Flashcards in Embryogenesis Deck (94):
What do oviduct epithelial cells do?
Make them briefly immobile
Stabilise sperm after capacitation
When do sperm reacquire mobility?
How is oocyte moved to the ampulla?
Swept along by cilia and muscle contractions
Where does fertilisation take place?
What three events does fertilisation require?
1. Activation of sperm
2. Establishment of diploidy
3. Activation of ovum
What induces sperm activation?
1. Progesterone from cumulus cells
2. Binding to zona proteins on oocyte
What does sperm activation cause?
1. Hyperactive tail movement - whiplash
2. Acrosome reaction
What promotes hyperactive sperm tail movement?
Opening of calcium channels to activate PKC
What is the acrosome reaction?
1. Fusion of outer acrosome and sperm plasma membranes
2. Phospholipase C and SNARE proteins involved
3. Release of hyaluronidase and exposure of acrosin on inner acrosome membrane
What are the zona pellucida proteins?
What do ZP1, ZP3 and ZP4 do?
1. Bind capacitated sperm
2. Induce acrosome reaction
What pathway does ZP3 activate?
Gi protein-coupled receptors on sperm
What does ZP2 do?
Weakly binds capacitated sperm to allow progression through zona
What allows sperm to burrow through zona pellucida?
1. Acrosin - protease
2. Physical movement
What space does sperm enter once it penetrates zona?
Where does membrane fusion occur in the mouse?
Equatorial region away from female pronucleus
Enveloped by microvilli
What does the microvillus-free area of the oocyte signify?
Position of female pronucleus
What are the three proteins essential for binding?
1. Izumo1 (sperm)
What is Izumo1?
Ligand on sperm essential for binding
What is Juno?
Izumo1 receptor on oocyte
What is CD9?
Tetraspanin on oocyte
Binding partner for Juno
What is syngamy?
Combining of chromosomes
How is the oocyte activated?
1. Wave of calcium release across oocyte from site of sperm fusion
2. Initiated by PLCζ enzyme from sperm
3. Helps to prevent polyspermy
What is the cortical reaction?
Release of cortical granules stimulated by calcium waves
Fill perivitelline space
Ovastatin cleaves binding sites on ZP3/4 and ZP2 so that sperm can no longer bind
Zona proteins become cross-linked
Juno receptor exocytosed
When does the secondary oocyte re-enter meiosis?
After the cortical reaction
How is the sperm pronucleus formed?
1. Nuclear envelope breaks down
2. Sperm protamines replaced with histones to decondense chromatin
How is the single diploid nucleus formed?
1. Pronuclear membranes break down
2. Chromosomes become orientated on common mitotic spindle
What does the oocyte contribute to the zygote?
2. Golgi apparatus
5. Endoplasmic reticulum
6. Maternal cytoplasm
7. Centrosomes and mitotic spindles
What does the sperm contribute to the zygote?
3. Small non-coding RNAs
4. Proteins targeted for destruction once they enter the oocyte
Why are sperm mitochondria degraded?
More susceptible to mutation through oxidative stress
mtDNA not protected by histones and no DNA repair mechanisms in sperm
Where are sperm naturally exposed to ROS?
1. Leukocytes in epididymis
2. Transit to female tract
3. Energetics of sperm movement
What is parthenogenesis?
Oocyte activation in absence of sperm fusion
Lacks male chromosomes and centriole
Gynogenetic diploid if polar body not expelled
At what point in human gestation do parthenogenetic embryos die?
During first cleavage as no centriole
What kind of development does a gynogenetic conceptus favour?
What kind of development does an androgenetic conceptus favour?
What are imprinted genes?
Genes only expressed by one parental allele
Silenced by methylation on the other parental allele
What is a hydatidiform mole?
All chromosomes of paternal origin due to loss of female pronucleus and duplication of male pronucleus
Absence of fetal tissue and overgrowth of placental tissues
Placental villi are oedematous
Give three examples of epigenetic modifications
1. DNA methylation
2. Histone methylation
What is the effect of DNA methylation?
Changes conformation of DNA to prevent access of transcription factors
Which pronucleus is more methylated at fertilisation?
When does DNA demethylation take place?
When does DNA remethylation take place?
Which loci are resistant to demethylation?
What are imprinted genes important for?
1. Placenta development
2. Reproductive behaviour
What is the conflict theory?
Paternal genes drive growth of placenta whilst maternal genes constrain fetal growth to prevent nutrient depletion and facilitate delivery
What is the morula?
