Early fetal development Flashcards
how can time spent in embryo-fetal development be measured
3 different methods:
fertilization age
gestational age
carnegie stage
what is fertilisation age
how is it measured
the number of days/weeks that have passed since fertilization occured
it is difficult to know the exact date on fertilisation when conceiving using natural methods (i.e. not IVF)
therefore the date of fertilisation is inferred as being 1 day after ovulation occured (as fertilisation must occur within 24 hrs of ovulation, ovulation date will be 14 days after the start of the period menses bleeding)
(not very useful measure practically)
what is gestational age
how is it measured
number of days/weeks that have passed since the start of the last menstrual period
can be determined in 3 ways:
if woman knows date of last period (when bleeding first started) - will be number of days since then
or can minus 14 days from the fertilization date
or can do early obstetric ultrasound and compare embryo to size charts
what is the difference between gestational and fertilisation age
both measure time spent in embryo-fetal development
fertilisation age is always 14 days longer than gestational age
fertilisation age - time elapsed since fertilisation occurred
gestational age - time elapsed since start of last menstrual period
what is carnegie stage
how is it measured
made up of 23 stages of embryonic development
measures embryo development based on features it has not time spent in utero (i.e. structures or features that are present or absent)
allows direct comparison of developmental rates and events between species
covers time period from fertilisation date to approx 60 days post fertilisation
how is carnegie stage different from gestational age and fertilisation age
fertilisation and gestational age measure embryonic-fetal development based on time
carnegie stage measures embryonic-fetal development based on embryo features not time
also only covers time period from fertilisation date - 60 days post fertilisation
how can pregnancy be divided according to embryological development
embryogenic stage: 14-16 days post-fertilization
first trimester
embryonic stage: 16-50 days post fertilization
first trimester
fetal stage: 50-270 days post fertilization (approx. 8-38 weeks)
second and third trimester
(each trimester is 12 weeks)
what occurs in the embryogenic stage of fetal development
time period between fertilisation date to 14/16 days post fertilisation
early embryo being formed from fertilised oocyte
characterised by formation of 2 distinct cell types:
pluripotent embryonic cells - contribute to fetus
extraembryonic cells - contribute to support structures e.g. placenta
(1st trimester)
what occurs in the embryonic stage of development
time period between 16-50 days post fertilization
establishment of germ layers
differentiation of tissue types
establishment of body plan
(still 1st trimester)
what occurs in the fetal stage of development
50-270 days post fertilisation (approx 8-38 weeks)
major organ systems are now present
migration of some organ systems to their final location
extensive growth + acquisition of fetal viability - ability of fetus to survive outside womb
(2nd and 3rd trimesters)
when does a fetus become an embryo
roughly at the end of 1st trimester
(transition from embryonic to fetal phase)
describe the first few days of life
ovulation - oocyte released from ovary into fallopian tube
oocyte fertilised by sperm → zygote (made up of 1 cell)
zygote undergoes mitotic divisions → cleavage stage embryos (made up of 2 then 4 then 8 cells)
cleavage stage embryo undergo mitotic divisions → morula (made up of over 16 cells)
morula undergoes mitotic divisions → blastocyst (made up of over 200 cells)
all occurring whilst oocyte then early embryo is migrating along fallopian tube to uterus
zona pellucida remains intact throughout whole process → mitotic divisions are occurring within restrictions of zona pellucida
label the stages of cell development in the first few days of life
note: intact zona pellucida
what is the maternal to zygotic transition
when does it occur
what are its main features
it is when development control is transferred from mother to embryo
before the transition, no embryonic genes are transcribed, maternal mRNA and maternal proteins synthesised + stored during oocyte development (pre-ovulation) are responsible for allowing zygote to divide to form embryo (i.e. to go from 1 → 2 → 4 cells)
after the transition occurs the zygote’s genome is activated → its own genes are transcribed, therefore reliance on maternal mRNAs + proteins are lost, embryo undergoes increased protein synthesis + maturation of organelles - especially mitochondria and golgi (as they are involved in metabolism and protein synthesis + distribution)
occurs when the embryo is mitotically dividing from 4 to 8 cells (cleavage stage embryos period)
what is the importance of maternal mRNAs and protein development before ovulation
very important
because embryo is reliant on the maternal mRNA and proteins to undergo its first few mitotic divisions (as none of its own genes are transcribed until the maternal to zygotic transition at the 4 to 8 cell stage)
