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Early Embryonic Dvl.
Organisation of the Body
| Question | Answer |
|---|---|
| 3 main processes in embryonic development | Cleavage - lost of cells from one cell Gastrulation - folding of tissues to form 3D shape Neurulation - making nerves, spinal cord and brain |
| Von Baer's Law | Early embryonic development is similar across many species (embryos have a similar body plan) but becomes divergent as development proceeds |
| Model animals | Mice Zebra fish - transparent embryos Chick - large embryo Fruit flies Nematode worms |
| Human menstrual cycle | Hormonal regulation by the pituitary and ovary lead to follicular development and ovulation at day 14 Hormone production by the corpus luteum and oestrogenic hormones play a critical post-ovulatory role in preparing the uterine lining for implantation |
| Oocyte maturation | During maturation in the few days before ovulation, meiosis 1 (halted in eggs) is completed in the dominant follicle (containing 15-20 oocytes) Meiosis 2 is halted in metaphase 500 fold increase in volume of cells surrounding the oocyte |
| What is the zona pellucida | A glycoprotein shell surrounding the oocyte inside follicular cells |
| Fertilisation | 1% of sperm get through cervix Oocyte moves into oviduct and is fertilised in the ampullary region Sperm require capacitation (Ca dependant removal of glycoprotein coat) and acrosome reaction on binding to zona pellucida to produce degrative enzymes |
| What happens at fertilisation | Female and male pronuclei form Formation of second polar body triggered by sperm penetration Sperm penetration causes cortical granules to release their contents (lysosomal enzymes), rendering zona pellucida impenetrable to further sperm |
| What does fertilisation lead to | Restoration of diploid number of chromosomes Determination of embryo sex Initiation of cleavage When pronuclei meet they divide and form 2 cells via cleavage |
| Early embryo cell potency | Totipotent - can form all cells in embryo and surrounding tissue |
| Cleavage in early development (frogs) | In frogs - reaches around 4000 cells dividing every 30 mins No G1/G2 to allow rapid division No growth as there are no nutrients to the embryo Cells do not specialise easily |
| Cleavage in humans | 12-24 hour cell cycle Zygotic transcription at 4-8 cell stage At 16 cells a morula forms with clear ICM/trophoblast in the first differentiation event This occurs via compaction (tight junctions between cells) Movement down oviduct |
| Dizygotic twins | Two separate blastocysts, no communication between each embryo May share a placenta but must have separate blood supplies If blastocysts implant separately they are not connected If the blastocysts implant close together the chorionic sac may connect |
| Monozygotic twins - one blastocyst | Two ICMs form in one egg Share a placenta and share a blood supply One embryo may develop faster than the other due to competition for the shared blood supply Share a chorionic sac |
| Monozygotic twins - blastocyst splits | Cells at the morula stage divide in 2 The embryos behave as dizygotic twins - are frequently misidentified Can have separate or fused placentas and chorionic sacs |
| In vitro Fertilisation | Relies on the fact that embryos can be frozen (resistant to teratogenic insult) Not all embryos implant - 4/5 will be used leading to multiple pregnancies Can remove one cell for genetic screening |
| Process of IVF | Hormonal stimulation of mature oocyte with gonadotrophins to trigger super ovulation Oocytes placed in a petri dish and fertilised Zygotes cleaved to 4 or 8 cell stage Embryos transferred into the uterine cavity with a catheter |
| What is IVM | In Vitro Maturation of immature oocytes Potentially avoids costly hormone treatment that may have side effects Successful in mouse but primarily used when IVF is inappropriate for a woman |
| Cloning | A way of producing animals with a particular genome Enucleate egg Add nucleus of an unaltered somatic cell that is treated with transcription factors to reverse epigenetic changes Trigger cleavage and allow development |
| Embryonic stem cells | Can be used to produce mouse models of human disease Can alter genetics of embryos to create disease models Also in humans from ICM cells or pre-meiotic germ cell precursors Potential for generating new tissues/organs Ethics of genome manipulation |
| Blastocyst formation | Inner cell mass - embryo Trophoblast - extraembryonic placenta Fluid through zona pellucida and trophoblast via Na+ pump driving osmosis of water into intracellular space of ICM to form the blastocoel Hatching from zona pellucida to allow implantation |
| Implantation | Hatched blastocyst is adherent -embeds in gland openings typically on anterior or posterior wall of uterus Triggers decidual reaction e.g. endodermal stroma has many vessels and gland to produce glycogen and mucus Immunologically safe site for the fetus |
| Ectopic implantation | Implantation outside the uterus Often in oviduct or even outside the reproductive system The embryo will not reach term Can be life-threatening for the mother as it can lead to haemorrhage e.g. rupture of the oviduct |
| Hydatidiform moles - embryo from 2 male genomes | Trophoblast grows with little or no embryonic tissue High levels of HCG made by trophoblast produce benign or malignant tumours Embryos from 2 male haploid genomes only give placental tissue Embryos from 2 female genomes form only embryonic tissue |
| Genomic imprinting and Pronuclear non-equivalence | Genes essential for embryonic development are silenced in the paternal genome Genes essential for placental development are silenced in the maternal genome This is mediated by DNA methylation |
| Effects of IGF-2 mutations | Involved in fetal growth Mutations in the paternal genome leads to Prader-Willi syndrome Mutations in the maternal genome leads to Angelman syndrome |
| Chromosome abnormalities | At birth 7% of major birth defects are due to chromosomal abnormalities, whilst 8% are caused by specific gene mutation 50% of all conceptions give rise to spontaneous abortions 50% of these have major chromosomal abnormalities |
| Examples of chromosomal abnormalities | Triploidy - extra pronucleus Monoploidy - loss of a pronucleus Trisomy - extra chromosome Monosomy - loss of a chromosome Translations, insertions, deletions etc |
| When do chromosomal abnormalities arise | Usually during gametogenesis Increased risk with parental age |
| What causes trisomies and monosomies | Non-disjunction The failure of homologous chromosomes or sister chromatids to correctly separate in meiosis Most common trisomy is Down syndrome (chromosome 21) |
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