First cleavage divisions
Cells are totipotent
Cells can be removed for genetic diagnosis
What is maternal cytoplasmic inheritance?
First cell division under influence of maternal mRNA and proteins generated in oocyte during folliculogenesis
When is transcription of embryonic genome initiated?
4-8 cell stage
Until what point does maternal RNA persist?
What is compaction?
1. Creating cell polarity
2. Initiating cell specialisation
Where is concanavalin A expressed?
Junctions between cells of morula
How is the blastocoel formed?
Establishment of ion gradients causes influx of water
What does the blastocyst consist of?
Trophectoderm and inner cell mass
What is the trophectoderm?
Trophoblast contributes to formation of placenta
What is the inner cell mass?
Forms embryo and contributes mesoderm to placenta
At what stage of development does the embryo implant?
What are Cdx2 and Oct4
Transcription factors that determine cell fate
What is the role of Oct4?
What is the role of Cdx2?
Trophoblast lineage determinant
Where is Oct4 expressed?
8-cell stage: nuclei of all blastomeres
Blastocyst stage: inner cell mass
Where is Cdx2 expressed?
8-cell stage: nuclei of all blastomeres
Blastocyst stage: trophoblast
What regulates Cdx2 expression?
Degree of cell-cell contact
Activated by Yap and Tead4
What is hippo?
Active in non-polarised cells
Phosphorylates Yap, preventing it from entering the nucleus - inhibits Cdx2 expression
After how many days does conceptus enter uterus?
3.5 days after fertilisation
What may be the effect of chlamydia infection?
Scarring and narrowing of Fallopian tube
What is the pO2 in the Fallopian tube?
Where does the zygote get its nutrition from?
1. Secretions from cells of oviduct
2. Then endometrial glands
What is the source of energy for the fertilised ovum?
What is the source of energy at the 2-cell stage?
Pyruvate and lactate
What is the source of energy at the 8-cell stage?
Glucose and essential amino acids
What is the source of energy at the blastocyst stage?
How does the blastocyst hatch from the zona pellucida?
Where does implantation occur?
Near fundus of uterus
What is placenta praevia?
Placenta covers cervical opening
Associated with increased risk of antepartum haemorrhage
How does the conceptus implant into the uterus?
Invasive interstitial implantation
Completely embedded in uterine wall
What does receptivity of the endometrium involve?
1. Downregulation of inhibitory mechanisms
2. Upregulation of factors promoting attachment
What is the window of implantation?
Period of progesterone dominance
Progesterone and oestrogen ready the uterus for implantation
Oestrogen prepares the conceptus for implantation
Burst of oestrogen synthesis required for implantation
Give five features of the endometrium in the pre-receptive window
2. High progesterone receptor expression
3. Thick mucin glycoprotein coat
4. Negative charge on surface membrane
5. Long microvilli
Give seven features of the endometrium in the receptive window
1. Safe for embryos to implant
2. Progesterone receptors decrease
3. Thinner mucin glycoprotein coat
4. Loss of negative charge on surface membrane
5. Pinopodes to absorb uterine fluid
7. Shorter/absent microvilli
What is the effect of reduced glycosylation of MUC1?
Allows protein-protein interactions between blastocyst and uterine lining, leading to implantation
Give three actions of LIF
1. Acts on trophoblast and uterine lining
2. Leads to upregulation of HB-EGF on uterine epithelium
3. Induces uterine α2 expression, and trophoblast integrins α6 and β5 expression
What does invasion of endometrium involve?
At what point after fertilisation does implantation take place?
Fully embedded after 11 days
What are decidual cells formed from?
Spindly stromal cells
Give two features of decidual cells
1. Large and round
2. Secrete cytokines
What cytokines do decidual cells secrete?
1. IGF-binding proteins
What are the three regions of the decidua?
1. Basalis - beneath conceptus
2. Capsularis - over conceptus
3. Parietalis - around the rest of the conceptus
What is amplification?
Extensive decidualisation after implantation
What is menstruation?
Shedding of decidual cells because they cannot de-differentiate if fertilisation does not occur
What secretes human chorionic gonadotropin?
What is the role of hCG?
1. Maintains corpus luteum so that it continues secreting progesterone to prevent menstruation
How long is the corpus luteum maintained?
10 weeks until placenta produces sufficient progesterone
What is the percentage of conceptuses that become live births?