Early embryology Flashcards
What system is used to stage development of human embryos?
Carnegie stages stage 1: zygote - - - - Stage 23: 60days
What structures are present in a fertilised zygote?
stage 1 in carnegie
2 (haploid) pronuclei in zygote: one from sperm and one from egg
zygote is surrounded by zona pellucida,
ZP contains the perivitelline space
perivitelline space contains the polar body (x2-3) which were formed from 2 meiotic divisions)
What does the zona pellucida contain in the fertilised zygote?
- cumulus cells
- excluded spermatozoa
What is the origin of the polar bodies?
- Germ cells duplicate gDNA, but arrest in the prophase of meiosis I
- selected oocyte resumed meiosis I and first polar body forms after telophase I
- oocyte arrests in metaphase II of meiosis II
- fertilisation leads to completion of meiosis II
- second meiotic division produces the second polar body
What must occur before the development of the embryo proper?
conceptus must implant and then generate the “germ disc”
this takes ~10d
What is the fimbriated infundibulum?
funnel shaped termini of uterine tubes
catch the oocyte once it is ovulated or released from the ovary
What are the cleavage stages in embryology?
1-cell zygote -> 2-cell zygote -> 4-cell zygote
division occurs by mitosis
What is the main difference between 1-cell zygote and the 2-cell/4-cell zygote?
1-cell: haploid pronuclei
2-cel/4-cell: diploid nuclei
What is the morula?
12-16 cells
Day 3
What changes occur between a morula to a blastocyst?
COMPACTION
loose cells get tightly joined together to form epithelial junctions (fluid tight barrier)
forms a blastocele (fluid filled cavity located inner to the inner cell mass)
ZP DISSOLVES
What is a (early) blastocyst?
Early blastocyst
32-64 cells
Day 4/5
What is a trophoblast?
Late blastocyst
Days 6/7
contains trophectoderm
formation of the epiblast and hypoblast from the bilaminar disc in the ICM
What is the trophectoderm?
these make up the extra-embryonic mesoderm and line the periphery of the blastocyst
they will not contribute of the embryo itself
How does embryonic development work?
progressive, sequential restriction of cell fate
long term changed are controlled by epigenetic modification (reversible)
What does the bilaminar germ disc go on to form?
HYPOBLAST: will not go on to form embryonic tissues
EPIBLAST: some of this sill go on to form embryonic and foetal tissues
What tissues do the blastocyst go on to form?
Blastocyst-> ICM + trophoblast
ICM -> epiblast + hypoblast
What does the epiblast go on to form?
AMNIONIC ECTODERM
(extra-embryonic)
PRIMITIVE ECTODERM
(= embryonic epiblast)
-> embryonic ectoderm
-> primitive streak
What does the hypoblast go on to form?
Extraembryoic endoderm -> yolk sac
What does the primitive streak go on to form?
- > EMBRYONIC ENDODERM
- > EMBRYONIC MESODERM
What does the primitive streak go on to form?
- > EMBRYONIC ENDODERM
- > EMBRYONIC MESODERM
- > extra embryonic mesoderm
What importance cell fate decisions must be made pre-implantation in mammalian embryos?
decision 1: in morula
decision 2: in blastocyst development
separation of extra-embryonic lineages vs the pluripotent embryonic progenitors
What is the general group of TFs that control the 2 pre-implantation cell fate decisions?
Yamanaki TFs
can induce iPSCs from fibroblasts etc (cause dedifferentiation)
e.g.
ICM: nanog, Oct4, Sox2, Sal4
Trophoectoderm: Tead4, Cdx2, Elf5
What occurs during pre-implantation (week1) of embryological development?
- single oocyte released at ovulation, guided by fimbriated infundibulum into the ovarian tube
- fertilisation occurs in the ampulla
- reductive divisions occurs at 16-24hr in the ZP
- Early blastomeres are totipotent and embryo can regulate
- ICM cells are pluripotent
- Morula undergoes compaction and format the blastocele (blastocyst forms)
- Blastocyst contains trophectoderm and ICM
8a. blastocyst must hatch from ZP pre-implantation
8b. TE forms trophoblast
What is reductive division?
cells get smaller as they divide in the embryo
occurs 16-24hr post fertilisation
= cleavage into morula
What is meant by ‘regulation of the embryo’ in early blastomere phase?
loss of some material or cells will be compensated for
there will be no phenotypic differences in the final foetus etc
What is the significance of the pre-implantation development
- allows fertilisation and pre-implantation in vitro (e.g. IVF)
- 1-2 blastomeres can be removed for screening and pre-implantation Dx
- ICM cells can differentiate into any adult cell type (but not placenta)
- In vitro culture of ICM cells (ES embryonic cells)
- somatic cells can eb re-programmed and placed into enucleated egg to form adult animal (Dolly)
- iPSC research
What are the normal sites for implantation and hatching from the ZP?
upper uterus
What sites are considered ectopic for embryo implantation?
