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.
Define these terms:
- zygote
- cleavage
- zona pellucida
Zygote: fertilised ovum
Cleavage: the zygote after mitotically diving
Zona pellucida: sperm’s glycoprotein shell that protects the blastomere from other sperm
Define these terms:
- morula
- ovary
- fallopian tube
- uterus
Morula: ball of cells formed after the blastomeres compact
Ovary: part of the female’s reproductive organs where the ova is released
Fallopian tube: tube that connects the ovaries to the upper part of the uterus. Egg usually fertilised in the ampulla.
Uterus: Reproductive organ in the lower part of the female pelvis where a conceptus can implant and develop into a fetus.
Discuss the blastocyst formation and the initial stages of implantation.
Blastocyst = morula + blastocoele
Inner mass becomes embryoblast.
Outer mass becomes trophoblast.
Blastocyst hatches out of zona pellucida to expand and implant in the uterine epithelium.
Define these terms:
- blastocyst
- trophoblast
- embryoblast
Blastocyst: structure after morula has developed a blastocoele
Trophoblast: outer cell mass flattened to form blastocyst wall
Embryoblast: inner mass of cells
Define these terms:
- Implantation
- Cytotrophoblast
- Syncytiotrophoblast
Implantation: syncytiotrophoblast adheres + invades uterine epithelium
Cytotrophoblast: inner layer of cells connected to the yolk sac.
Syncytiotrophoblast: outer layer of cells that becomes support tissue.
Describe the embryonic/bilaminar disc formation and the initial division within it.
Bilaminar disc made from the embryoblast differentiating into the epi/hypoblast. The epiblast cells form the amniotic cavity whereas the hypoblast forms the primitive yolk sac.
Define these terms:
- bilaminar disc
- Germ layer
- Ectoderm
- Mesoderm
- Endoderm
Bilaminar disc: differentiation of the embryoblast and now consists of the epi/hypoblast
Germ layer: made of 3 layers and make up the trilaminar disc.
Ectoderm: apical layer of the trilaminar disc
Mesoderm: middle layer of the trilaminar disc
Endoderm: basal layer of the trilaminar disc
Define these terms:
- primitive streak
- Gastrulation
- Trilaminar embryonic disc
Primitive streak: It is a divot in the epiblast disc formed at start of gastrulation on epiblast surface.
Gastrulation: beginning of week 3 where bilaminar disc becomes trilaminar disc due to the epiblast differentiating.
Trilaminar embryonic disc: epiblast differentiation into 3 layer disc (ectoderm, mesoderm, endoderm) that displaces the hypoblast.
List the ectodermal derivatives
CNS
Skin
Organs
List the mesodermal derivatives
CVS
Muscles
Bone
Cartilage
List the endodermal derivatives
Internal structures
Epithelial linings
Gland parenchyma
