Devo - Lecture 3 Embryogenesis Week 1

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What are the 3 primary germ layers of the embryo?: -Ectoderm
-Endoderm
-Mesoderm
What are the 2 types of embryonic tissues in the early embryo?: -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?: 1. Epithelium
2. Nervous tisue
3. Connective tissue
4. Muscle
Into what 2 periods is prenatal development subdivided?: 1. Embryonic period
2. Fetal period
How many weeks is each prenatal development period?: Embryonic = weeks 1-8
Fetal = weeks 9-38
What are 9 things that happen in the embryonic period?: 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?: -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?: 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?: -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?: 1. Cell divison
2. Genome activation
3. Blastocyst formation
4. Blastocyst development
What characterizes the cell divisions during week 1?: -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?: 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?: 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?: -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?: 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?: 1. Keeps blastomeres together
2. Prevents the embryo from being recognized as foreign
3. Prevents premature implantation
What does hatching from the ZP allow?: -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?: 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
-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?: 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?: 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.

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