- lower part of uterine body or cervix
- ovary
- ampulla of uterine tube
- peritoneum (retrograde ovulation)
What happens when hatching from the ZP goes wrong?
incomplete dissolving of ZP
semi-encapsulated embryo, where ZP is thin around the blastocele
What occurs at the implantation of the embryo?
[Day 9]
embryo usually implants in the posterior uterine wall
enzymatic degradation of epithelial tissue in endometrium needed for implantation
implantation site is sealed by a fibrin plug
What structures are present at full implantation?
bilaminar germ disc: epiblast and hypoblast layers
amniotic cavity is now fully formed
inferior to the bilaminar germ disk is the blastocele
lacunae form in the uterine endometrium (pools of maternal blood which will go on to form placental vessels)
glands in uterine endometrium become more highly secretory
What occurs 12-14 days post-fertilisation gestation?
Day 12
spaces appear in the extra-embryonic reticulum
this will go on to become the chorionic cavity
Day 14
these spaces grow and merge to forth extra embryonic coelom
(= chorionic cavity)
The blastocele has become the yolk sac
What is the function of the yolk sac?
provides nutrients to the developing embryo
particularly efficient at mediating transfer of nutrients from mother to baby
What processes underly implantation of the embryo?
- syncytiotrophoblast (surrounds the embryo) invades the uterine wall
- trophoblasts then secrete hCG, which maintains the CL to secrete P and maintain pregnancy
- ICM divides to generate epiblast (ectoderm) and hypobast (extra-embryonic/primitive endoderm). This forms the amniotic and chorionic cavities.
- Embryo is ready for gastrulation
- Differential regulation of maternal/paternal genes (IMPRINTING: methylation to epigenetic control expression)
What is the significance of the implantation stage of embryo development?
- usually occurs in posterior uterine wall (if it occurs in ectopic sites, then risk of rupture)
- morning-after pill and IUDs interfere with implantation
- 70% of blastocysts will implant
What is the probability of a blastocyst implanting?
70% OVERALL
- 40% abort over week 2
- 15% abort over week 3
- 50% of spontaneously aborted embryos are chromosomally abnormal
What is a hydaditiform mole?
= molar pregnancy
expression of paternal genes only
(either from imprinting or physical accumulation of paternal nuclear material only in the embryo)
=> contains only trophoblast cells only and no egg/embryo
(can be complete or partial molar pregnancy)
When does gastrulation occur?
usually ~ week 3
What occurs in gastrulation?
- > starts at the causal end of the future embryo and works towards cranial pole
1. Initiated at the primitive groove -> the primitive streak [@ Henden’s node)
2. caused by thickening of the epiblast/ectoderm in midline of bilaminar disk
3. epiblast undergoes EPITHELIAL-TO-MESENCHYMAL transition
4. Cells migrate between epiblast-hypoblast to generate mesoderm and endoderm
5. Mesodermal and endodermal migration
6. formation of axial, paraxial, intermediate and lateral mesoderm
What tissues form from the ectoderm?
- outer surface (dermis)
- CNS
- neural tube and crest
What tissues form from the endoderm?
- digestive tube
- pharynx
- trachea
all of the (epithelial) linings and glandular tissues
What tissues form from the mesoderm?
- paraxial: somites -> muscle and skeleton
- intermediate: urogenital
- lateral; heart, spleen, blood
- head: skull, teeth
What are the rates and associations of twinning?
rate: 15/1000 (UK)
- associated with lower birth weight and shorter pregnancy (~37wks)
What is the rate of triplets?
1:10,000 in the UK
Where are the highest/lowest twining rates globally?
highest: C. Africa
lowest: Asia, S. America
How are dizygotic twins formed?
formed from 2 separate oocytes
~66% of all twins
What are the different types of twins?
dizygotic: fraternal (66%)
monozygotic: can be identical (33%)
When are the different types of twins formed?
BEFORE BLASTOCYST (<5d) dichorionic diamniotic (DCDA)
AT ICM FORMATION (5-7d) monochorionic diamniotic (MCDA)
AT EPIBLAST FORMATION (10d)
monochorionic monoamniotic
AT PRIMITVE STREAK (14d)
monochorionic monoamniotic
What is the structure of dizygotic (non-identical) twins?
2 oocytes are fertilised
each one implants separately
2 amnions, 2 chorions
What is the structure of monozygotic (identical) twins?
A 2-cell zygote (within the same ZP) splits into 2 blastocysts
2 blastocysts implant separately
2 amnions, 2 chorions
What are the 2 types of monochorionic twins?
