early foetal development Flashcards
what is fertilisation age (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)
what is gestational age
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 charts
menstrual period = 28 days long, ovulation occurs halfway through this
what is 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 happens in the embryogenic stage (14-16 days post-fertilisation)
establishing the early embryo from the fertilised oocyte
determining two populations of cells : pluripotent embryonic cells (contribute to fetus) and extraembryonic cells (contribute to the support structures eg placenta)
what happens in the embryonic stage (16-~50 days post fertilisation)
establishment of the germ layers and differentiation of tissue types
establishment of the body plan
what happens in the fetal stage (~50 to 270 days post fertilisation or~8 to ~38 weeks)
major organ systems are now present
migration of some organ systems to final location
extensive growth and acquisition of fetal viability (survival outside the womb)
how are the 3 trimesters organised and what happens at the end of the first trimester
the 1 and 2 make up the first trimester and the 3 is part of the second and third trimester
the transition from embryo to fetus occurs roughly at the end of the first trimester
summary of STEPS from oocyte to blastocyst
1) start off with an ovulated oocyte(single cell) > fertilisation where it meets sperm and
becomes zygote
2) zygote undergoes mitotic divisions (cleavage divisions)
3) cleavage stage embryos (2-8 cells)
4) 8th embryo proceeds with further mitotic divisions > giving a structure called the
morula at the 16 cell stage
5) morula progresses and forms blastocyst (200-300 cells)
6) development happens as oocyte and embryo migrates along fallopian tube
7) zona pellucida (protein shell surrounding oocyte at ovulation) is present in all stages -
all divisions are occurring within zona pellucida
until what stage are the genes of the embryo transcribed
4-8 cell stage
what is the embryo dependent on to get through the first few divisions
embryo is dependent on maternal mRNA and proteins to get through the first divisions
these mRNAs and proteins are synthesised and stored during oocyte development (pre-ovulation)
failure to synthesise, store or interpret these mRNAs can proteins during oogenesis can impair embryonic development
what happens in the maternal to zygotic transition
transcription of embryonic genes (zygotic genome activation)
increased protein synthesis
organelle (mitochondria, Golgi) maturation - metabolism and protein synthesis
what is compaction
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 zona pellucida
hard protein shell inhibiting polyspermy and protects early embryo
what is the inner cell mass
pluripotent embryonic cells that will contribute to the final organism
what is trophectoderm
extra embryonic cells that contribute to the extra embryonic structures that support development eg placenta
what is the blastocoel
fluid filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity (centre)
once embryo reaches this stage > developmental potential becomes limited as it is still retained within the zona
what happens during hatching (day 5-6)
to implant, the blastocyst must escape the zona pellucida
enzymatic digestion
cellular contractions
weaken points of zona pellucida
what happens during peri-implantation events (day 7-9)
embryo has undergone initial connection with endometrium
trophectoderm lineage separates further into syncytiotrophoblast and cytotrophoblast
trophoblast cells fuse to form syncytiotrophoblast
syncytiotrophoblast invasion destroys local maternal cells in the endometrium and destroys capillaries
creates interface between embryo and maternal blood supply
cytotrophoblast cells remain individual to provide source of syncytiotrophoblast cells