Early Foetal Development Flashcards
fertilisation age
measured from the time of fertilization (assumed to be +1 day from last ovulation)
difficult to know time of fertilization exactly (unless IVF)
gestational age
calculated from the time of the beginning of the last menstrual period (LMP)
Determined by fertilization date (+14 days) if known, or early obstetric ultrasound and comparison to embryo size charts.
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 fertilization age in humans
embryogenic stage is in what trimester?
first
When is the embryogenic stage?
14-16 days post fertilisation
What happens in the embryogenic stage?
establishing the early embryo from the fertilized oocyte
Determining two populations of cells: pluripotent embryonic cells (contribute to fetus)
Extraembryonic cells (contribute to the support structures eg placenta)
embryonic stage is in what trimester?
first
when is the embryonic stage?
16-50 days post fertilisation
What happens in the embryonic stage?
Establishment of the germ layers and differentiation of tissue types
Establishment of the body plan
fetal stage is in what trimester?
second and third
when is the fetal stage?
8 to 38 weeks
What happens in the fetal stage?
Major organ systems now present
Migration of some organ systems to final location
Extensive growth and acquisition of fetal viability (survival outside the womb)
outline the journey from fertilisation to developing into blastocyst
ovulated oocyte > zygote > cleavage stage embryos (2-8 cells) > morula (16+ cells) > blastocyst (200-300 cells)
The genes of the embryo are not transcribed until?
4-8 cell stage
The embryo is dependent on what to get through the first divisions?
maternal mRNAs and proteins
What occurs in the maternal-to-zygotic transition?
transcription of embryonic genes, increased protein synthesis, organelle maturation
Compaction starts the?
formation of the first two cell types
When does compaction occur?
3-4 days post fertilisation
What is compaction?
outer cells become pressed against the zona, change from spherical to wedge shaped, outer cell connect via tight gap ix/desmosomes, become polarised, form diffusion barrier
Blastocyst formation establishes what two cell types?
pluripotent embryonic cells that contribute to final organism, extra embryonic cells that contribute to the structures that support development
What is the zona pellucida?
hard protein shell inhibiting polyspermy and protects the early embryo
What is the blastocoel?
fluid filled cavity formed osmotically by trophoblast pumping Na+ ions into cavoty
What is the trophoectoderm?
where the extra embryonic cells are
What process does the blastocyst go through to implant?
hatching (day 5-6), must escape zona pellucida
How does the blastocyst escape the zona pellucida?
enzymatic digestion
cellular contraction
Morula is separated into?
inner cell mass
trophoectoderm
trophoblast cells fuse to form?
syncitiotrophoblast
What does syncitiotrophoblast invasion cause?
destroys local maternal cells in the endometrium > creates interface between the embryo and the maternal blood supply
Why do cytotrophoblast cells remain individual?
to provide a source of syncitiotrophoblast cells
Inner cells mass further separates into?
epiblast
hypoblast
What is the epiblast?
where the fetal tissues are derived
What is the hypoblast?
this forms the yolk sac (extraembryonic structure)
What is the final stage before gastrulation?
bi-laminar embryonic disc formation
When does bi-laminar embryonic disc formation occur?
day 12+
some cells become separated from the epiblast to form what?
amniotic cavity
syncitiotrophoblast secretes what important hormone? why is it important?
hCG > important for pregnancy testing
What is gastrulation?
differentiation of bi laminar disc into three germ layers
When does gastrulation happen?
day 15
the development of what determines the axis of the embryo?
primitive streak
The primitive streak expands at _____ to create the ________.
cranial end primitive node (contains depression called primitive pit > primitive groove)
After formation of the primitive groove, outline the process that occurs?
invagination >epiblast cells migrate to primitive streak > invasion of hypoblast > displacement of the cells > replaced by definitive endoderm
After formation of the definitive endoderm, the remaining cells of the epiblast are referred to as?
ectoderm (most exterior distal layer)
Germ layer between endoderm and ectoderm is called?
mesoderm
What organs are derived from the endoderm?
