Early Foetal Development Flashcards
what are the different methods of measuring time in embryo-foetal development?
fertilisation age
gestational age
Carnegie stage
what is the fertilisation age?
(also known as conceptual age)
- Measured from time of fertilisation (assumed to be +1 day from last ovulation)
- Difficult to know time of fertilisation exactly (unless IVF)
what is the gestational age?
- Calculated from the time of the beginning of last menstrual period (LMP)
- Determined by fertilisation date (+14days) if known, or early obstetric ultrasound and comparison to embryo size charts
what is the Carnegie stage?
- 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 phases of ovarian cycle?
stages based on embroylogical development?
- germinal stage
- embryonic stage
- foetal stage
what happens during the germinal stage?
(14-16 days post-fertilization):
- establishing the early embryo from the fertilized oocyte
- Determining two populations of cells:
- pluripotent embryonic cells (contribute to fetus)
- Extraembryonic cells (contribute to the support structures eg placenta)
what happens during the embryonic stage?
(16-~50 days post fertilization):
- Establishment of the germ layers and differentiation of tissue types
- Establishment of the body plan
what happens in the foetal stage?
(~50 to 270 days post-fertilization or ~8 to ~38 weeks):
- 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 are the stages in the first few days from fertilisation to blastocyst?
- Zygote undergoes a series of mitotic divisions (cleavage divisions) to form a 2 cell embryo, 4 cell embryo and 8 cell embryo
- 8 cell embryo proceeds with further mitotic divisions à morula à blastocyst
- The zona pellucida (protein shell) that surrounds the embryo is present for all stages
when does the maternal to zygote transition occur?
- 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 during embryo compaction?
- Starts formation of first 2 cell types
- Around the 8-cell stage or later:
- Outer cells become pressed against zona
- 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
what is the composition of the blastocyst?
what is required for the blastocyst to implant?
must escape the zona pellucida via hatching (day 5-6)
this is done via:
enzymatic digestion
cellular contraction
what occurs during peri-implantation events?
(day 7-9)
trophectoderm lineage separates further
inner cell mass separates
what does the trophectoderm lineage separate into?
- trophoblast cells fuse to form syncitiotrophoblast
- Syncitiotrophoblast invasion destroys local maternal cells in the endometrium
- Creates interface between embryo and maternal blood supply (breaks down capillaries)
- cytotrophoblast cells remain individual and divide to provide source of syncitiotrophoblast cells
what do inner cell mass seperate into?
- epiblast: from which the fetal tissues will be derived.
-
hypoblast: which will form the yolk sac (extraembryonic structure)
- important in gut development and early haematopoiesis
what is the final stage before gastrulation?
bilaminar embryonic disc formation
what happens during bi-laminar embryonic disc formation?
day 12+
- Some epiblast cells become separated from the epiblast by the formation of a new cavity – the amniotic cavity.
- The epiblast cells on the top create a structure called the Amnion
- These amnion cells will contribute to the extra-embryonic membranes.
- This leaves a two-layer disc of epiblast and hypoblast, sandwiched between cavities. (bilaminar embryonic disc)
- This epiblast will give rise to the fetal structures and organs
- Embryo is now ready for gastrulation
what are the cell lineages from the initial morula?
what occurs during gastrulation?
- 13+ days after fertilisation:
- Thickened structure form along midline in the epiblast= primitive streak
- Near caudal end of bilaminar embryonic disc
- The primitive streak divides the major body axis of the embryo including
- the primitive streak it expands to form the primitive node which contains a circular depression= primitive pit
- The depression continues along the midline of the epiblast towards the caudal end of the streak= primitive groove
- Cells of the epiblast migrate inwards towards the streak
- Detach from the epiblast and slip beneath into the interior of the embryo = invagination
- The first cells to invade through the primitive groove invade the hypoblast and displace its cells
- The hypoblast cells are replaced by the new proximal cell layer known as definitive endoderm
- The remaining cells of the epiblast are known as the ectoderm
- Forms the most exterior distal layer
- Some of the invaginated epiblast cells remain in the space between ectoderm and definitive endoderm
- These cells form a germ layer= mesoderm
- When the formation of definitive endoderm and mesoderm is complete, the epiblast cells no longer migrate towards primitive streak
- Through gastrulation the ectoderm continues to form from the cranial to caudal end of embryo establishing 3 distinct primary germ layers
what organs form in the different germ layers?
when does the notochord form?
- day 14
- Notochord is a rod-like tube structure formed of cartilage-like cells
- Forms along the embryo midline, under the ectoderm
how does the neural groove form?
- 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
what happens once the neural groove is formed?
- Neural folds move together over neural groove
- Ultimately neural folds fuse, forming a hollow tube (neural tube)
- Neural tube overlaid with epidermis (ectoderm)
- Migration of the neural crest cells from folds
-
Neural tube initially open at each end
- Closure at head end: ~ day23
- Closure at tail end: ~ day27
- Closure at head end precedes formation of brain structures
how do somites form?
- blocks of paraxial mesoderm condense and bud off in somite pairs
- one of each pair on either side of the neural tube
- Somitogenesis commences 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 in embryo total
what tissues is derived from somites?
sclerotome: vertebrae and rib cartilage
dermomyotome which subdivides to form:
- dermatome: gives rise to dermis of skin, some fat and connective tissue of neck and trunk
- myotome: forms the muscles of the embryo
how is the primitive gut formed?
- 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