so if these maternal mRNAs or proteins are not synthesised, stored or interpreted correctly during oogenesis → can lead to impaired embryonic developement
when would the maternal to zygotic transition occur
what are the features associated with this
4 to 8 cell stage
what is compaction
what are its main features
when does it occur
process which starts the formation of 2 distinct cell lineages
occurs around the 8 cell stage (then divides to form morula)
as more cells form due to mitosis the outer cells of embryo are pressed against the zona pellucida → change in shape from spherical to wedge-shaped
these outer cells then develop connections with each other through tight gap junctions + desmosomes → this means that there is a diffusion barrier between the inner and outer cells of embryo
outer cells also become polarised (apical + basal polarity)
2 distinct cell populations in embryo - inner and outer
how do the cell populations formed in compaction contribute to further embryonic development
morula → blastocyst
following compaction the inner + outer cells reorganize themselves as cell division continues to allow formation of blastocoeal cavity (inner cells clump to form a mass together, outer cells become shell-like layer)
2 distinct populations
inner cells - pluripotent embryonic cells → contribute to fetus development
outer cells = trophectoderm - extra embryonic cells → contribute to extraembryonic structures which support development e.g. placenta
what is the zona pellucida
what is its function
hard protein shell surrounding embryo
prevents polyspermy and protects early embryo
limits embryonic development by restricting size until hatching occurs
how does blastocyst form
inner cells have formed a mass together (pluripotent embryonic cells)
trophoblasts (extra embryonic cells) which form outer layer actively pump Na+ ions into cavity
this creates osmotic potential for water to enter → fluid filled blastocoel cavity forms
(zona pellucida is still present)
what is hatching
why is it necessary
necessary because zona pellucida limits the developmental potential of the embryo by limiting the size to which it can grow (i.e. number of cells which can form)
escape of blastocyst from zona pellucida shell - achieved through combination of enzymatic digestion + cellular contraction by embryo → weaken a point of zona pellucida which allows blastocyst to extrude itself out of the shell → implant in endometrium
blastocyst needs to hatch, otherwise it can’t implant in endometrium
label this
draw out process of pre-implantation development
describe the cellular changes that occur in the peri-implantation period (7-9 days post fertilisation)
trophectoderm forms 2 cell lineages - syncytiotrophoblasts and cytotrophoblasts
syncytiotrophoblasts are invasive cells → invades endometrium by destroying maternal endometrial cells, ultimately degrades capillary endothelial cells → creates interface between embryo and maternal blood
cytotrophoblasts - continue to divide to provide a source of syncytiotrophoblasts
inner cell mass forms 2 cell lineages as well - hypoblasts and epiblasts
epiblasts - cells which the fetal organs + tissues will be derived from
hypoblasts - cells which will form the yolk sac (located on underside of epiblasts cells, facing cavity)
label the cells
what are the functions of each
describe the cellular changes that occur in peri-implantation period (12+ days post fertilisation)
syncytiotrophoblasts continue to invade maternal endometrium (cytotrophoblasts continue dividing to be a source of syncytiotrophoblasts)
epiblasts become separated by formation of amniotic cavity:
epiblasts above the amniotic cavity → form amnion → gives rise to extra embryonic structures
epiblasts below the amniotic cavity and the layer of hypoblasts form bilayer embryonic disc
formation of the bilayer embryonic disc signifies that the embryo is ready for gastrulation
label the diagram
what are the functions of the cells
what is the significance of this
draw flow diagram of the cell lineages that form during the transition from morula to blastocyst
what is the bilaminar disc
when does it form
what is its significance
2 layers of cells - epiblast and hypoblasts
formed around 12+ days post fertilisation when amniotic cavity forms and separates the epiblasts into 2 different populations
formation of the disc signifies that embryo is ready for gastrulation
describe the process of gastrulation
(occurs in 3rd week post-fertilisation)
thickened structure forms in midline of embryo near caudal end → this is the primitive streak
at cranial end primitive streak expands to form primitive node
primitive pit (circular depression) forms in primitive node
primitive pit extends along midline towards caudal end of streak → forms primitive groove
once primitive groove has formed there is invagination of epiblasts through primitive groove
the epiblasts that migrate through the groove then invade hypoblasts and replace them → forms definitive endoderm
epiblasts which continue to migrate through primitive groove once endoderm has formed, deposit to form a new layer (between the epiblasts and endoderm) → mesoderm
once endoderm and mesoderm have formed the remaining epiblasts which did not migrate is called the ectoderm
whole process is the formation of 3 germ cell layers from bilaminar disc
