Early fetal developement Flashcards
How is time (age) measured in fetal development
Fertilisation age
Gestational age
Carnegie age
Describe and explain fertilisation age in fetal development
(also known as conceptual age):
- measured from the time of fertilisation (assumed to be +1 day from last ovulation)
- difficult to know time of fertilisation exactly (unless IVF)
Describe and explain Gestational age in fetal development; i.e. how is it calculated
calculated from the time of the beginning of the last menstrual period (LMP)
• determined by fertilisation date (+14 days) if known, or early obstetric ultrasound and comparison to embryo size chart
Describe and explain Carnegie age in fetal development; i.e. how is it calculated
- 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 fertilisation age in humans
what are the stages of embryo-fetal development.
Explain them in detail
- Embryogenic Stage (14-16 days post-fertilisation):- 1st trimester
- establishing the early embryo from the fertilised oocyte
- determining two populations of cells:
- pluripotent embryonic cells (contribute to fetus)
- extra-embryonic cells (contribute to the support structures eg placenta)
- Embryonic stage (16-~50 days post fertilisation)- 1st trimester
- establishment of the germ layers and differentiation of tissue types
- establishment of the body plan
- Fetal stage (~50 to 270 days post-fertilisation or ~8 to ~38 weeks):- 2nd and 3rd trimester
- major organ systems now present
- migration of some organ systems to final location
- extensive growth and acquisition of fetal viability (survival outside the womb)
What establishes early embryo from fertilised oocyte?
two populations of cells:
pluripotent embryonic and extra-embryonic cells
Which stages of development correspond to the trimesters of pregnancy?
First trimester = embryogenic and embryonic stages
Second & third trimester = fetal stage
the transition from embryo to fetus occurs around end of 1st trimester
Describe what happens to the egg in the first few days of life. Draw it
- start with ovulated oocyte (1 cell)
- fertilised by sperm
- becomes a zygote (1 cell)
- zygote undergoes mitotic cleavage division (2-8 cells)
- these are cleavage stage embryos
- 8 cell embryo divides again to form the morula (16+ cells)
- morula develops into a blastocyst (200-300 cells)
> developmental trajectory occurs as the egg migrates along the fallopian tube into the uterus
> zona pelucida is present for all of these stages!
What is the first major developmental event in the embryo?
Maternal to zygotic transition - 4-8 cell stage
- Until 4-8 cell stage, the genes of the embryo are not transcribed
- Embryo is dependent on maternal mRNAs and proteins to get through the first divisions
- These mRNA and proteins are synthesized and stored during oocyte development (i.e. pre-ovulation)
- Failure to synthesise, store or interpret these mRNAs and proteins during oogenesis can impair embryonic development
What occurs at the maternal to zygotic transition?
4-8 cell stage
- Transcription of embryonic genes (zygotic genome activation)
- Increased protein synthesis
- Organelle (mitochondria, Golgi) maturation
What is the second major developmental event in the embryogenic (stage)?
Explain it in detail
Compaction - Around the 8-cell stage or later
- starts the formation of the first two cell types
- outer cells become pressed against zona pellucida
- change from spherical to wedge-shaped
- outer cells connect to each other through tight gap junctions and desmosomes
- forms barrier to diffusion between inner and outer embryo
- outer cells become polarised (apical and basolateral membranes)
What structures are formed by the process of compaction?
Explain their features
Compacted morula
- 2 distinct cell populations:
- Inner (pink in diagram)
- Outer (green in diagram)
Blastocyst
- Inner (pink) cells and outer (green) reorganise
- formation of the blastocoel cavity
Describe the structures of the blastocyst and their functions
Zona Pellucida:
- Hard protein shell inhibiting polyspermy and protects early embryo
Inner cell mass
- Pluripotent embryonic cells that will contribute to the final organism
Trophoectoderm
- Extra-embryonic cells that contribute to the extraembryonic structures that support development
Blastocoel:
- Fluid-filled cavity formed osmotically by trophoblast (outer cell) pumping Na+ ions into cavity, and water follows
What must occur to allow implantation of the blastocyst?
