Embryology Flashcards
Define ‘oocyte’
Cell in ovary which undergoes meiotic division to form ovum
Define ‘zygote’
Fertilized ovum (diploid cell); oocyte + sperm
Define ‘blastocyst’
Hollow sphere of cells with a cell inner cell mass, just before implantation
Define ‘monozygotic’
Twins derived from a single ovum (identical twins)
Define ‘dizygotic’
Twins derived from two ovum (fraternal twins)
Define ‘cytotrophoblasts’
Interior cell layer of blastocyst
Define ‘syncytiotrophoblast’
Outer layer of cells of blastocyst; no plasma membrane but many nuclei; invasive into uterine wall
Define ‘embryo’
Implanted bundle of cells less than 8 weeks after conception
Define ‘fetus’
Implanted cells more than 8 weeks after conception
Describe the embryonic and fetal periods of development
Embryonic period (0-8 weeks): Establishment of organ systems
Fetal period (9 weeks to birth): Growth and maturation of organ system; stuff just gets bigger
Describe the first week of human development and pathogenesis and outcome ectopic pregnancies
First week:
- Day 0: oocyte released from ovary; fertilization occurs in uterine tube
- Day 2: Zygote completes first division (2 cells)
- Day 3: 4 cells and 8 cells; size stays constant
- Day 4: Morula (solid ball of cells)
- Day 5-6: Embryo “hatches” from zona pellucida; increases size of mass
- Day 7: Blastocyst forms and implants
Ectopic pregnancy locations:
- Ectopic pregnancies are located outside uterine walls
- Most common ectopic location is ampulla of uterine tube (dilated part that curves over ovary)
Explain how home pregnancy tests work
Pregnancy test detect elevated hCG (human chorionic gonadotropin) in maternal urine and blood.
- Trophoblast produces hCG
- hCG maintains corpus luteum
- corpus luteum produces progesterone
Describe process of in vitro fertilisation
Two methods:
- oocyte is washed over by sperm
- single sperm is injected into oocyte
Embryo is placed in uterus after 3 days
Describe process of genetic screening
- Cells can be removed from early embryo (before 8 cell stage) since remaining cells are totipotent (no side effects from removal.
- Cell can be screened from genetic diseases
Describe multiple gestations
- Identical twins: single fertilized zygote divides into two
- Fraternal twins: two fertilized zygotes
Describe molar pregnancy
- Occurs when sperm fertilizes an egg, but maternal nucleus is lost (empty egg with only paternal DNA)
- Results in a placenta but NO embryo.
Describe gastrulation phase
- Two layered embryos is converted into three layers (Epiblast [outer layer] and hypoblast [inner layer])
- Mesoderm cells fill between layers
Describe the three primary germ layers
1) Ectoderm -> outer layer - epidermis and CNS
2) mesoderm -> middle layer - dermis, muscles, connective tissue, cardiovascular system, reproductive
3) endoderm -> inner layer - lines viscera, gastrointestinal tract
Describe neurulation phase
- Embryo has three layers
- Neural plate forms (thickening of ectoderm)
- Neural groove forms as walls of neural plate grow upward
- Neural tube forms when neural groove closes in middle region
- Anterior and posterior neuropore close later
Describe organogenic period
- Organs of body are developed from three layers
- occurs from week 4 to week 8
Describe fetal period
- 9 weeks to 38 weeks
- fetus grows from 3cm long to 36 cm long
- Start: villi around all uterine wall
- Middle: villi restricted to maternal attachment site
- End: uterine cavity obliterated
- hCG decreases and progesterone and estrogen increase
Describe fetus sampling
- Chorionic villous sampling (8-10 weeks): small piece of placenta is removed and cultured
Describe fetal anomaly scan
- 18 weeks
- look for congenital malformations
- can determine sex
Define primary prevention
address root of cause of birth defect (e.g. drug or behvaiour that causes defect)
Define secondary prevention
aim to detect and treat birth defect early on
Define anencephaly
Failure of head end of neural tube to close; baby born without skull covering brain
Define spina bifida
Failure of lower end of neural tube to close; baby born with spinal cord exposed
Describe role of folic acid in neural tube defect prevention
- Folic acid reduced risk of neural tube defects by 50~70%
- Found in dark green vegetables, liver, and beans
- Added to all bread
- Women aren’t aware they are pregnant when neural tube is developing. Need folic acid before awareness
Describe time frame for major malformations
- Organs are most sensitive to damage during the early stages of development (critical development time)
- After critical development, damage will cause minor defects (e.g. abnormal size)
- Most malformation damage happens during 3 to 8 weeks
Describe the different types of defects
- Genetic origin (25%): e.g. Down syndrome (trisomy 21); Holt-Oram syndrome
- Potentially preventable cause (10%): e.g. maternal diabetes or infection; drugs or alcohol
- Unknown cause (65%): note that large majority of birth defects have unknown causes
Describe categorization of risk of drugs in pregnancy
A -> No evidence of harmful effects
B -> animal studies; no human studies; may or may not have effects
C -> May have effects
D -> suspected on causing damage
X -> high risk of damage; NO NEED TO TAKE THESE WHILE PREGNANT
Describe birth defect agent: thalidomide
- Severely malformed limbs
- Not teratogenic in rats and mice, but was in humans
- Very common drug and caused 10000 malformed children
Describe birth defect agent: Isotretinoin (accutane)
- High risk of severe congenial malformation
- Vitamin A analogue
- Treats acne so no reason to take during pregnancy
Describe birth defect agent: anticonvulsants
- Increase risk of birth defect by 2-4 fold
- Increase risk of spina bifida by 5-10 fold
- Convulsions can injury fetus too so it’s a trade-off
Describe birth defect agent: alcohol
- Increase risk of mental retardation, and major birth defects (usually heart defects)
- Caused by excessive maternal alcohol in take
- no safe limit of alcohol is proven: some say NO alcohol, some say low amount is acceptable
- If blood alcohol level doesn’t reach excessive levels, it’s alright
Describe detection of teratogenic drugs
- All new drugs are tested in animals before release:
1) must be two different species
2) maximum tolerated dose is used, and fetus is examined before birth - All that is tested is that it does NOT cause increase in chance of defect
- Information is collected from human after release:
1) 100 human pregnancy with no defects = does NOT cause 4-fold increase in defects (95% significance)
2) 600 human pregnancy with no defects = does NOT cause 2-fold increase in defects (95% significance)
