1a Early Fetal Development Flashcards
What is the fertilization age?
The amount of time which has elapsed since fertilizaton
Why is fertilization age difficult to measure?
Time can elapse between intercourse and fertilization
In what instance can an exact fertilization age be established?
IVF
What is the gestational age?
The fertilization day + 14 days
How many stages of embryo development are there in the carnegie scale?
23
What does the Carnegie scale show?
Embryo features not time
Which time measurement can be used for comparing between species?
Carnegie
What window of time does the Carnegie scale show?
60 days
How many days post fertilization does the embryogenic stage last?
14 to 16 days
What is the name of the stage 16-50 days post fertilization?
Embryonic stage
What occurs in the embryogenic stage?
Established the early embryo from the fertilized oocyte, ad determination of two separate cell populations - he pluripotent cells and extra-embryonic cells
Which two types of cells are developed in the embryogenic stage?
Pluripotent embryonic cells and extra-embryonic
What happens in the embryonic stage of development?
Establishment of germ layers and differentiation of tissue types
When does the embryo to foetus transition occur?
During the first trimester
In what stage is the body plan established?
Embryonic stage
What happens to the organs in the fetal stage?
Migration of some organ systems to their final location
What is meant by fetal viability?
The ability of the foetus to survive outside the womb
What is the time period of the fetal stage?
50-270 days
Which stages of embryo-fetal development are in the first trimester?
Embryogenic and embryonic
What is the name given to the fertilized oocyte?
Zygote
When the embryo goes from 2 to 8 cells, what is this called?
Cleavage stage embryos
When is the embryo considered a morula?
When it has 16+ cells
What characterizes a blastocyst?
200 to 300 cells
And a fluid filled cavity
What is the embryo dependent on to get through the first few stages of development?
Maternal mRNAs and proteins
At what point do the embryos own genes get transcribed?
The 4 to 8 cell stage
What is the transcription of the embryonic genes called?
Zygotes genome activation
What happens in the zygote genome activation stage?
Increased protein synthesis
Organelle maturation
Which organelles mature during zygote genome activation?
Mitochondria and golgi
What is the maternal to zygote transition?
This is the transition between the embryo being dependent on mothers mRNAs and proteins, to transcribing its own genes
When are maternal mRNA and proteins synthesized and stored?
During oocyte develoment
What is compaction?
When the cells bind tight to each other and pull in
What starts the formation of the first two cell types?
Compaction
When does compaction occur?
Around the 8 cell stage or later
What happens when the outer cells become pressed against the zona pellucida?
They become pressed against the zona and consequently change shape from rounded to wedge shapes
How do the outer cells connect to each other?
Through desmosomes and tight junctions
What is the name given to the outer layer during blastocyst formation?
Trophectoderm
What type of cells does the inner cell mass contain?
Pluripotent stem cells
What are trophectoderm cells?
Extra-embryonic cells that contribute to the extra-embryonic structures like the placenta
What is the purpose of zona pellucida?
A hard shell protein to prevent polyspermy and protect the early embryo
What is the blastocoel?
A fluid filled cavity in the center of the blastocyst
How does the blastocoel form?
The trophoblast pumps Na+ ions into the cavity which water then follows with osmosis
What process occurs during day 5-6 in development?
Hatching - this is where the blastocyst breaks free from the zona pellucide
How does the blastocyst escape the zona pellucida?
Enzymatic digestion or cellular contractions - weakens the zona pellucida so much to allow the blastocyst to extrude itself out the shell
Where does the blastocyst implant itself following extrusion from the shell?
The uterine endometrium
What forms in the first stage of the separation of embryonic cell lineages?
Inner cell mass and the trophectoderm
What events happen from day 7 to 9?
Peri-implantation events
What does the trophectoderm layer separate into?
Syncytiotrophoblast and cytotrophoblast
What does the syncytiotrophoblast do?
It invades the uterine endometrium and starts to degrade the capillaries found there
Creates an interface between embryo and maternal blood supply
What does the inner cell mass divide into?
The epiblast and the hypoblast
What is derived from the epiblast?
fetal tissues and organs
What is derived from the hypoblast?
The yolk sak and other extra-embryonic structures
What is the final stage before gastrulation and occurs on day 12?
The bilaminar embryonic disc formation
How does the amniotic cavity form?
Some epiblast cells become separated from the epiblast
What is the amnion?
An extra-embryonic membrane
When is the embryo ready for gastrulation?
When the bilaminar disc of the epiblast and hypoblast sandwiched between the amniotic cavity forms
What does the syncytiotrophoblast start to produce at the bilaminar disc stage?
hCG = human chorionic gondadotrophin
What substance is detected during pregnancy tests?
The detection of the beta subunit of hCG in the maternal blood and urine
What is gastrulation?
The process whereby the bilaminar embryonic disc undergoes reorganization to form a trilaminar disc, forming the three primary germ layers
What are the three primary germ layers?
Ectoderm
Mesoderm
Endoderm
What structure forms along the midline of the epiblast around day 15 post fertilisation?
Primitive Streak
Towards which end of the embryo does the tail of the primitive streak originate from?
The caudal end
What is formed when the primitive streak expands?
The Primitive pit
What happens in the process of invagination?
