5.4 - Embryogenesis Flashcards
Define Teratology.
- the study of causes and biological processes leading to abnormal development
- become prominent in 1960s due to Thalidomide
Define critical periods
- time of increased sensitivity in development
- during weeks 1-2, the embryo is not susceptible to teratogens, which is beneficial because this is usually when the mother is unaware that she is pregnant
- weeks 3-8 (Embryonic period) - generally a period of high sensitivity
- weeks 8-38 (Fetal period) - organs are undergoing maturatio, so they’re not quite as vulnerable to major congenital anomalies
- exception: CNS - longer period of high sensitivity
Define congenital birth defects
- leading cause of infant mortality in US
- causes:
- genetics
- “environment” (intrauterine environment, infection, drugs, diet)
- unknown (65% of defect have no known or identifiable cause)
- multifactorial
- risks: periods o higher/lower susceptibility
Define embryonic stem cells
- derived from inner cell mass of embryo
- are pluripotent: can form virtually any cell or tissue type
Describe Fertilization
- when male gamete (sperm) uniting with female gamete (oocyte)
- resulting in fertilized egg (zygote)
- occurs typically in ampullary region of uterine tube
List the 3 results of fertilization
1 - restoration of diploid number of chromosomes
- determined by sperm
#3 - initiation of cleavage
- necessary for zygote to persist
- without fertilization. the oocyte degenerates 24 hours after ovulation
What is the morula?
- 16-cell stage of zygote
- forms after several rounds of mitosis, which follows fertilization
What is the blastocyst and when does it form?
- around day 5
- a hollow ball of cells formed from morula
What is the inner cell mass?
- cluster of cells inside blastocyst
- cells that will eventually form body of embryo
- not yet specialized; phenoenal ability to differentiate
What is (normal and abnormal) implantation and when does it occur?
- day 6
- when blastocyst implants into endometrial layer in lateral or posterior uterine wall (normally)
-
(abnormally) - implants outside of uterus
- “ectopic” or “extra-uterine” pregnancy
- 95% implant in ampullary region of uterine tube
- 5% implant in abdominal cavity, specifically attach to peritoneum in rectouterine cavity
What is a teratogen?
Any substance that causes birth defects.
What is an example of a teratogen that caused phocomelia?
Thylidamide that was used to treat morning sickness.
What are the two periods and timeframes of development?
Fertilization and gestational
How many weeks is a full term child considered in the context of the Fertilization timeline?
38 weeks, begins at conception, Embryology is taught using days “postfertilization”. Thus pregnancy is concidered 38 weeks.
How many weeks is a full term child considered in the timeframe Gestational?
40 weeks, begains two weeks earlier at the begining of last menstrual period.
Define Trophoblast
Differentiates into 2 layers; the cytotrophoblast and the syncytiotrophoblast during the second week.
What two layers does the embryoblast form?
Epiblast and hypoblast
What 2 layers does the extraembryonic mesoderm split into?
The somatic and splanchnic layers
Which layer produces hCG?
Produced by the syncytiotrophoblast.
What two cavities are large during the first 2 weeks but completely disapper by the 3rd month?
Chorionic cavity and the yolk sac cavity.
When and what is Gastrulation?
Occurs during the thrid week.
Gastrulation converts the bilaminar embryo into trilaminar embryo.
What is Holoprosencephaly?
- Injury to the anterior midline of germ disc.
- Cyclops (one eye)
- High doses of alcohol or genetics.
What is Caudal dysgenesis?
Injury to the caudal aspect of the germ disc.
Mermaid/fused legs
-Genetics and environmental insults largest causes.
What is Sacrococcygeal tumors?
Remnant of the primitive streak.
How do conjoined twins normally occur?
Splitting of the primitive node
What three germ layers form the adult tissues?
Ectoderm, Mesoderm, and Endoderm
What does the Ectoderm form?
Neural system, skin, appendages that relate to external environment.
What does the mesoderm later form?
Musculoskeletal tissue, genitourinary system, body wall and membranes that line cavities.
What does endoderm later form?
Foregut, midgut and hindgut, GI tract, and appendages (liver, pancreas), respiratory tract, bladder.
Dizygotic (fraternal twins)
2/3 of twins
Result from two oocytes fertilized by differnet spermatozoa
Totally differnet genetic constitutions
May be same or opposite sex
Monozygotic twins (identical)
Result from splitting of the zygote at various stages of development.
Can happen prior to implantation
Often the inner cell mass (blastocyst stage) splits
Will be same sex and have same genetic constitution.
Conjoined twins
- At later stages of development, partial splitting of the primitive node and streak may result in formation of conjoined twins.
- The type of twins formed depends on when and to what extendt abnormalities of the node and streak occur.
Define the placenta
The placenta is an organ that connects the developing being to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mothers blodd supply.
Do the blood types of the mother and child mix?
The blood supplies of the mother and embryo do not mix.
Blood types may not be compatible
Mothers blood pressure might damage embryo
Define the Umbilical Cord
Connects the embryo with the placenta
Takes blood from the embryo to the placenta and back again. Cut at birth to separate the mother and baby.
Define embryology
The branch of biology and medicine concerned with the study of embryos and their development.
Define Embryogenesis
The process by which the embryo is formed and develops, until it develops into a fetus.
