early foetal development Flashcards

1
Q

what is fertilisation age (also known as conceptual age)

A

measured from the time of fertilisation (assumed to be +1 day from last ovulation)
difficult to know time of fertilisation exactly (unless IVF)

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2
Q

what is gestational age

A

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

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3
Q

what is carnegie stage

A

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

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4
Q

what happens in the embryogenic stage (14-16 days post-fertilisation)

A

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)

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5
Q

what happens in the embryonic stage (16-~50 days post fertilisation)

A

establishment of the germ layers and differentiation of tissue types
establishment of the body plan

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6
Q

what happens in the fetal stage (~50 to 270 days post fertilisation or~8 to ~38 weeks)

A

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)

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7
Q

how are the 3 trimesters organised and what happens at the end of the first trimester

A

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

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8
Q

summary of STEPS from oocyte to blastocyst

A

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

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9
Q

until what stage are the genes of the embryo transcribed

A

4-8 cell stage

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10
Q

what is the embryo dependent on to get through the first few divisions

A

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

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11
Q

what happens in the maternal to zygotic transition

A

transcription of embryonic genes (zygotic genome activation)
increased protein synthesis
organelle (mitochondria, Golgi) maturation - metabolism and protein synthesis

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12
Q

what is compaction

A

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

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13
Q

what is the zona pellucida

A

hard protein shell inhibiting polyspermy and protects early embryo

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14
Q

what is the inner cell mass

A

pluripotent embryonic cells that will contribute to the final organism

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15
Q

what is trophectoderm

A

extra embryonic cells that contribute to the extra embryonic structures that support development eg placenta

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16
Q

what is the blastocoel

A

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

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17
Q

what happens during hatching (day 5-6)

A

to implant, the blastocyst must escape the zona pellucida
enzymatic digestion
cellular contractions
weaken points of zona pellucida

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18
Q

what happens during peri-implantation events (day 7-9)

A

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

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19
Q

what does differentiation of the inner cell mass give rise to

A

epiblast - from which fetal tissues will be derived

hypoblast - which will form the yolk sac (extraembryonic structure)

20
Q

what happens during Bi laminar embryonic disc formation (day 12+)

A

the bilaminar (2 layer) embryonic disc is the final stage before gastrulation:
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 2 layer disc of epiblast and hypoblast - sandwiched between cavities
embryo is now ready for gastrulation

21
Q

what cells secreted hCG

A

syncytiotrophoblast secretes hCG

detection of beta hCG subunit in blood/urine is basis of pregnancy testing

22
Q

big big flashcard for STEPS for gastrulation (dont memorise just read this one ig bc i have a summary one somewhere here)

A

the process whereby the bilaminar embryonic disc undergoes reorganisation to form a trilaminar disc

end of the second week of development, the bilaminar disc has formed
through the 3rd week of development > bilaminar disc differentiates to establish 3 primary germ layers in a process known as gastrulation
~15 days after fertilisation - thickened structure forms along midline of epiblast near the caudal end of bilaminar embryonic disc
this is called the primitive streak
the formation of the streak defines the major body axis of the embryo including cranial and caudal ends
as well as the left and right sides of the embryo
at the cranial end of the embryonic disc, the primitive streak expands to create a primitive node containing a circular depression - primitive pit
this depression continues along the midline of the epiblast towards the caudal end of the streak forming a primitive groove
once formed, cells of the epiblast migrate inwards towards the streak, detach from the epiblast, and slip beneath into interior of embryo (invagination)
first cells to invaginate through the primitive groove invade the hypoblast and displace its cells
hypoblast cells are completely replaced by a new proximal cell layer > referred to as definitive endoderm
by day 16 - majority of hypoblast has been replaced
the remaining cells of the epiblast are now referred to as the ectoderm and forms the most exterior distal layer
some of the invaginated epiblast cells remain in the space between the ectoderm and the newly formed definitive endoderm forming a germ layer called the mesoderm
once formation of definitive endoderm and mesoderm is complete > epiblast cells no longer migrate towards the primitive streak
throughout gastrulation the ectoderm continues to form from the cranial to caudal end of the embryo establishing 3 distinct primary germ layers

23
Q

summary for gastrulation

A

the first cells to invaginate through the primitive groove form the definitive endoderm
the remaining cells of the epiblast are called the ectoderm
cells that remain in the space between the ectoderm and definitive endoderm form a layer called the mesoderm

