Lecture 3: Early Embryology pt.1 Flashcards
What is embryology?
Study of molecular, cellular & structural factors involved in progression from a single cell (pre-embryonic stage) to a baby (40 weeks)
What happens during fertilisation?
Oocyte released from ovary and travels along fallopian tube.
Fertilisation by the sperm occurs in the ampulla and produces a zygote.
This zygote eventually divides into a blastocyst.
Implants itself into the upper posterior uterine wall.
How long are sperm and oocytes viable for?
Sperm = 5 days Oocyte = 1 day
Why is ovulation typically described as 7 days?
Sperm is viable for 3 days whereas oocytes as only viable for 1 day. Thus, there are 3 days before & after the 1 day (the oocyte is viable) for a more likely chance of successful fertilisation.
3-1-3 = 7 days
Order these in formation order… morula, blastocyst, blastomeres, zygote.
- Zygote
- Blastomeres
- Morula
- Blastocyst
What is the purpose of the zona pellucida?
Sperm’s glycoprotein acts as a protective shell for the blastomere against other sperm. Prevents more than 2 sets of DNA.
Define the following terms:
- pre-embryonic/germinal period
- embryonic period
- fetal period
Pre-embryonic/germinal period: fertilisation - end of week 2.
Embryonic period: 3rd week to end of 8th week.
Fetal period: 9th week till birth at 38 weeks (40 including conception)
Describe what happens in the 1st week of human development.
- 30hrs after fertilisation
- day 2
- day 3
- 30hrs after fertilisation: mitotic cleavage into 2 blastomeres
- day 2: sperm’s zona pellucida protects blastomeres from other sperm
- day 3: blastomeres divide into morula
Describe what happens in the 1st week of human development.
- day 4: compaction into blastocyst (1st cavity)
- day 5 hatching so blastocyst can enlarge and implant
- day 6/7: conceptus implantation begins
Describe the 2nd week of human development.
- day 8
- day 9/10
- day 11
- Day 8: morula differentiates- embryoblast (epiblast + hypoblast) & trophoblast (cytotrophoblast + syncytiotrophoblast)
- Day 9/10: conceptus implants in uterine lining & establish maternal blood flow
- day 11: yolk sac moves from cytotrophoblast
Describe the 2nd week of human development.
- day 12
- day 13
- day 14
- day 12: syncytio. makes contact with endometrium capillaries so bathed in maternal blood for O2, glucose.
- day 13: 2° yolk sac formed by pinching off from 1° yolk sac
- day 14: extraembryonic mesoderm spaces merge into a chorionic/embryonic cavity
Describe the events of the 3rd week of human development. Try to include the following:
- primitive streak
- gastrulation
- Epiblast differentiation
- flat-disked epiblast develops primitive streak: signalling gastrulation
- cells invaginate in the primitive streak so the differentiate + displace hypoblast
- Epiblast differentiates ectoderm, mesoderm, endoderm
Describe the 4th week of human development.
- Gastrulation created notochord from mesoderm’s middle clusters of cells
- Neuroectoderm forms a thickened neural plate which keeps thickening so curls into neural tube.
- Embryonic folding occurs where you have cephalocaudal folding (head then tail fold) then lateral folding
What is important about gastrulation?
Establishes L+R side of the body.
If the ciliated cells become immobile then they will not beat the primitive node’s signals to the left. This is called side specific signalling.
This would cause situs invertus where the body’s internal structure is mirrored (eg: heart more towards RHS)
Describe the notochord and its purpose.
Solid rod of mesoderm cells that define the midline.
They convert the ectoderm into neuroectoderm for neurulation which leads to the CNS formation.
What are the 5 areas of the mesoderm?
Somatic mesoderm paraxial mesoderm Intermediate mesoderm Splanchnic mesoderm Intraembryonic coelom
What does the ectoderm eventually develop into?
Organs, epidermis, nervous system
What does the mesoderm eventually develop into?
CVS Muscles Bone Supporting tissue cartilage
What does the endoderm eventually develop into?
Epithelial lining of GI, respiratory tract
Consequences of embryonic folding.
- Ensures embryo fully within amniotic cavity
- Ectoderm on outside and endoderm on the inside
- Pulls connecting stalk around to the abdomen side
Define the pre-embryonic period.
first 2 weeks after fertilisation
Define the embryonic period.
3rd -8th week after fertilisation
Define the fetal period.
9th-38t/40th week after fertilisation
Discuss the changes taking place as a zygote travels down the fallopian tube.
After fertilisation on the ampulla, zygote moves via cilia.
As the zygote moves it cleaves into 2 blastomeres.
Blastomeres mitotically divide into a morula.
Compaction separates the inner mass into an embryo/trophoblast and makes a blastocoele.
