12-24 of pregnancy)
- Production related to embryo size
Why is interferon tau expressed in days 12-24 of pregnancy?
- Normally would have luteolysis occuring in this period
- IFNT prevents luteolysis
Describe the actions of interferon tau
- Acts on endometrium to suppress OXTR expression
- stimulates protein synthesis critical for embyro
- Antiviral action
- Suppresses PGF2a pulsatile secretion
- Indirectly maintains progesterone
Describe the role of oestradiol in maternal recognition of pregnancy in pigs
- Secreted before implantation
- Re-direct flow of PGF2a away from uterine vein but does not prevent PGF2a secretion
- Some interferon present but not anti0luteolytic
Describe PGF2a redirection in pigs
- Excreted through uterine glands into lumen rather than being absorbed into veins then to CL
- Embryo in lumen is secreting enzymes that metabolise PGF2a
Describe maternal recognition of pregnancy in horses
- Oxytocin stimulates luteolytic PGF2a
- Embryo suppresses OXTR
- No blastocyst interferon
- But signal unknown
- Pregnant mare serum gonadotrophin (eCG) is NOT the signal (production after luteolysis)
Describe the blastocyst migration in horses
- Blastocyst does not elongate, remains spherical and unattached
- Migrates through whole of uterus and horns before implantation
- Critical for MRP and establishment of pregnancy
- Distributes MRP signal
- Stimulated by prostaglandins
- Blastocyst produces oestradiol
- 3 low MW proteins that suppress PGF2a prod.
What is the maternal recognition of pregnancy factor in primates?
Chorionic gonadotrophin (hCG in humans)
Describe PGF2a in primates
- Luteolytic
- Produced by CL
- CL stimualtes own regression (need signal from embryo to stimulate this)
Describe the action of chorionic gonadotrophin
- hCG produced by syncytiotrophoblast cells
- Acts on LH receptor on CL
- Suppresses PGF2a
- Absorbed by venous supply, acts directly on CL
Describe the role of prolactin and placental lactogen in maternal recognition of pregnancy in rodents
- Oestrous cycle~4 days
- Mating leads to (psuedo)pregnancy
- Prolactin secretion, extends luteal phase 10-14 days
- Can be caused by sterile mating
- Placental lactogen is later MRP (takes over once prolactin secretion finished)
In what species does MRP occur before implantation?
- Ruminants
- Pigs
- Mare
What species have no MRP?
- Dogs
- Ferrets
- Roe deer
In what species does MRP occur mid-gestation?
- Rabbits
- Rodents
When does MRP occur in the cow?
(days)
And the ewe?
Cow: Day 16-1
Ewe: day 13-14
When does MRP occur in the mare?
Day 14
When does MRP occur in sows?
Day 12-13
When does MRP occur in goats?
Day 17
When does MRP occur in humans?
Day 7-12