24
Q

what does the endoderm give rise to

A

GI tract
liver, pancreas
lung
thyroid

25
Q

what does the ectoderm give rise to

A

CNS and neural crest
skin epithelia
tooth enamel

26
Q

what does the mesoderm give rise to

A

blood (endothelial cells, RBCs and WBCs)
muscle (smooth, skeletal and cardiac)
gonads, kidneys and adrenal cortex
bone, cartilage

27
Q

what is the notochord

A

notochord is a rod like tube structure formed of cartilage like cells

28
Q

what happens during notochord formation (day ~13+)

A

forms along the embryo midline, under the ectoderm
forms from primitive streak towards head end of embryo
acts as a key organising centre for neurulation and mesoderm development (releases growth factor signals and organising mesoderm development - vasculature)
neural plate (thickened ectoderm)

29
Q

what happens during neurulation - forming the neural tube and CNS?

A

notochord signals direct the neural plate ectoderm to invaginate forming neural groove
creates 2 ridges (neural folds) running along the cranio-caudal axis
neural crest cells specified in neural folds

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
importantly it is the notochord (in green) which acts as the major signalling organiser for controlling the process of neurulation

30
Q

when do the closures of the neural ends occur

A

closure at the head end ~d23
closure at the tail end ~d27
closure at the head end precedes formation of brain structures

31
Q

failure of neural tube closure is a common developmental defect
what are the 2 defects

A

anencephaly (failure of closure at the head end) - absence of most of the skull and brain
and
spina bifida (failure of closure at the tail end) - open neural tube at birth usually lower spine

32
Q

what do cranial neural crest cells give rise to

A

cranial neurons, glia, lower jaw, middle ear bones (ossicles) and facial cartilage

33
Q

what do cardiac neural crest cells give rise to

A

aortic arch/pulmonary artery septum, large arteries wall muscular connective tissue

34
Q

what do trunk neural crest cells give rise to

A

trunk NC : dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes

35
Q

what do trunk neural crest cells give rise to

A

dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes

36
Q

what do vagral and sacral neural crest cells give rise to

A

parasympathetic ganglia and enteric nervous system ganglia
defects of neural crest migration/specification lead to diverse birth defects including pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut

37
Q

what is somitogenesis

A

segmentation of the body axis

formation of somites

38
Q

what are the steps for somitogenesis

A

somites = arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord
blocks of paraxial mesoderm condense and bud off in somite pairs
one of each pair either side of the neural tube
somitogenesis commences at the head end and progresses down the long axis of the embryo to tail end
rate of budding or appearance of somite pairs is species specific as is the number of pairs
humans 1 pair/90mins - 44 pairs
less defined as they move down

39
Q

what tissues do somites form

A

somites initially form 2 types of embryonic tissue:
sclerotome : vertebrae and rib cartilage
dermomyotome : which in turn sub divides into
dermatome = gives rise to the dermis of the skin, some fat and connective tissues of
neck and trunk
myotome = forms the muscles of the embryo

40
Q

how does the primitive gut form in the embryo and then what is it patterned into

A

the primitive gut arises from 2 types of folding in the embryo :
ventral folding - where the head and tail ends curl together
lateral folding - where the 2 sides of the embryo roll
this pinches off part of the yolk sac to form the primitive gut
the primitive gut is then patterned into foregut, midgut and hindgut

41
Q

what does the foregut give rise to

A

oesophagus, stomach, upper duodenum, liver, gallbladder, pancreas

42
Q

what does the midgut give rise to

A

lower duodenum and remainder of small intestine, ascending colon and first two thirds of transverse colon

43
Q

what does the hindgut give rise to

A

last third of the transverse colon, descending colon, rectum and upper anal canal

44
Q

when does the foetal heart system come about

A

begins as tube of mesoderm around d19 beating and pumping blood commences around d22
fetal heartbeat detected from ~6 weeks gestational age

45
Q

how and when does the foetal lung system come about

A

arise from the lung bud and endodermal structure adjacent to the foregut in the 4th week of development
lung bud splits into 2 at the end of the 4th week and progressively branches through development

46
Q

how do the gonads form in embryo

A

forms from mesoderm as bipotential (not committed to testis or ovary) structures known as gonadal/genital 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 gene leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2