early fetal development Flashcards
what are the 3 ways of measuring embryo fetal development
fertilisation age
gestational age
carnegie stage
what is the fertilisation age
measured from time of fertilisation - ammued 1 day after last ovulation
difficult to know time unless ivf
what is the gestational age
calculated from time of beginning of last menstrual period
determined by fertilisation date + 14 days if known
or can use early obstetric ultrasound and compare to embryo size charts
what is the carnegie age
the 23 stages of embryo development based on features than time
allows comparison of development rates between species
covers window of 0-60 days
what are the stages we can divide pregnancy into
embryogenic stage
embryonic stage
what is the embryogenic stage
14-16 days post fertilisation
establishing early embryo from fertilised oocyte
determining 2 types of cells:
pluripotent embryonic and extraembryonic cells
what is the embryonic stage
16 - 50 days post fertilisation
establishment of germ layers and differentiation of tissue types
establishment of body plan
what is the foetal stage
50-270 days post fertilisation
major organs present
migration of some organs to final location
extensive growth and acquisition of fetal viability - survival outside womb
what trimester do embryogenic and embryonic stages take place
1st trimester
when does fetal stage take place
2nd and 3rd trimester
8+ weeks
what is the zygote
single cell after fertilisation
what are the cleavage stage embryos
2-8 cell embryos
what is the morula
16 cells+
what is a blastocyst
200-300 cells
how does embryo get through first divisions
until 4-8 cell stage genes of embryo arent transcribed
embryo is dependent on maternal mRNAs and proteins which are stored during oocyte development
what happens during maternal to zygotic transition
transcription of embryonic genes
increased protein synthesis
maturation of organelles e.g mitochondria and golgi
what is compaction
formation of first 2 cell types
around 8 cell stage + outer cells become pressed against zona
change from spherical to wedge shaped
what properties do cells pushing on zona gain
outercells connect to each other through tight junctions and desmosomes
forms barrier to diffusion between inner and outer embryo
outer cells become polarised
what two cells does blastocyst formation establish
inner cell mass - pluripotent embryonic cells
outer cells - trophectoderm - extraembryonic cells that contribute to extraembryonic that support development
what is the zona pellucide
hard protein shell inhibiting polyspermy and protects early embryo
what is the blasctocoel
fluid filled cavity formed osmotically by trophoblasts pumping na+ ions into cavity
hatching
escape of blastocyst from zona pellicuda done throguh enzymatic digestions and cellular contractions
can then implant into endometrium
when does peri implantation take place
7-9 days approx
what happens during peri implantation
trophectoderm lineage separates further:
- trophoblast cells fuse to form syncitiotrophoblasts which invade local maternal cells in endometrium
- creates interface between embryo and maternal blood supply
what are cytotrophoblasts
cells remain individual to provide source of syncitiotrophoblast cells
what does inner cell mass separate into
epiblast - from which fetal tissues will be derived
hypoblast - form the yolk sac(imp in haematopoeisis) - extraembryonic structure
when does bi laminar embryonic disc formation take place
day 12+
= final stage before gastrulation
what happens during bilaminar embryonic disc stage
syncitiotrophoblast continue to expand into endometrium
some cells from epiblast become separated by formation of amniotic cavity
what is the amnion
cells that continue to contribute to extra embryonic membranes
what is two layered /bilaminar disc
epiblast and hypoblast
what does syncitiotrophoblasts secrete
human chorionic gonadotrophin ,hcg used for pregnancy testing
what is gastrulation
change from single layer of epiblast cells to complex structure containing multiple layers
what is the first step in gastrulation
around 15 days thickened structure forms along midline in epiblast near caudal end of bilaminar disc = primitive streak
what does the primitive streak define
major axis of embryo incl cranial and caudal ends and left and right side
what takes place at cranial end of embryonic disc
primitive streak expands to create primitive node
which contains circular depression - primitive pit
what is primitive groove
when depression continues along midline of epiblast towards caudal end of streak
what is invagination