Hatching (day 5 - 6)
In order to implant the blastocyst must escape zona pellucida by:
> Enzymatic digestion
> Cellular contractions
This weakens a point of the zona allowing blastocyst to extrude itself
Describe the first separation of embryonic cell lineages (blastocyst stage)
Morula divides into
- inner cell mass = embryonic tissues
- trophectoderm = extra-embryonic tissues like placenta
What occurs to the blastocyst in peri-implantation (explain all the proceses involved)
~ day 7-9
Trophectoderm lineage separates further:
- Trophoblast cells fuse to form syncytiotrophoblast
- Syncytiotrophoblast invasion destroys local maternal cells in the endometrium
- Creates interface between embryo and maternal blood supply (breaks down capillaries and hence bathe in maternal blood)
- Cytotrophoblast cells remain individual to provide source of syncitiotrophoblast cells
Inner cell mass separates further into:
- Epiblast: from which the fetal tissues will be derived.
- Hypoblast: which will form the yolk sac (extraembryonic structure)
What is the yolk sac derived from?
hypoblast
What is the function of the yolk sac?
important in gut development and early haematopoesis
Outline the second separation of embryonic cell lineages (peri-implantation stage)
Inner cells → epiblast + hypoblast
Trophoblast → syncitiothrophobast + cytotrophoblast
Describe and explain the final embryogenic stage before gastrulation.
DRAW what happens in this stage
what secretes hCG?
Bi-laminar embryonic disc formation (day 12+)
- syncitiotrophoblast continues to expand into the endometrium.
- some cells become separated from the epiblast by the formation of a new cavity - the amniotic cavity
- these amnion cells will contribute to the extra-embryonic membranes.
- this leaves a two-layer disc of epiblast and hypoblast, sandwiched between cavities
- embryo is now ready for gastrulation.
Syncitiotrophoblast secretes hCG
- Detection of beta hCG subunit in blood/urine is basis of pregnancy testing
Describe the third separation of embryonic cell lineages (bi-laminar disc)
epiblast gives rise to amnion and epiblast (that gives rise to fetus
Summarise the process of gastrulation
1. Formation of primitive grove
- thickened structure forms along midline in epiblast at the caudal end
- this is the primitive streak
- primitive streak expands to create primitive node
- → primitive pit
- • continues to caudal end to form primitive groove
2. Invagination
• cells of epiblast migrate inwards towards streak
• detach from the epiblast
• move to the interior of the embryo
• cells invade and displace the hypoblast
3. Formation of germ layers
- hypoblast replaced with cells forming the definitive endoderm
- remaining cells (ventral) of the epiblast are the ectoderm
- some invaginated cells remain in ectoderm and endoderm = mesoderm
- ectoderm continues to form from cranial to caudal end
What defines the major body axes of the embryo?
primitive streak
it divides the fetal body into:
- cranial and caudal ends
- right from left
outline the 4th stage separation of embryonic cell lineages
what is the function of the notochord
Acts as a key organizing centre for neurulation and mesoderm development.
> releases growth factor signals for CNS formation
> particularly for development of musculature
Neurulation occurs after gastrulation.
Describe this process
day 13
Notochord helps to form the neural plate (thickened ectoderm)
- Notochord signals direct the neural plate ectoderm to invaginate forming neural groove
- Creates two ridges (neural folds) running along the cranio-caudal axis
- Neural crest cells specified in neural folds, they move upwards
- Neural folds move together over neural groove
- Ultimately neural folds fuse, forming a hollow tube
- Neural tube overlaid with epidermis (ectoderm)
- Migration of the neural crest cells from folds to differentiate as it migrates
label this diagram
green- notochord
large Purple ring - neural tube
After neurulation, what is the next step in embryo development.
Explain this process
Somitogenesis: segmentation of the body axis
Blocks of paraxial mesoderm condense and bud off in somite pairs
One of each pair either side of the neural tube and notochord.
Starts at the head end and progresses down the long axis of the embryo
Rate of ‘budding’ or appearance of somite pairs is species-specific, as is the number of pairs.
- Humans 1 pair/90 min, 44 pairs
How many pairs of somites are formed between week 4 and 5
Descirbe any differences in visibility of somites
Week 4- (3-4 pairs of somites)
Week 5 (30 pairs)
somites pairs are more visible at the head end than the tail end; it starts forming from the head
What tissues do somites initially form? give their features
Mesodermal tissues specifically:
Sclerotome: vertebrae and rib cartilage
and the
Dermomyotome, which in turn sub-divides to form:
- Dermatome: gives rise to dermis of the skin, some fat and connective tissues of neck and trunk
- Myotome: forms the muscles of the embryo
Describe the formation of the gut tube formation.
when does its start
after Day 16.
The primitive gut arises from two types of folding in the embryo:
- Ventral folding: where the head and tail ends curl together
- Lateral folding: where the two sides of the embryo roll
This pinches off part of the yolk sac to form the primitive gut
Primitive gut is then patterned into foregut, midgut and hindgut