MONOZYGOTIC, DIAMNIONIC
- one morula but 2 ICMs and disks
- ICM splits and forms 2 embryonic disks
- 2 amnions in a shared chorion
MONOZYGOTIC, MONOAMNIOTIC
- one blastocyst
- Forms 2 primitive streaks
- amnion and chorion are both shared
What are the 2 routes to conjoined twins?
Both start from one morula/blastocyst;
- One germ disk -> partially divided primitive streak
- Partially divided ICM
What is neurulation?
occurs at WEEK 3-4
- whole brain, spinal cord, cranial and spinal nerves form from the neural tube
- cranial and spinal ganglia form from the neural crest
- neural plate os neuroectoderm (induced from epiblast and by the notochord)
What sequence does neurulation occur in?
neural plate -> neural groove -> neural folds -> neural tube (and neural crest migration)
How does the neural plate form?
notochord stimulated ectoderm (made from epiblast) to form the neuroectoderm
In which direction does the primitive streak form?
in the caudal to cranial direction
will have stretched out ~ ¾ of the way across the disk of embryo
What are the first steps in neurulation?
notochord signalling to overlying ectoderm to form the neural plate
rolling of neural plate medially
DAY 18-19
What occurs to mediate neural tube closure?
DAY 20
- neural folds rise out of the plane of the disk
- neural folds meet and fuse to create the neural tube
DAY 22
- Anterior neuropore (cranial pole) is patent
- Posterior neuropore is patent (caudal pole)
- surface ectoderm covers neural tube
What occurs in the same time-frame as neurulation?
(other form-shaping processes)
- gut formation
- body folding (“silk purse model)
What is the “silk purse” model of folding in embryology?
WEEK 4-5
IMPORTANT IN FORMING THE UMBILICAL CORD FROM THE YOLK SAC
neck of purse string correlates to region where amnion finishes/comes together on the stalk
- septum and heart move from the margin to the centre
- yolk sac, allantois and stalk make the umbilical cord
- prochordal and cloacal plates delimit the gut tube
What is the allantois?
- forms from the yolk sac
- develops to form the umbilical cord
- helps to perform gas exchange and handle wast removal
What are defects of neural tube morphogenesis?
Both defects have perinatal lethality
- Anencephaly
- Cranioachischisis
What are the main types of spina bifida?
(caudal defects)
- meningocele (exposed meninges)
- myelomeningocele: fluid filled sac where meninges and nervous tissue protrude
- occulta: filaire of the neural arches to form around the neural tube. Usually asymptomatic but identified by a tuft of hair around the failed closure point
- myeloschisis aparta: complete expo of the neural tube without skin covering
What is anencephaly?
caused by CRANIAL neuropore failure
perinatal lethality
also craniorachischisis
What is spina bifida?
Caused by CAUDAL neuropore failure
What is the prevalence of neural tube defects?
0.5-2 per 1000
most common CNS malformation
How may neural tube defects be picked up in utero?
leakage of ALPHA FOETOPROTEIN (aFP) associated with NTDs
can measure serum levels at ~15-16wks
What are the different causes of NTDs?
NTDs = neural tube defects
- genetic: polygenic vs. monogenic
- environmental: e.g. sodium valproate and other epilepsy medications
- multifactorial (genetic and geographical for e.g.)
- dietary: known is folate and malnutrition
What is the genetic understanding of NTDs
NTDs = neural tube defects
genes governing :
- the induction of hinge points by the notochord
- folding via actin/microtubule based cell wedging
What does the neural crest go on to form?
- PNS (Schwann cells, neuroglia, autonomic NS)
- adrenal medulla
- melanocytes
- facial cartilage
- dentine (teeth)
What can defects in the neural crest lead to?
- Hirschprung’s disease
- aganglionic megacolon
What is aganglionic megacolon?
congenital absence of enteric ganglionic cells
What is convergence-extension process?
causes migration of peripheral cells into the midline of the embryo
allows lengthening of the neural plate in order for neural tube folding to occur
When does the neural plate lengthen and narrow?
during neurulation
Days 18-20
What is the relationship between folate and neural tube defects?
dietary folate has influence on NTDs
folate (+B12) reduced NTD incidence
current advice is 400mcg/day dose prior to conception
OR
5mg/day dose if NTDs are recurrent issue
alternatively eat fortified foods
What are the other developmental benefits of taking folate supplements?
reduced palate and heart defects
Why does folate reduce developmental defects in utero?
involved in 1-Carbon metabolism
What other supplements may help reduce risk of NTDs?
inositol
research ongoing
What is somitogenesis?
is the process by which somites form.
Somites are bilaterally paired blocks of paraxial mesoderm
form along the anterior-posterior axis of the developing embryo
What do somites go on to form?
Mesodermal origin
make the bulk of our body
e.g. axial skeleton and most muscle
responsible for relationship between bone to muscle and nerve to blood vessels along the trunk
and dermatome formation
How many somite pairs are made in total?
