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Devo - Lecture 3 Embryogenesis Week 1


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What are the 3 primary germ layers of the embryo?
What are the 2 types of embryonic tissues in the early embryo?
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?
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:
-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?
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?
What will the hypoblast become?
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

How many births in USA are:
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
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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