Devo - Lecture 3 Embryogenesis Week 1
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- What are the 3 primary germ layers of the embryo?
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-Ectoderm
-Endoderm
-Mesoderm - What are the 2 types of embryonic tissues in the early embryo?
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-Epithelium
-Mesenchyme - What major difference distinguishes embryonic tissue from adult?
- The ability to transform between epithelium and mesenchyme, given the proper stimulus.
- What ability do adult tissues have?
- The ability to self-renew via stem cells
- What are the four primary tissue types found in human bodies?
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1. Epithelium
2. Nervous tisue
3. Connective tissue
4. Muscle - Into what 2 periods is prenatal development subdivided?
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1. Embryonic period
2. Fetal period - How many weeks is each prenatal development period?
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Embryonic = weeks 1-8
Fetal = weeks 9-38 - What are 9 things that happen in the embryonic period?
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1. Fertilization -> zygote formation
2. Blastocyst forms from zygote cleaving
3. Implantation
4. 3-layer flat embryo forms
5. Axes determined
6. Segmentation of embryo
7. Folding of embryo
8. Limbs/head/trunk form
9. Organs/organ systems form - What happens in the Fetal period?
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-Fetus grows in size
-Organs mature and grow - What is the difference between gestational and fertilization age?
- Gestastional age is 2 weeks longer than fertilization because starts at LNMP.
- What is the definition of an embryo?
- An unborn human in the first 8 weeks of development
- What is the Perinatal period?
- 22 weeks -> First month after birth.
- What is the neonatal period?
- the first month after birth.
- How many stages is the first week broken into? What days correspond?
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Stage 1 = Day 1
Stage 2 = Day 1.5-3
Stage 3 = Day 4
Stage 4 = Day 5-6 - How does the embryo change during the first week?
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-Goes from being unicellular to multicellular
-Develops a cavity
-Travels through oviduct
-Implants in uterine wall - What happens to the zona pellucida durnig the first week?
- It disappears by the 4-6 day.
- What are the 4 major molecular and cellular events during cleavage?
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1. Cell divison
2. Genome activation
3. Blastocyst formation
4. Blastocyst development - What characterizes the cell divisions during week 1?
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-Rapid mitosis, no G1/G2
-No cell growth - ZP restricts it
-Asynchronous blastomere division
-Cytoplasm:Nucleus ratio goes from 300:1 -> 3-6:1 - What causes the first cell division in an embryo?
- A program written by maternal oocyte during oogenesis.
- When does the embryo's own genome start to direct mitosis?
- When it is activated at the 2-4 cell stage
- When do paternal genes start to be expressed?
- After embryonic genome activation, 2-4 cell stage.
- What significant morphological change occurs as the embryo reaches the morula, 16-cell stage?
- Compaction
- What causes compaction?
- The increased surface area of contact between blastomeres.
- At what cell stage (number) does blastocyst formation occur?
- 8-16 cells
- What distinguishes the blastocyst formation?
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Development of 2 cell lineages:
-Trophoblast
-Inner cell mass - What is the inner cell mass?
- Embryoblast - the real stuff that we care about.
- What exactly is the blastocyst?
- A fluid filled vesicle lined by trophoblast cells
- What capability is conferred to the embryo once the blastocyst forms?
- The ability to implant in the uterine wall.
- What type of tissue is the trophoblast layer of cells lining the blastocyst?
- Epithelial - cells joined by junctions, polarized, and covering the cavity lining.
- What type of tissue will the trophoblast become?
- Extraembryonic
- What controls trophoblast and extraembryonic tissue development?
- Genomic imprinting - paternal genes are favored.
- What is another example of genomic imprinting?
- -X chromosome inactivation
- How do X chromosomes get inactivated in female somatic vs. trophoblast cells?
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Trophoblast: paternally derived X is selected for inactivation
Somatic cells: inactivation is random -
What is the polarity of
-Trophoblast tissue
-Inner cell mass -
Trophoblast = polarized
ICM = initially non polarized - What happens to the ICM as it develops?
- Forms a Bilaminar Disc
- What does the bilaminar disc consist of?
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-Hypoblast
-Epiblast - What will the hypoblast become?
- Extraembryonic
- What will the Epiblast become?
- Embryonic + Extraembryonic
- What develops as a result of the Bilaminar disc formation?
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Embryo's Axis of Polarity
-Ventral= bottom epiblast layer
-Dorsal = upper epiblast layer - What 3rd thing (other than trophoblast and ICM development) occurs to the embryo during trophoblast development?
- Hatching from the ZP
- What are the functions of the ZP while the embryo develops?
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1. Keeps blastomeres together
2. Prevents the embryo from being recognized as foreign
3. Prevents premature implantation - What does hatching from the ZP allow?
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-Small increase in embryo size
-Ability to implant in uterine wall. - What is potency?
- The full range of developmental capabilities available to a blastomere.
- What is nondisjunction?
- The failure of chromosomes to separate correctly - results in cells with different genomes.
- What 3 develpmental potentials do cleavage-stage embryos posses?
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1. The potential to REGULATE.
2. Potential for Cell Diversity
3. Potential for Twinning - What is Regulation?
- The ability to develop normally when parts have been removed or added.
- What happens to the embryo's potential for regulation?
- It decreases
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-What term denotes full ability to regulate? What stage has it?
-What denotes decreased ability to regulate? What stage? -
Totipotent - 0-8 cell stage
Pluripotent - 16-cell stage - What condition results from an embryo's cells containing different genomes? What causes this?
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Chimerism, aka Mosaicism
-Nondisjunction -
How many births in USA are:
-Twins
-Triplets -
Twins = 1/90 births
Triplets = 1/8000 births - What causes monozygotic twins to form? What fraction of twins result from this process?
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Blastomere/ICM separation
1/3 - What causes dizygotic twins to form? What fraction?
- Double ovulation and fertilization; 2/3
- What causes conjoined twins to form?
- Separation of blastomere/ICM (monozygotic twinning), but failure of the embryo to separate COMPLETELy.