Early fetal development Flashcards
1
Q
What is fertilisation age/conceptual age?
A
- measured from time of fertilisation (assumed to be 1 day after last ovulation, as generally fertilisation has to occur w/in 24h of ovulation)
- weeks or days post-fertilisation
- difficult to know time of fertilisation exactly unless IVF
2
Q
What is gestational age?
A
- calculated from the time of the beginning of the last menstrual period
- determined by early obstetric ultrasound comparing to embryo size charts
- always 14 days longer than fertilisation age bc it goes from start of menstrual period rather than point of ovulation or fertilisation
3
Q
What is the Carnegie staging system?
A
- 23 stages of embryo development based on embryo FEATURES, not time
- allows comparison of developmental rates between species
- covers window of 0-60 days post-fertilisation in humans
4
Q
What is the embryogenic stage?
A
- runs from point of fertilisation to about 14/16 days post fertilisation
- early embryo established from fertilised oocyte
- characterised by formation of 2 cell types: pluripotent embryonic cells- contribute to foetus, and extra embryonic cells- contribute to support structures e.g. placenta
5
Q
What is the embryonic stage?
A
- runs from about 16 days post-fertilisation to about 50 days post fertilisation
- establishment of germ layers and differentiation of tissue types
- establishment of body plan
6
Q
What is the foetal stage?
A
- runs from about 50 days to 270 days post fertilisation (~8-~38 weeks)
- major organ systems now present, although some may be in different place
- migration of some organ systems to final location
- extensive growth and acquisition of fetal viability (ability to survive outside womb)
7
Q
What stages correspond to the first trimester?
A
embryogenic and embryonic stages
8
Q
What stage corresponds to the second and third trimesters?
A
foetal stage
so transition from embryo–> foetus occurs roughly at end of 1st trimester
9
Q
What happens in the first few days of life?
A
- ovulated oocyte, single cell
- undergoes fertilisation–> becomes zygote
- undergoes mitotic divisions–> cleavage stage embryos 2-4-8 cell embryo
- further divisions–> morula (16+ cells)
- progresses to form blastocyst (200-300 cells)
- all happens as it migrates along Fallopian tube into uterus
N.B. zona pellucida present for all these stages^
10
Q
What is the maternal-to-zygotic transition?
A
- occurs at 4-8 cell stage
- before this, none of the genes in the embryo are transcribed and the development/division is dependent on maternal mRNAs and proteins
- these mRNAs and proteins are synthesised and stored during oocyte development (before ovulation)
- at transition, embryonic genes get transcribed, increased protein synthesis, and maturation of organelles e.g. mitochondria and Golgi
11
Q
What is compaction?
A
- occurs around 8-cell stage or later
- gives first 2 cell types (inner and outer)
- outer cells become pressed against zone pellucida–> change from spherical to wedge-shaped
- tight junctions and desmosomes form between these outer cells, connecting them and forming a barrier to diffusion between inner and outer embryo
- outer cells also become polarised (apical and basal)
12
Q
What is the role of the zona pellucida?
A
- protect early embryo
- prevent polyspermy
13
Q
What is the structure of the blastocyst?
A
- contained w/in zona pellucida
- inner cell mass (from inner cells): gives rise to pluripotent embryonic cells that will contribute to final organism
- trophectoderm (from outer cells): gives rise to extra-embryonic cells that contribute to extra embryonic structures that support development e.g. placenta
- blastocoel: fluid-filled cavity formed osmotically by trophoblasts pumping Na+ ions into centre of embryo (water then follows)
14
Q
What is hatching?
A
- around day 5-6
- blastocyst secretes enzymes and cellular contractions weaken a point in the shell
- -> blastocyst escapes from the zona pellucida shell
- blastocyst can now implant in endometrium
15
Q
What differentiation events occur when the embryo has undergone its initial connection with the endometrium (peri-implantation events ~day 7-9)?
A
- trophectoderm lineage separates into syncitiotrophoblasts and cytotrophoblasts
- sycitiotrophoblast is invasive–> invades endometrium and degrades local maternal cells, breaking down capillaries–> so syncitiotrophoblasts are bathed in maternal blood
- cytrotrophoblast cells continue to divide to provide more syncitiotrophoblast cells - inner cell mass separates into epiblasts and hypoblasts
- epiblasts will form foetal tissues
- hypoblasts will form yolk sac (extraembryonic structure)
16
Q
When does bi-laminar embryonic disc formation occur?
A
around day 12 post fertilisation
17
Q
What occurs during bi-laminar embryonic disc formation?
A
- some epiblast cells separate from main block, forming the amniotic cavity–> epiblast cells along the top give rise to the amnion (an extra-embryonic membrane)–> whereas the epiblasts below the amniotic cavity give rise to foetal structures/organs
- hypoblast remains sitting under the epiblast (the one below amniotic cavity)–> 2-layer disc of epiblast+hypoblast, in the middle between cavities - BI-LAMINAR EMBRYONIC DISC
- now ready for gastrulation
18
Q
What cells secrete hCG?
A
syncitiotrophoblasts secrete hCG (detection of beta hCG subunit in blood/urine = pregnancy test)