Cells of the epiblast migrate towards the streak, detach and slip beneath in into the interior of the embryo
What is the first germ layer to be formed?
The endoderm
How does the endoderm form?
When epiblast cells invaginate and displace the hypoblast cells, forming a new layer called the endoderm
How does the ectoderm form?
The remaining hypoblast cells which haven’t been replaced by epiblast cells then form the ectoderm
How does the mesoderm form?
When some of the invaginated epiblast cells remain in the space between the endoderm and the ectoderm
What happens when the mesoderm forms?
The epiblast cells no longer migrate towards the primitive streak
When is gastrulation complete?
When all three layers have been formed
What organ systems does the endoderm give rise to? GILLT
GILLT
GI Tract
Lungs
Liver
Trachea
What organ systems does the mesoderm give rise to?
BMBG
Blood
Muscle
Bone
Gonads, Kidney and Adreal cortex
What systems does the ectoderm give rise to?
CNS and neural crest
Skin epithelia
Tooth enamel
What is the notochord?
A rod-like tube structure formed of cartilage-like cells that acts as a key organising center for neuralation and mesoderm development
What does the notochord do?
Releases growth factor signals that are important for neuralation
Where is the notochord relative to the neural plate?
The notochord lies below the nueral plate
What affect does the notochord have on the neural plate?
it sends signals up from the notochord to move up through the embryo and direct the neural plate to invaginate forming the neural groove
What is the neural groove?
A groove frmed through the formation of two neural ridges (neural folds) that run alongside the cranio-caudal axis
What cells are found in the neural folds?
Neural creast cells
What forms when the neural folds move together and eventually fuse?
Neural tube
What overlays the neural tube?
Epidermis - which is ectoderm derived
What are the two hallmarks of the neural crest cells?
They are plastic and highly migratory
Around what day does the neural tube close at the head end?
23
Around what day does the neural tube close around the tail end?
27
What two conditions are common with the closure of the neural tube?
Anencephaly
Spina Bifida
What causes anencephaly?
Failure of the neural tube to close at the head end
What is spina bifida and what does it cause?
Neural tube defect caused when the neural tube is open at birth as the tail end has not closed
What do cranial neural crest cells help to derive?
Cranial neurones, glia, lower jaw, middle ear bones and facial cartilage
What do cardial neural crest cells help to derive?
aortic arch, pulmonary artery septum, large arteries walls, musculoconnective tissue
What do trunk neural crest cells help to derive?
Dorsal root ganglion, sympathetic ganglia, adrenal medulla, aortic arch clusters, melanocytes
What do vagral and sacral neural crest cells help to derive?
Parasympathetic ganglia, and enteric nervous system ganglia
What can defects of neural crest migration / specification lead to?
Pigmentation disorders, deafness, cardiac and facial defects, failure to innervate gut
What is somitogenesis?
The formation of somites
What are somites?
Blocks of mesoderm along the axis of the embryo
How do somites form?
As development progresses, there is synchronized budding of the paraxial mesoderm from both sides at the same time, forming somites
Where does somitogenesis commence?
At the head end of the embryo
What is the rate of somite budding in humans?
1 pair every 90 minutes
how many somite pairs do humans have?
44 pairs
Where are the more defined somites found?
Towards the head end
What two types of tissues do somites initially form?
Dermomyotome and sclerotome
What is the sclerotome?
The vertabrae and rib cartilage
What does the dermamyotome subdivide to form?
The dermatome and myotome
What does the dermatome give rise to?
The dermis of the skin, fat, connective tissues of the neck and trunk
What does the myotome give rise to?
Muscles of the embryo
At what point does formation of the gut occur?
Day 16+
What arises from the two types of folding in the embryo?
the primitive gut
What are the two types of folding which occur to give rise to the primitive gut?
Ventral - where the head and tail ends curl together
Lateral - Where the two sides of the embryo roll
What is the yolk sac derived from?
The hypoblast
What is formed when part of the yolk sac is pinchd off?
The primitive gut
What happens to the primitive gut after it is formed?
Patterened into the foregut, hindgut and midgut
What does the foregut derive?
Esophagus, stomach, upper duodenum, liver, gall bladder, pancreas
What does the midgut derive?
Lower duodenum, remainder of small intestine, ascending colon. first 2/3rds of the transverse colon
What does the hindgut derive?
Remaining third of transverse colon, descending colon, rectum and upper anal canal
When is the first fetal heart beat detected?
6 weeks gestational age
How does the heart form?
Begins as a tube of mesoderm around day 19
What do the lungs arise from?
The lung bud in the 4th week of development
What do the gonads form from?
The mesoderm
How does the embryo become a masculine embryo?
The presence of the SRY gene directs the gondal cells to become Sertoli cells, which triggers testes development, Leydig formation and testosterone production
How do embryos become feminine?
Absence of SRY gene leads to gonadal cells adopting a granulosa cell fate and ovary development
During compaction, how do the outer cells bind to each other?
Outer cells connect to each other through tight gap junctions and desmosomes
What characterises a compacted morula?
The formation of two individual cell types - trophectoderm and inner cell mass
What is the zona pellucida?
Hard protein shell inhibiting polyspermy and protects early embryo
