Early Foetal Development Flashcards
what 2 ways can we measure time in embryo-foetal development?
fertilisation age
gestational age
what is fertilisation age (conceptual age) and what are the implications of this and how it can be measured accurately?
age measured from the time of fertilisation (assumed to be +1 day from last ovulation)
it is difficult to know the time of fertilisation exactly (unless IVF)
what is the gestational age?
the age calculated from the start of the last menstrual period
how long does the menstrual period last?
28 days
which day of the menstrual period does ovulation occur?
day 14
how many days longer is gestational age compared to fertilisation age?
14 days longer
what are the carnegie stages of embryo development? (how many and what are they based on)
23 stages of embryo development
based on embryo features (not time)
covers the window of 0-60 days fertilisation age in humans
what does the carnegie stages allow?
allows comparison of different developmental rates between species
what is the embryogenic stage (14-16 days post-fertilisation)?
Formation of early embryo from the fertilised oocyte
Formation of pluripotent embryonic cells (contributes to foetus)
Formation of exrtaembryonic cells (contributes to support structures e.g., placenta)
what is the embryonic stage (16-50 days post-fertilisation)?
Establishment of the germ layers and differentiation of tissue types
Establishment of body plan
what is the foetal stage (50-270 days post-fertilisation/ 8 to 38 weeks)?
Major organ systems present
Migration of some organ systems to final location
Extensive growth and acquisition of foetal viability (ability to survive outside the womb)
what is a trimester of pregnancy?
The 12-14 week blocks in which pregnancy are divided up into
There are 3 trimesters, 1st, 2nd and 3rd trimesters
which trimester of pregnancy do the embryogenic and embryonic stages correspond to?
1st
Which trimester of pregnancy does the foetal stage correspond to?
2nd and 3rd
Briefly describe the first few days of life starting from an oocyte to a blastocyte?
ovulated oocyte fertilised–> unicellular zygote–> 2,4,8 cell embryos (cleavage stage embryos)–> morula (16 cell embryo)–> blastocyst (200-300 cells)
- This development is happening as the oocyte and the early embryo is migrating along to fallopian tube and into the uterus where it can implant
what is the zona pellucida?
protein shell of egg cell
At which stages of this development is the zona pellucida present?
present for all of the stages forming the blastocyst (from ovulated oocyte to blastocyst)
Functions of the zona pellucida?
Protects early embryo
Inhibits polyspermy (prevents multiple sperm cells fertilising the oocyte)
Up until the maternal-zygotic transition, what is the development of the embryo and the divisions of the embryo dependent on + when does it occur?
Maternal mRNAs and proteins to get through the first divisions, since the genes of the embryo aren’t transcribed yet
Occurs before the 4-8 cell stage
When are maternal mRNAs and proteins synthesised and stored?
during the process of oocyte development, pre-ovulation
What can failure to synthesise and store maternal mRNAs and proteins during oogenesis lead to?
Impaired embryonic development
What happens at the maternal-to-zygotic transition?
transcription of embryonic genes (zygomatic genome activation)
increased protein synthesis
organelle (mitochondria, Golgi) maturation
When does the maternal-to-zygotic transition occur?
at the 4-8 cell stage
What is compaction?
the embryo developmental event in which the embryo forms 2 cell lineages, of spherical (inner) and wedge-shaped (outer) cells
When does compaction occur?
around the 8 cell stage or later
How do the spherical cells become wedge-shaped in compaction?
outer cells become pressed against the zona pellucida
How do the outer cells connect to each other (2) and what do they form? And what do the outer cells become (hint they don’t become different types of cells)?
Through tight gap junctions and desmosomes
Forms a barrier to diffusion between inner and outer embryo
Outer cells become polarised
What cell type do the inner cells give rise to?
Pluripotent embryonic stem cells
What cell type does the trophectoderm give rise to?
extra-embryonic cells
What is the trophectoderm?
the outer cells of the embryo/ the trophoblasts
what is the blastocoel?
a fluid filled cavity in the middle of the embryo
how is the blastocoel formed?
the trophoblasts/ the outer cells pump Na+ ions into the centre of the embryo, so water moves in by osmosis
What is hatching and why is it needed?
The blastocyst escapes the zona pellucida
It is needed in order for the embryo to progress further
How does hatching occur?
Enzymatic digestion (blastocyst secretes enzymes which break down the zona pellucida)
Cellular contractions
What happens if the blastocyst can’t escape the zona pellucida?
It can’t undergo implantation into the endometrium
When do peri-implantation events occur?
occurs 7-9 days after hatching, when the blastocyst connects with the endometrium
What does the trophectoderm separate/ differentiate into?
Syncytiotrophoblasts and Cytotrophoblasts
What do syncytiotrophoblasts do?
Invades the uterine endometrium, destroying local maternal cells
Establishes maternal blood supply to embryo by breaking down capillaries
What do cytotrophoblasts do?
continue to divide to provide a source of syncytiotrophoblasts
What does the inner cell mass separate/ differentiate into?
epiblast and hypoblast
What do epiblasts form?
foetal tissues are derived from it
What do hypoblasts form?
forms the yolk sac (an extraembryonic structure important in gut development and early haemopoiesis)
When is the bi-laminar embryonic dis formation?
days 12+, the final stage before gastrulation
What happens to the epiblast at the bi-laminar embryonic disc formation stage?