44 somite pairs
What is contained within the chorda-mesoderm?
most centrally located
contains notochord
What does the paraxial mesoderm go on to form?
located either side of the chord-mesoderm
goes on to form:
- head
- somite (sclerotome, myotome, dermatome)
What does the intermediate mesoderm go on to form?
located between the paraxial mesoderm and the lateral mesoderm
forms the kidney, urogenital tract and gonads
What does the lateral mesoderm go on to form?
the most peripheral mesoderm
forms the splanchnic, somatic and extra-embryonic structures
How does somitogenesis occur?
- somites are laid down sequentiall
- in a cranial to caudal fashion
- controlled by developmental clock (circadian) gene expression
What do epithelial somites differentiate into?
- sclerotome: vertebrae and ribs
- myotome: epimere (skeletal muscles), hypomere (abdominal wall muscles), limb muscle
- dermatome: dorsal dermis
- syndetome: tendons
What kind of tissue do somites begin as?
initially epithelial in nature
before differentiation into the different -tome types
What is the origin of the vertebrae and the ribs embryologically?
- sclerotome forms and condenses around the neural tube
- chondrification occurs locally at WEEK 6
- ossification occurs at WEEK 9 (continues until end of puberty)
- WEEK 8: complex cartilaginous thoracic vertebrae and ribs
What does the sclerotome go on to form?
- vertebral bodies
- neural arches
- proximal ribs
What does somite differentiation determine?
vertebra-muscle-nerve relationships
How does somite differentiation occur?
sclerotome is divided into caudal and cranial halves
this occurs via the outgrowth of neural tube nerve to connect to the myotome
fusion of the caudal half of the sclerotome to the cranial half of the inferior sclerotome = vertebrae formation
What is the structure of the somite dermatome in utero?
neural tube (central/medial) epithelial somite (lateral)
What is the structure of the differentiated dermatome?
neural tube (centrally/medially located)
Lateral/adjacent to this are the caudal and cranial sclerotomes
which are separated by the neural tube nerve
Lateral to the sclerotomes are the myotome
How are the myotomes arranged in adult dermatome?
segmental arrangement down the trunk of the body
Where do tendons originate from embryologically? What tissues do they connect in development?
originate from syndetome (paraxial mesoderm)
connect the epaxial and hypaxial muscle fibres
How does innervation of the skin occur embryologically?
[WEEK 6: formation of the dermatome]
- face is supplies by 3 divisions of the trigeminal nerve
- at week 6: limb bud not yet rotated so segmental pattern of primitive dermatomes are evident
- cord is centred on T10 so this is where belly button will be
- hind limbs have higher segmental level that the anus
Which cranial nerve is the trigeminal nerve?
Fifth cranial nerve
CN V
What does the dermatome go on to form?
WEEK 6
- dermis
- subdermal connective tissue
Why is the development of the dermatome important?
closely related to the muscular and skeletal development
shingles (varicella infection) mimics the dermatome segmental structure
also useful in testing spinal nerve function
What is teratogenesis?
formation of congenital malformation
What proportion of births have a structural anomaly?
2-3% of births
most common: cardiovascular
functional and late onset disease: % unknown
What are the causes of structural anomalies in utero?
genetic: 50%
environmental: 10%
unknown: 40% - polygenic and multifactorial
How many known teratogens are there?
~20 classes of known human teratogen
- some are receptor mediated
- of all toxicants only a few re teratogens
What common medications are teratogenic?
- ACEi
- chemo
- EtOH
- retinol, vitamin A
- sodium valproate
- warfarin
- tetracyclines
- streptomycin
What are some types of medications that are teratogenic ligand-activated TFs?
e. g. oestrogen
- retinoic acid (vitamin A derived)
- glucocorticoid
- corticosteroids
- steroids
- androgens (eg. progestins)
- oestrogen
- thyroid medications
Why are retinoids considered teratogenic?
(second most potent teratogen after thalidomide)
- normally, retinoids are morphogens
- however, prolonged expo to excess retinol can be hazardous
- And some synthetic retinoids have very long half-life (~1y)
What are the key determinants of teratogenesis?
- timing of exposure
- pharmacokinetics
- genotype
How may genotype determine teratogenesis?
SNPs in ADH1B
contribute to Foetal EtOH syndrome susceptibility
What are the pharmacokinetic determinants of teratogenesis?
- dose-dependent effect
- threshold for effect
- peak concentration and length of expo is important
- interaction of teratogen with placenta
What are the timing determinants of teratogenesis?
- pre-organogenesis (<2wks) are v. rare
- most structural defects occur during organogenesis (first 2wks)
- later defects: urogenital and cranial
- 2nd and 3rd trim: functional and growth defects
What is the PACT approach to minimising birth defect risk in pregnancy?
P: plan ahead
A: avoid harmful substances
C: choose a healthy lifestyle
T: talk to your doctor, antenatal care