GI tract liver pancreas lung thyroid
What organs are derived from the ectoderm?
CNS and neural crest
skin epithelia
tooth enamel
What organs are derived from the mesoderm?
blood (endothelial, RBC, WBC)
muscle (smooth, skeletal, cardiac)
gonads, kidneys, adrenal cortex, bone, cartilage
When does notochord formation occur?
day 13+
What is notochord?
rod-like tube structure formed of cartilage-like cells
Where is the notochord found?
along the embryo midline under the ectoderm
The notochord acts as?
a key organizing centre for neurulation and mesoderm development
What is the neural plate?
thickened ectoderm
What is neurulation?
forming the neural tube and CNS
Outline what happens in neurulation.
neural plate invaginates > forms groove > creates two ridges (neural folds) > neural crest cells specified in folds > folds fuse > overlaid with ectoderm > migration of neural crest cells from folds
The neural folds run along what?
cranio-caudal axis
The neural tube is initially open at each end. When does the head end close?
day 23
The neural tube is initially open at each end. When does the tail end close?
day 27
Closure of the head end of the neural tube precedes the formation of?
brain structures
List examples of neural tube defects.
anencephaly
spina bifida
What is anencephaly?
(absence of most of the skull and brain) arises from failure to close at the head end
What is spina bifida?
(open neural tube at birth, usually lower spine due to failure to close tail end– varying severity
Neural crest cells
Ectoderm-derived, plastic and migrate extensively during development
List types of neural crest migration
cranial NC
cardiac NC
trunk NC
vagral and sacral NC
cranial neural crest cells specify to?
cranial neurones, glia, lower jaw, middle ear bones (ossicles), facial cartilage
cardiac neural crest cells specify to?
aortic arch/pulmonary artery septum, large arteries wall musculoconnective tissue
trunk neural crest cells specify to?
dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
vagral and sacral neural crest cells specify to?
parasympathetic ganglia and enteric nervous system ganglia
Defects of neural crest migration/ specification lead to birth defects including?
pigmentation disorders
deafness
cardiac and facial defects and failure to innervate the gut
what is somitogenesis?
formation of somites
what are somites?
arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord
outline somitogenesis
Blocks of paraxial mesoderm condense and bud off in somite pairs > One of each either side of the neural tube > commences at the head end + progresses down
how many somite pairs in humans/rate of budding?
44 pairs
1 pair/90 minutes
Somites form what two types of embryonic tissue?
sclerotome
dermomyotome
sclerotome forms?
vertebrae and rib cartilage
dermomyotome sub divides to form?
dermatome
myotome
dermatome gives rise to?
dermis of the skin
some fat + connective tissues of neck and trunk
myotome gives rise to?
muscles of the embryo
when does formation of the gut tube occur?
day 16+
the primitive gut arises from what types of folding?
ventral folding
lateral folding
the folding _____________ to form the primitive gut.
pinches off part of the yolk sac
primitive gut is patterned into?
foregut
midgut
hindgut
foregut derivatives
oesophagus, stomach, upper duodenum, liver, gallbladder, pancreas
midgut derivatives
lower duodenum and remainder of small intestine, ascending colon and first two-thirds of transverse colon
hindgut derivatives
last third of the transverse colon, descending colon, rectum and upper anal canal
heart begins pumping blood around?
day 22 (detectable about 6 weeks gestational age)
when do lungs arise? split into two?
4th week
end of 4th week
continues to branch throughout development
how do gonads arise?
bipotential structures (gonadal/genital ridges) > specify into male/female genitals
how do XY embryos develop gonads?
presence of SRY gene on Y chromosome > gonadal cells become Sertoli cells > triggers testis development, Leydig cell formation, testosterone production
how do XX embryos develop gonads?
absence of SRY gene > gonadal cells adopt granulosa cell fate + ovary development
ovary development requires reinforcement by?
FOXL2