Some cells become separated from the epiblast by the formation of the amniotic cavity
The new cells formed are the amnion cells
Why do we call it bi-laminar embryonic disc?
Epiblast layer sits on top of hypoblast layer
What do the syncytiotrophoblasts secrete at the bi-laminar formation stage?
hCG (human chorionic gonadotrophin)
What is the basis of pregnancy testing?
detection of the beta subunit of hCG
Embryonic cell lineages (morula to amnion)
What is gastrulation?
the process whereby the bi-laminar embryonic disc differentiates to form a trilaminar disc (Formation of the 3 germ layers)
Summarise gastrulation. (6)
- epiblasts of bilaminar embryonic disc form primitive streak
- primitive streak elongates and forms primitive node
- primitive node contains circular depression which eventually forms a primitive groove
- invagination occurs (cells separate from epiblast)
- the first cells to invaginate through the primitive groove form the definitive endoderm, (the hypoblast is displaced by spreading endoderm)
- the remaining cells of the epiblast form the ectoderm (the exterior layer)
- the cells in the space between the endoderm and the ectoderm form the mesoderm
embryonic cell lineages (post-gastrulation)
What organs does the endoderm give rise to? (5)
- GI tract
- Liver
- Pancreas
- Lung
- Thyroid
What organs does the ectoderm give rise to?
- CNS and neural crest
- Skin epithelia (epidermis)
- Tooth enamel
What organs does the mesoderm give rise to? (5)
- Blood (endothelial cells, RBCs and WBCs)
- Muscle (smooth, muscle, cardiac)
- Gonads
- Kidneys and adrenal cortex
- Bone and cartilage
What is notochord and what does it do?
A rod-like tube structure formed of cartilage-like cells, which forms along the embryo midline, under the ectoderm
It acts as a key organising centre for neurulation and mesoderm development
How does the notochord organise neurulation?
by controlling the neural plate ectoderm
Which germ layer is the notochord derived from?
mesoderm cells
What is neurulation?
the transformation of the neural plate into the neural tube
Summarise neurulation?
- signals from notochord cause invagination of neural place ectoderm, forming neural groove
- forms 2 neural folds containing neural crest cells
- neural folds fuse together over neural groove, forming a hollow tube
- neural tube overlaid with epidermis (which is ectoderm derived)
- migration of the neural crest cells from folds
For the CNS development to continue, what must happen to the open ends of the neural tube?
closure at the head end and the tail end
Which closure of the neural tube happens first?
the head end (23 days)
tail end (27 days)
Which 2 conditions/ developmental defect can failure of neural tube development lead to?
spina bifida
anencephaly
What are the types of neural crest cells?
Neural crest cells- ectoderm-derived, plastic and migrate extensively during development:
Cranial NC
Cardiac NC
Trunk NC
Vagral and sacral NC
What are the cranial NC derivatives?
Cranial NC: cranial neurones, glia, lower jaw, middle ear bones (ossicles), facial cartilage
What are the cardiac NC derivatives?
Cardiac NC: aortic arch/ pulmonary artery septum, large arteries wall musculoconnective tissue
What are the trunk NC derivatives?
Trunk NC: dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
What are the vagral and sacral NC derivatives?
Vagral and sacral NC: parasympathetic ganglia and enteric nervous system ganglia
What is somitogenesis?
Formation of somites
What are somites?
paired blocks of paraxial mesoderm, with one of each pair on each side of the neural tube
Is the rate of budding into somites species-specific?
yes
What is the rate of budding in humans, and how many somite pairs do we form?
1 pair every 90 mins
44 pairs
Which 2 types of embryonic tissue do somites form?
sclerotome and dermomyotome
What do sclerotomes give rise to?
vertebrae and rib cartilage
What 2 subtypes does dermomyotome divide to form and what do they give rise to?
dermatome (gives rise to dermis of skin, some fat and connective tissue of neck and trunk)
myotome (gives rise to muscle of the embryo)
How does formation of the gut tube occur?
ventral folding and lateral folding of the embryo, pinches off the yolk sac to form the primitive gut
What is ventral folding?
where the head and tail ends curl together
What is lateral folding?
where the 2 sides of the embryo roll
What are the 3 derivatives of the primitive gut?
foregut, midgut, hind-gut (all structures formed are endoderm derived)
What does the foregut give rise to?
oesophagus, stomach, upper duodenum, liver, gallbladder, pancreas
What does the midgut give rise to?
lower duodenum and remainder of small intestine, ascending colon and 2/3 of transverse colon
What does the hindgut give rise to?
last 1/3 of the transverse colon, descending colon, rectum and upper anal canal
From which germ layer is the heart derived from?
mesoderm
From which week is the foetal heartbeat detectable?
from 6 weeks gestational age
From which germ later are the lungs derived from?
endoderm (lungs arise from lung bud (an endodermal structure))
From which germ later are the gonads derived from?
mesoderm
The gonads are initially bipotential, what does this mean?
Not committed to testes or ovary
The presence of which gene triggers testes development?
SRY gene on Y chromosome
Causes gonadal cells to become sertoli cells which triggers testes development
Absence of SRY gene leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2
From which germ later is the epidermis derived from?
ectoderm
From which germ later is the dermis derived from?
mesoderm
