Fertilization/Cleavage divisions Flashcards
When does cleavage occur?
cleavage: days 1-5
When does implantation occur?
implantation: days 6-9, approx 6 days after ovulation
Where is the mammalian egg normally fertilised?
ampulla of the oviduct/fallopian tube
What are the two phases at which oogenesis is halted?
primary oocyte halted at prophase 1 until puberty;
secondary oocyte halted at metaphase 2 until fertilised
When is spermatogenesis initiated?
only at puberty
Average sperm count?
40-100 million spermatozoa (decreasing in Western world)
Purpose of the acrosome?
at front of head of spermatozoa - bursts when in contact with receptors in zona pellucida = release of enzymes digesting ZP
What prevents polyspermy?
Granules release content when sperm fuse with oocyte plasma membrane, modifying ZP
Embryos with maternal Xsomes only?
- gynogenetic embryos
- poorly developed trophoblast
Embryos with paternal Xsomes only?
- androgenetic embryos
- over-developed trophoblast, poorly developed embryo
What are hydatidiform moles?
- formed when trophoblast grows uncontrollably
- leads to miscarriage, tumour
- complete moles: only paternal Xsomes
- partial moles: 2 sets of paternal Xsomes and 1 set of maternal Xsomes
Describe cleavage divisions
- mitosis in fertilised egg as it travels down fallopian tube to uterus
- not accompanied by cellular growth
- large egg splits into smaller cells until they reach normal size
- cleavage divisions take 12-24h, last 5 days in humans
Describe compaction
- 16-cell morula
- cells maximise contacts with each other
- outside pop called trophoblast (polar)
- inner pop called inner cell mass (non-polar)
- formation of blastocoel (fluid filled cavity), displacing ICM to 1 pole
- embryo now blastocyst
How is regulative development a cause of twins?
- embryo can be divided into individual blastomeres at 2-cell and 4-cell stage, going on to form identical clowns
- reg. dev. responsible for monozygotic/identical twins
- 30% formed 2-cell stage, the rest at blastocyst stage
- proof that blastomeres are totipotent
Define tetragametic chimera
- 2 embryos fuse and form normal sized tetragametic chimera (formed by 2 sperm and 2 eggs)
- can happen when fraternal twins fuse to form single embryo
What are the functions of the trophoblast?
- implantation
- differentiate chorion
- immunosuppression
- endocrine gland (chorionic gonadotrophin, progesterone…)
What is the role of HCG (chorionic gonadotrophin)?
-maintain pregnancy by maintaining steroid production of ovary
-produced by trophoblast during 1st trimester,
maintaining production of progesterone by ovary (required to maintain uterine wall, prevent menstruation)
-in later pregnancy, progesterone produced by trophoblast, no HCG=no ovarian production
What is the role of the ZP?
-prevent embryo from implanting into fallopian tube
( + prevent polyspermy through its modification)
-when in uterus, embryo hatches from ZP so it can implant (secretion of emzymes = hole in ZP)
What does the ICM give rise to?
- during implantation: epiblast and hypoblast
- epiblast later gives rise to 3 germ layers
