early foetal development

Question 1

what is fertilisation age (also known as conceptual age)

Answer 1

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

Question 2

what is gestational age

Answer 2

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

Question 3

what is carnegie stage

Answer 3

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

Question 4

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

Answer 4

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)

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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)

Question 7

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

Answer 7

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

Question 8

summary of STEPS from oocyte to blastocyst

Answer 8

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

Question 9

until what stage are the genes of the embryo transcribed

Answer 9

4-8 cell stage

Question 10

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

Answer 10

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

Question 11

what happens in the maternal to zygotic transition

Answer 11

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

Question 12

what is compaction

Answer 12

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

Question 13

what is the zona pellucida

Answer 13

hard protein shell inhibiting polyspermy and protects early embryo

Question 14

what is the inner cell mass

Answer 14

pluripotent embryonic cells that will contribute to the final organism

Question 15

what is trophectoderm

Answer 15

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

Question 16

what is the blastocoel

Answer 16

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

Question 17

what happens during hatching (day 5-6)

Answer 17

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

Question 18

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

Answer 18

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

Question 19

what does differentiation of the inner cell mass give rise to

Answer 19

epiblast - from which fetal tissues will be derived

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

Question 20

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

Answer 20

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

Question 21

what cells secreted hCG

Answer 21

syncytiotrophoblast secretes hCG

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

Question 22

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

Answer 22

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

Question 23

summary for gastrulation

Answer 23

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

Question 24

what does the endoderm give rise to

Answer 24

GI tract
liver, pancreas
lung
thyroid

Question 25

what does the ectoderm give rise to

Answer 25

CNS and neural crest
skin epithelia
tooth enamel

Question 26

what does the mesoderm give rise to

Answer 26

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

Question 27

what is the notochord

Answer 27

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

Question 28

what happens during notochord formation (day ~13+)

Answer 28

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)

Question 29

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

Answer 29

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

Question 30

when do the closures of the neural ends occur

Answer 30

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

Question 31

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

Answer 31

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

Question 32

what do cranial neural crest cells give rise to

Answer 32

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

Question 33

what do cardiac neural crest cells give rise to

Answer 33

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

Question 34

what do trunk neural crest cells give rise to

Answer 34

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

Question 35

what do trunk neural crest cells give rise to

Answer 35

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

Question 36

what do vagral and sacral neural crest cells give rise to

Answer 36

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

Question 37

what is somitogenesis

Answer 37

segmentation of the body axis

formation of somites

Question 38

what are the steps for somitogenesis

Answer 38

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

Question 39

what tissues do somites form

Answer 39

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

Question 40

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

Answer 40

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

Question 41

what does the foregut give rise to

Answer 41

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

Question 42

what does the midgut give rise to

Answer 42

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

Question 43

what does the hindgut give rise to

Answer 43

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

Question 44

when does the foetal heart system come about

Answer 44

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

Question 45

how and when does the foetal lung system come about

Answer 45

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

Question 46

how do the gonads form in embryo

Answer 46

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