Early embryology Flashcards
What creates a zygote?
Fertilisation between female oocyte and male sperm
What happens to zygote?(think expansion)
Mitotic cell divisions (cleavage) making daughter cells (blastomeres)
What are blastomeres?
Daughter cells formed during cleavage of zygote
What surrounds the oocyte?
Thick glycoprotein layer called the zona pellucida
What does sperm bind to?
Zona glycoprotein, ZP3, is responsible for sperm binding
How do sperm degrade the ZP?
Once bound to ZP3 they undergo an acrosome reaction which produces enzymes that can degrade the ZP.
At what cell stage of cleavage do cells lose their totipotency?
16 cell
What stages of the cell cycle do not occur in cleavage?
G1/G2
What is the cell mass called at the 16 cell stage?
Morula
What is compaction, what does it do?
TJs form between the cells, blastomeres align, separates inner cells from outer cells
What is the inner cell mass and the outer cell mass called?
Inner cell mass - embryoblast
Outer cell mass - trophoblast
What does inner cell mass do?
Form embryo proper
What does outer cell mass form?
Contributes to placenta
What causes Inner cell mass to be displaced to one side (what’s this side called)?
Fluid accumulation collecting in blastocele cavity - flattens inner cell mass into embryonic pole/embryoblast
What is the blastocele?
A fluid cavity that forms in the intracellular spaces of the inner cell mass
When is implantation?
Day 6
What happens to uterus after implantation?
Endometrial cells adjacent to implanted blastocyst undergo decidual reaction
Uterine lining becomes swollen and vascular - trigger secretion of GFs, glycogen and mucus
What is the decidual reaction?
Causes uterine lining adjacent to implanted embryo to become swollen and vascular - trigger secretion of GFs, glycogen and mucus
What happens to the trophoblast layers after implantation?
Trophoblast differentiates into two layers, invade uterine wall
What are the two populations of trophoblast?
Inner layer of mononuclear cells (cytotrophoblast)
Highly invasive outer layer with no cell boundaries (syncytiotrophoblasts)
What sort of trophoblastic cells make hCG?
Syncytiotrophoblast
What does hCG do?
Maintain corpus luteum
What combine to form placenta?
Trophoblasts and uterine tissue
When is implantation complete?
Day 13
What is an ectopic pregnancy (where do most occur)?
Implant outside of uterus (mostly in uterine tube)
What do trophoblasts do after invading uterine wall (i.e. ECM)?
Break down ECM between endometrial cells
What is the cellular difference between cytotrophoblast and syncytiotrophoblast?
Cytotrophoblast is mitotic
Syncytiotrophoblast is multinucleate
Describe what happens to inner cell mass during implantation?
Differentiate into epiblast and hypoblast
What is hypoblast adjacent to?
Blastocele/blastocyst cavity
What cell shapes are the epiblast vs hypoblast?
Hypoblast (cuboidal)
Epiblast (columnar)
Where does amniotic cavity form?
Epiblast, splits it into two layers
What happens to epiblast layer after it’s split by amniotic cavity?
Epiblast layer adjacent to cyotrophoblasts will form amnion
What does amnion do?
Lines amniotic cavity and eventually envelops embryo
What compromises bilaminar germ disc?
External epiblast
Hypoblast
When does the bilaminar germ disc form?
Week 2
What do cells from hypoblast do?
Migrate to form extraembeyonic endoderm and mesoderm (not part of embryo)
What do extraembryonic endoderm and extraembryonic mesoderm do?
Migrate from hypoblast to line blastocele cavity which converts cavity to primary yolk sac then secondary yolk sac
How does the blood supply to the syncytiotrophobast originate?
Vesicles present in the syncytiotrophoblast fuse forming large lacunae
Lacunae become continuous with maternal capillaries (which when dilated become known as sinusoids), maternal blood then enters the lacunae.
As placenta develops how does cytotrophoblast develop?
Cytotrophoblast develops increasing levels of villi invaginating into the syncytiotrophoblast
Extra-embyronic mesoderm lining cytotrophoblast and amnion is?
Extra embryonic somatic mesoderm
Extra-embyronic mesoderm lining covering yolk sac is?
Extra-embyronic splanchnic mesoderm
Compare speeds of bilaminar disc comparted to trophoblast?
Growth of bilaminar disc slow compared to trophoblast
Cavities form in the extraembryonic mesoderm, they?
Develop/join form chorionic/extraembryonic cavity
What does extraembryonic cavity surround, what’s the exception?
