Early Fetal Development Flashcards
What are the 3 ways to measure embryo-fetal development time?
Embryo-fetal developmental progression can be measured in different ways:
- Fertilization age (conceptual age) = measured from the time of fertilisation (assumed to be +1 day from last ovulation)
- Gestational age: calculated from start of last period (day 0), and ovulation happens 14 days after this
- Carnegie stage: based on presence or absence of certain embryo features
Why is fertilisation age not very practical?
Usually a period of time has elapsed after fertilisation so days/ weeks post fertilisation, this is v difficult to know in practice
Variaiblity in time between intercourse and fertilisation in natural conception
Can be inferred if we know time of ovulation
Only can know exact time if using IVF
What is gestational age?
Calculated from the time of the beginning of the last menstrual period (LMP)
Determined by fertilization date (+14 days) if known, or early obstetric ultrasound and comparison to embryo size charts
What is carnegie stage?
Based on embryo features rather than time
23 stages of embryo development based on embryo features not time
Allows comparison of developmental rates between species
Covers the window of 0-60 days fertilization age in humans
What are the 3 stages of pregnancy?
Embryogenic stage
Embryonic stage
Fetal stage
What is the Embryogenic Stage?
14-16 days post-fertilization - part of first trimester
Form early embryo from fertilised oocyte and formation of 2 cell types:
Pluripotent embryonic cells (contribute to organs of fetus)
Extraembryonic cells (contribute to the support structures e.g. placenta)
What is the Embryonic Stage?
16-50 days post fertilisation - part of first trimester
Establishment of germ layers, differentiation of tissue types, and body plan - major organ systems are specified
What is the Fetal Stage?
50-270 days post-fertilisation (8-38 weeks) - so transition from embryo to fetus occurs roughly at end of first trimester, and fetal stage makes up the 2nd and 3rd trimesters
Major organ systems now present but these are now migrating to their final location
Extensive growth and acquisition of fetal viability - ability of foetus to survive outside the womb
What is the developmental trajectory for the first few days of life?
Ovulated oocyte after fertilisation = zygote
Zygote undergoes mitotic division (cleavage) = 2 cell embryo to 4 cell embryo to 8 cell embryo (these are cleavage stage embryos)
8 cell embryo proceeds with further mitotic divisions to form a morula (16 cell)
Morula forms an blastocyte (around day 5)
This developmental trajectory is happening as the oocyte/ early embryo is travelling along the fallopian tube and into the uterus to be implanted
The zona pellucida (hard, protein shell) that surrounds the oocyte at ovulation is present at all these stages so all these mitotic divisions are happening within the constriction of the ZP
What is the first major developmental event?
Maternal-to-Zygotic transition
What is the Maternal-to-Zygotic transition?
Occurs at the 4-8 cell stage
We start to get transcription of embryonic genes and lose the reliance on maternal mRNA and proteins - i.e. embryotic genes take over and begin transcription
Embryo increases protein synthesis and organelle (mitochondria, Golgi) maturation
How does the cell divide prior to Maternal-to-Zygotic transition?
No genes are transcribes
Divisions dependent on maternal mRNA and proteins
What is the second major developmental event?
Compaction
What happens during Compaction?
Early events (compaction and blastocyst formation) separate the embryo into embryonic (inner cell mass) and extra-embryonic cells (trophectoderm)
Compaction occurs at the 8 cell stage
Gives us our first 2 cell lineages (2 distinct cell types):
- Outer cells: tightly bound wedge-shaped outer cells (green)
As outer cells become pressed against zona they change from spherical to wedge-shaped.
- Inner cells: shielded from outside (pink)
Outer cells become polarised and connect to each other through tight gap junctions and desmosomes = to form barrier to diffusion between cells inside the embryo and the outside of the embryo
What is the third major developmental stage?
Blastocyte formation stage - establishes the 2 cell types
How do the 2 types of cells formed in the Compaction stage develop further in the Blastocyte formation stage?
These 2 cells types develop further:
Outer cells form outer shell of blastocysts
Inner cells clumping together at one end and there is formation of the blastocoel cavity
Once this happens the morula is a blastocyst
What is the zona pellucida and why is it important?
Hard protein shell inhibiting polyspermy (multiple sperm fertilisation) and protecting the early embryo
What is the purpose of the inner cell mass at the blastocyte formation stage?
Gives rise to the pluripotent embryonic cells that will contribute to the final organism
What is the purpose of the outer cell mass at the blastocyte formation stage?
AKA the Trophoectoderm/ trophoblast:
Give rise to extra-embryonic cells that contribute to the extraembryonic structures that support development e.g. placenta
What is the formation of the Blastocoel at the blastocyte formation stage?
Fluid-filled cavity formed osmotically
Trophoblasts pump Na+ ions into centre of the embryo, and water follows this osmotically to form large fluid-filled space in middle
What is hatching and why is it important?
Escape of the blastocyst from the zona pellucida shell - around day 5-6
Essential for further development and implantation (into endometrium)
How does hatching occur?
Blastocyst secretes enzymes = enzymatic digestion
And cellular contractions of the embryo
Both contribute to weaken zona pellucida
Once the embryo has implanted into the endometrium, it undergoes peri-implantation events.
What are the peri-implantation events?
Further differentiation of the embryo - 7-9 days post fertilisation
- Trophectoderm lineage separates
- Inner cell mass separates
What does the trophectoderm lineage separate to?
Trophectoderm lineage separates into the syncitiotrophoblast and the cytotrophoblast
The trophoblast cells fuse to form syncitiotrophoblast, which is invasive - invades uterine endometrium and starts to degrade endometrium cells to breakdown capillaries = allows syncitiotrophoblasts to bathe in maternal blood
This creates interface between embryo and maternal blood supply
Cytotrophoblast cells remain individual cells and contrinue to divide (proliferate) to add cells to the syncitiotrophoblast cells
What does the inner cell mass separate to?
Epiblast and hypoblast
Epiblast = from which the fetal tissues and organs will be derived
Hypoblast = which will form the yolk sac (extraembryonic structure important in gut development + early haematopoesis), this lines the under-side of the epiblast and faces the cavity
What is the final developmental stage?
Occurs around day 12
Formation of the bilaminar (two-layer) embryonic disc is the final stage before gastrulation
At this stage:
Syncitiotrophoblast continues to expand into the endometrium to firmly implant embryo in endometrium and begin secreting hCG
Some of the epiblast cells become separated from the main block of epiblasts by the formation of a new cavity – the amniotic cavity.
The epiblast cells at the top of the amniotic cavity give rise to the amnion and these amnion cells will contribute to the extra-embryonic membranes
The epiblast cells below the amniotic cavity will be the ones that give rise to the fetal tissue structures and organs
There is a layer of hypoblast below the epiblasts which faces the blastocoel, together they form a bilaminar embryonic disc
At this stage it is ready for gastrulation