57.1 Fertilization and Implantation Flashcards
What are the steps that take place during fertilization?
Zona pellucida binding, acrosome reaction, cortical reaction.
What are some important structural and functional features that an oocyte must have in order to undergo successful fertilisation?
- Expanded cumulus cell matrix (inner layer = corona radiata) -> To aid passage into fimbria, protect zygote and provide an additional barrier to polyspermy
- Zona pellucida -> Provide a surface receptor for sperm, prevent polyspermy, protect preimplantation embryo and prevent fusion of embryos
- Initiated second meiotic division at ovulation
- Enough proteins, RNA, ribosomes, mitochondria, etc for development of the early embryo (only maternal gene expression until ~8 cell stage)
- Ability to decondense sperm nucleus
- Multiple mechanisms to prevent polyspermy (e.g. cortical reaction)
Label this oocyte.
What is the corona radiata, how is it formed and what is its function?
What it is:
- The innermost layer of the cells of the cumulus oophorus, directly adjacent to the zona pellucida
Function:
- Supply vital proteins to the cell
- Sperm must get through it to reach oocyte
Formation:
- Formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle
- Originates from the squamous granulosa cells present at the primordial stage of follicular development
- The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage
What is the zona pellucida and what is its function?
- A glycoprotein layer surrounding the plasma membrane of oocytes.
- It is secreted by both the oocyte and the ovarian follicles.
- The zona pellucida is surrounded by the corona radiata
- Functions:
- Provides a surface receptor for sperm
- Prevents polyspermy
- Protects preimplantation embryo + prevents fusion of embryos
How does an oocyte travel down the oviduct (Fallopian tube)?
- High levels of estradiol at the end of follicular phase and in luteal phase cause muscular activity in the oviduct
- Its fimbriated end becomes closely apposed to ovulation site
- Its folded, ciliated epithelium wafts ovulated oocyte & corona into tube
- Secretions from epithelial cells help nourish the developing embryo
What hormone is responsible for the movement of a oocyte down the oviduct (Fallopian tube) after release? When are the levels of this hormone high?
- Estradiol (a type of oestrogen)
- High levels are at the end of follicular phase and in the luteal phase
- They cause muscular activity in the oviduct, so that the fibriated end comes close to where the oocyte is
What cross-section is shown here?
Oviduct (Fallopian tube)
How can you monitor for when ovulation is happening?
- Basal body temperature rises within a few hours of ovulation
- Immunosticks with an anti-LH antibody (since there is a surge of LH)
What happens to body temperature just after ovulation?
It rises within a few hours.
Describe the epithelium of the oviduct (Fallopian tube).
- Fimbriae -> Ciliated columnar cells
- Secretory cells
What are some structural and functional features that spermatazoa must have in order for successful fertilisation to occur?
- Haploid complement of DNA
- Potential for strong motility
- Receptors for the zona pellucida and egg
- Ability to penetrate cumulus and zona pellucida
- Ability to fuse with the egg
- Ability to activate an egg
What is the first change that sperm undergo following ejaculation?
Capacitation in the female genital tract
What is capacitation of spermatazoa?
The physiological changes spermatozoa must undergo in order to have the ability to penetrate and fertilize an egg.
What happens during capacitation?
-Enzymes in the uterus cause the removal of glycoproteins and cholesterol from the sperm head (around the acrosome)
-Cell membrane becomes more fluid and permeable to calcium ions as a result
-Increased motility
How does the egg direct sperm to it?
- Atrial natriuretic peptide (ANP) may act as a chemoattractant
- Progesterone + Atrial natriuretic peptide (ANP) from cumulus cells stimulate motility & acrosome reaction
What is capacitation of spermatazoa dependent on?
Secretions of seminal vesicles and prostate
How long is sperm stored for in the female and where?
- For 5 days after intercourse
- Most in the isthmus and some in the cervix
What are some changes that occur during capacitation of spermatazoa?
- Acrosin is activated
- Motility is increased
- Loss of cholesterol from the sperm head
What is the cumulus oophorus?
- A cluster of cells (cumulus cells) that surround the oocyte.
- The innermost layer of these cells is the corona radiata.
- This layer of cells must be penetrated by spermatozoa in order for fertilization to occur
How many zona pellucida proteins are there in humans and which one do spermatazoa bind to first?
4 zona pellucida proteins -> Binds to ZP3 first on the plasmalemma covering the acrosome to trigger the acrosome reaction.
What is the acrosomal reaction and what occurs during it?
It is the reaction when the spermatazoon binds to the zona pellucida:
Acrosome membrane ruptures releasing lytic enzymes which digest the zona pellucida (hydrolyse to provide a path for sperm)
Exposes ZP2 receptor on sperm head
Draw the general structure of a spermatazoon.
Label this sperm.
What is the cortical granule reaction and what occurs during it?
It is the reaction that occurs after the spermatozoon fuses with the zona pellucida, in order to prevent polyspermy:
Fusion of the two membranes triggers an increase in intracellular calcium (released from smooth ER by IP3-DAG pathway)
Calcium wave causes cortical granules to release their contents into the zona pellucida
Induces coagulation and hardens the ZP to prevent penetration by another mature spermatozoa so the zygote remains diploid
How does fertilisation trigger the sequence of events that follow it?
Fertilisation causes Ca2+ transients in zygote:
- Wave spreads from point of fertilisation
- Wave initiated by PLCζ (zeta) which is on the internal membranes of sperm equator
- A few waves stimulate the cortical granule reaction
- Repeated waves stimulate the start of development, division etc.
What happens after fertilisation?
Cleavage - rapid mitotic divisions without cell growth - as the zygote moves down the fallopian tubes
How does the uterus prepare for pregnancy?
Prepared through the follicular phase:
Poliferative phase in first half of pregancy increases mass and cell number of the endometrium - ready for implantation in the secretory phase (maintained by progesterone until hCG takes over)
Spiral arteries are formed in proliferative phase
Increased number of endometrial glands to provide glycogen rich endometrial secretion that nourishes conceptus
What is expressed on uterine epithelial cells that needs to be downregulated for implantation to occur?
Uterine epithelial cells produce mucins which hide HS binding receptors needed for implantation
What phase of meiosis is the oocyte arrested in prior to fertilisation?
Metaphase II
Where does fertilisation occur typically?
At the isthmus-ampulla junction.
(On the diagram, the oocyte comes in from the top, while the sperm comes from the bottom)