Prim yolk sac and amniotic cavity except where germ disc connected to trophoblast by connecting stalk
Which 2 cavities form in week 2?
Amniotic and yolk sac cavities
How does umbilical cord form?
Embryo attached to its trophoblastic shell by connecting stalk, stalk develops into umbilical cord
What does gastrulation achieve?
3 germ layers established and basic body plan emerges.
When does gastrulation occur?
3rd week
How does gastrulation start?
Formation of midline structure primitive streak in epiblast
What end of bilaminar embryo disc is prim streak found?
Caudal end
What does primitive streak define?
Rostral caudal axis
Left right axis
Dorsal ventral axis
Where does primitive node develop relative to prim streak?
Rostral end of prim streak
What does primitive node do?
Neural inducing and organising properties.
Where do epiblast cells move (through) in gastrulation?
Primitive streak
What do first and second wave of epiblastic cells form?
First wave: displace hypoblast creates embryonic endoderm
2nd wave: spreads between endoderm and epiblast forms embryonic mesoderm
How is embryonic ectoderm formed from epiblast?
Epiblast cells that don’t go through streak
What structures does ectoderm form?
Skin and CNS
What does non neural ectoderm form?
Skin, tooth enamel, nails
What does mesoderm form?
Skeletal and connective tissue, blood and muscle
What are the 2 types of mesoderm?
Axial mesoderm and paraxial mesoderm
What does notochord do?
Pattern other structures
What stage of development does the notochord form in?
Gastrulation
What cells form the notochord?
Endoderm and hypoblast
How is the notochord formed?
Cells of the notochordal plate (hypoblast and endoderm) detach from the endoderm forming a solid cord of cells.
What does the notochord eventually become?
Nucleus palposus of the spine
What does paraxial mesoderm form in trunk?
37 pairs of somites
What does paraxial mesoderm form in head?
Somitomeres - contribute to head mesenchyme (i.e. muscles of face and bones of skull)
What does paraxial mesoderm do either side of notochord?
Condenses into segments
What are somites?
Paired balls of paraxial mesoderm
What do somites differentiate into?
Sclerotome
Dermamyotome (dermatome and myotome)
What part of somite is sclerotome formed from?
Ventromedial parts of somite
What do sclerotome cells do (where do they move and form)?
Move medially forming: Vertebral body (surrounding notochord)
Verebral arch (surrounds neural tube)
Annulus fibrosis
Ribs
What does dermatome contribute to?
Dermis (fat and connective tissue of neck and back)
What does myotome differentiate to?
Muscle
What does intermediate mesoderm do?
Differentiate into urogenital structures
What does lateral plate mesoderm line?
Divide to line intraembryonic coelom
What does lateral plate mesoderm form?
Somatic/parietal mesoderm
Splanchnic /visceral mesoderm
What is somatic/parietal mesoderm adjacent to?
Ectoderm
What does splanchnic/visceral mesoderm do?
Forms gut wall and vascular system
What is splanchnic mesoderm adjacent to?
Endoderm
What does somatic/parietal mesoderm form?
Dermis and limbs
Where is mesoderm absent in body (2 points) i.e. endoderm direct contact with ectoderm?
Buccopharngeal membrane (rostrally) and cloacal membrane (caudal)
What do buccopharyngeal membrane and cloacal membrane mark?
End of gut tube
Each somite forms its own…
Sclerotome, myotome and dermatome
Each myotome and dermatome retains its…
Innervation from its segment of origin regardless of where they migrate
What does endoderm form?
Gut tube, liver, lungs
How does notochord pattern neural tube?
Shh, pattern along dorsal ventral axis
What is a teratogen?
Substance that causes malformation in embryo/fetus
What is a critical period?
Period of development during which organ/system is susceptible to genetic/environmental factors
When generally is the most sensitive period for teratogenic effects?
3-8 weeks
Does a teratogen always have an effect on embryonic development, why?
No, teratogen may be administered before/after critical sensitive period
What is situs inversus?
Position of major organs is reversed
What axis of embryonic disc grows in gastrulation?
Rostral caudal axis
What direction do cells migrate to form primitive streak?
Rostrally
In what direction does primitive streak shorten/regress?
Towards caudal end
How does sclerotome fuse to form vertebrae?
The lower half of one sclerotome fuses with the upper half of the adjacent one to form each vertebral body.
Embryonic germ layer from which the notochord is derived
Epiblast
What can the notochord be classed as?
Mesoderm (axial)
Embryonic ectoderm gives rise to the…
Brain
Does the trophoblast become anything to do with the foetus?
No