(cells in different layers are exposed to differentiate environments → signals them to become different derm layers)
label the germ cell layer
fill in this table on the organs derived from each germ layer
what is the first major event which occurs after gastrulation
describe what happens
why is it significant
formation of the notochord - occurs around day 13+
notochord forms from primitive streak and grows along the midline towards the cranial end of the embryo
forms underneath the ectoderm
rod-like tube formed of cartilaginous-like cells
important as the notochord acts as a key organizing centre for neurulation (process of CNS development) and mesoderm development (especially important for muscles)
describe the process of neuralation
process in which neural plate forms the neural tube (which then goes on to form CNS) - directed by notochord
notochord acts as the major signalling organiser for neurallation
notochord (underneath ectoderm) sends signals to the neural plate above it (neural plate is an area of thickened ectoderm)
the signals from the notochord stimulate the neural plate ectoderm to invaginate → formation of neural groove and for 2 area of neural plate to rise up and form ridges → formation of neural folds (which run along cranio-caudal axis)
neural crest cells in neural folds are specified
over next few days neural folds move closer to each and fuse together over neural groove → forms hollow neural tube
as the neural tube forms the neural crest cells migrate away from folds
label this diagram
what process is occurring here
label the diagram
what process has just occurred to form this structure
what will happen next
neurulation - formation of neural tube from neural plate
can see that the neural tube has a complete lumen → therefore neural ridges have fused across length of the embryo and epidermis has formed over the top (derived from ectoderm) and that neural crest cells have migrated away from folds
now the ends of the neural tube need to close, head end will close at day 23, tail end will close at day 27
(neurulation begins in around day 13+ and continues into 4th week)
why is closure of the neural tube important
closure at head end = day 23
closure at tail end = day 27
head end needs to close in order for brain structures to form
failure of neural tube closure is common birth defect:
head end failure (much more drastic) - ancephaly
tail end failure - spina bifida
what are neural crest cells
what is their function
neural crest cells are multipotent ectoderm derived cells which are highly plastic and migrate extensively from neural folds during development:
they are classified according to where they end up in the embryo → lead to formation of various tissues in each location
what is the clinical significance of neural crest cells
defects of neural cell migration and specification are relatively common - problems with them going to the right location and developing into the right cell when they are there
can cause a diverse range of birth defects depending on which neural crest cells are affected
trunk NC → pigmentation disorders
cranial NC → deafness, facial defects
cardiac NC → cardiac defects
vagral and sacral NC → faecal defects + no gut innervation
what are somites
how do they form
pairs of condensed blocks of paraxial mesoderm
formed through somitogenesis:
paraxial mesoderm is found on either side of the neural tube (above notochord)
blocks of paraxial mesoderm condense in pairs (one on either side of neural tube) and bud off (appear) at the same time → synchronized budding to form somites
somitogenesis starts at cranial end and progress towards caudal end down long axis
occurs in segmented animals
rate at which the somites bud (appear) and the number of somite pairs is species specific
in humans 1 par buds every 90mins and there are 44 pairs
what is shown in this picture
what can be noted about their appearance
somites - paired blocks of condensed paraxial mesoderm on either side of the neural tube
in humans they will stop forming at 44 pairs
somites closer to the cranial end are more defined than the caudal → this makes sense as somite formation begins at the cranial end
why are somites important
how do they do this
they give rise to important mesodermal tissues
label this diagram
how does the gut form in the embryo
primitive gut forms around day 16+:
ventral folding - cranial and caudal ends of embryo fold inwards together
lateral folding - both side of embryo fold inwards together
ventral and lateral folding cause section of yolk sac (derived from hypoblasts) to be pinched off
this pinched off part forms primitive gut → then is subdivided into foregut, midgut and hindgut
what does the foregut give rise to
endoderm derived
what does the midgut give rise to
endoderm derived
what does the hindgut give rise to
endoderm derived
what germ layer is the heart derived from
when does it start to form
when does it start pumping blood
mesoderm
what germ layer are the lungs derived from
when do they start to form
endoderm
what germ cell layer are the gonads derived from
how do they become sex specific
mesoderm
draw flow chart of cell lineages that form from morula onwards
following gastrulation what are the major events that occur in order