cells of epiblast migrate towards streak, detach from epiblast and slip beneath it into interior of embryo
what do first cells to invaginate primitive groove do
invade the hypoblast and displace its cells
what is the definitive endoderm
hypoblasts are eventually replaced by new proximal layers
what is the ectoderm
remaining cells of epiblast
forms most exterior distal layer
what is the mesoderm
layer of cells invaginated between endoderm and ectoderm
once mesoderm and endoderm form
epiblast cells no longer migrate towards primitive streak
what does ectoderm do throughout gastrulation
continues to form from cranial to caudal end of embryo
what organs/tissues does endoderm give rise to
gi tract
liver, pancreas
lung
thyroid
what organs/tissues does ectoderm give rise to
cns and neural crest
skin epithelia
tooth enamel
what tissues does mesoderm give rise to
blood, endothelial cell,rbcs,wbcs
muscle
gonads, kidneys and adrenal cortex
bone,cartiliage
what is the first major event after gastrulation
formation of notochord
around day 13
what is the notochord
rod like tube structure formed of cartilage like cells
forms along embryo midline, under ectoderm
what is role of notochord
acts as key organising centre for neuralation and mesoderm development
what is the neural plate
thickened ectoderm
sits on top of embryo
what is neuralation
forming neural tube and cns
how does cns form
notochord signals to neural plate ectoderm to invaginate forming neural groove
creates 2 ridges running along craniocaudal axis
what does formation of neural fold create
neural crest cells which migrate away and populate variety of diff tissues
what happens when neural folds move together
neural folds fuse forming hollow tube
neural tube is overlaid with epidermis
migration of neural crest cells
when does closure of head and tail end of neural tube form
head end - around 23 days
tail end - around 27 days
closure at head end precedes formation of brain structure
what is anencephaly
failure of closure at head end = absence of most of skull and brain
what is spina bifida
open neural tube at birth
usually lower soine defect
what do neural crest cells do
become: cranial nc - cranial neurones,glia,lower jaw, middle ear bones,facial cartiliage cardiac nc trunk nc vagral and sacral nc
what can defects in nc migration cause
birth defects e.g pigmentation disorders, deafness, cardiac and facial defects and failure to innervate gut
what is somitogenesis
formation of somites which arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord
what occurs during somitogenesis
blocks of paraxial mesoderm condense and bud off in somite pairs
one of each pair either side of neural tube
where does somitogenesis commence
at head end and progresses down the long axis of embryo
rate of budding or appearance of somite pairs is species specific as is number of pairs
what do somites initially form
2 types of embryonic videos:
sclerotome
dermomyotome
what is the sclerotome
vertebrae and rib cartilage
what is the dermomyotome
in turn subdivides to form dermatome and myotome
what is the dermatome
gives rise to dermis of skin, some fat and connective tissues of neck and trunk
what is the myotome
forms muscles of the embryo
where does primitive gut arise
2 types of folding:
ventral folding - where head and tail ends curl together
lateral folding - where two sides of embryo roll
pinches off part of yolk sac
what is primitive gut is split into
foregut
midgut
hindgut
what does foregut develop into
eosophagus, stomach, upper duodenum, liver, gallbladder and pancreas
what does midgut develop into
lower duodenum and remainder of small intestine, ascending colon and first two thirds of transverse colon
what does hindgut develop into
last third of tranverse colon, descending colon, rectum and upper anal canal
where does heart form from and when
begin as tube of mesoderm around day 19, beating and pumping blood commences around day 22
fetal heartbeat detectable from approx 6 weeks gestational age
when does lung form and where from
arise from lung bud and endodermal structure adjacent to foregut in 4th week of development
lung bud splits into 2 at end of 4th week and progressively branches
when does gonads form and where from
forms from mesoderm as bipotential ( not committed to testis/ovary) structures known as gonadalgenital ridges
xy embryos - presence of sry gene on y chromosome directs gonadal cells to become sertoli cells, triggering testis development, leydig cell formation and testosterone production
xx embryos - absence of sry leads to gonadal cells adopting granulosa cell fate and ovary development, requires reinforcement of foxl2
