Early fetal development Flashcards
What is ‘Fertilisation age’?
Also known as conceptual age
Measured from the time of fertilisation
However it is difficult to know time of fertilisation exactly unless its IVF
How is time of fertilisation estimated?
+1 from last ovulation
What is gestational age?
Calculated from time of the beginning of the last menstrual period - LMP
Determined by fertilisation date (+14) if known, or early obstetric ultrasound and comparison to embryo size charts
What is Carnegie staging system?
23 stages of embryo development based on embryo FEATURES rather than time
Covers 0-60 days post fertilization
Why is the Carnegie system good
Allows comparison of developmental rates between specifie
How do we measure time in embryo-fetal development?
Embrogenic stage
Embryonic stage
Fetal stage
What is the embryogenic stage?
14-16 days post fertilisation
establish early embryo from the fertilized oocyte
determine two populations of cells as pluripotent
extraembryonic cells
What is the embryonic stage?
16-50 days
Establishment of germ layers and differentiation of tissue types
Establishement of the body plan
What is the fetal stage?
50-270 days post fertilization or 8-38 weeks
major organ systems now present
Migration of some organs to final location
Extensive growth andacquistion of fetal viability
What stages make up the first trimester?
Embryogenic and embryonic stage
What stages make up the second and third trimester?
Fetal stage
What changes occur in the first few days of life e.g. from oocyte to blastocyte
Ovulated oocyte 1 cells
Fertilisation
Zygote 1 cell
Cleavage stage embryos - 2-8 cells
Morula 16+ cells
Blastocyst 200-300
- during this is migrates from fallopian tube to uterus
What is the maternal to zygotic transition?
The embryo is dependent on material mRNA and proteins to get through first divisions
These are synthesised and stored during oocyte development which is pre-ovulation
Failure to synthesise, store or interpret these mRNAs and proteins during oogenesis can impair embryonic development
So maternal to zygotic transition including:
- transcription of embryonic genes known as ZYGOTIC GENOME ACTIVATION
- Increased protein synthesis
- organelle maturation
What does compaction start?
The formation of the first two cell types around the 8 cell stage or later
WHat does Compaction do?
Outer cells become pressed against zona
Change from spherical to wedge shapes
Outer cells connect to each other through tight gap junctions and desmosomes
Forms barrier to diffuson between inner and outer embryo
Outer cell becomes polarised.
Forms copacted morula and then blastocyst
How does a blastocyst make two cell types?
Inner cell mass : pluripotent embryonic cells
Trophoectoderm : extra embryonic cells that contribute to extraembryonic structures that support development
What does the Zona pellucida do?
Hard protein shell inhibiting polyspermy and protects early embryo
What is the blastocoel?
Fluid filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity
Why is hatching necessary?
Escape of blastocyst from the zona pellucida to allow further growth and implantation
How does hatching occur?
Enzymatic digestion
Cellular contraction from blastocyte
Creates a weak spot and comes out the zone pellucida leaving it empty
How do trophoectoderm lineage separate further?
Trophoblast cells fuse to form syncitiotrophoblast and cytotrophoblast
What do syncitiotrophoblast?
These cells invade and destroy local maternal cells in the endometrium
this creates interface between embro and maternal blood supply
What do the cytotrophoblasts do?
Remain sources to make syncitiotrophoblasts
What does the inner cell mass divide into?
Epiblast : from which fetal tissues will be derived
Hypoblast : which will form the yolk sac
What is the formation of the bi-laminar embryonic disk?
Some epiblast cells get seperated from the epiblast due to the amniotic cavity forming and then become the amnion membrane
The epiblast left below the cavity gives rise to the fetal structures. and the layer below is renamed the hypoblast which becomes the yolk sac
Hence why it is called a bi-laminar disk
What secretes HCG for pregnancy testing?
Syncitiotrophoblasts
What is gastrulation?
The process by which the bilaminar embryonic disk forms a trilaminar disk which is the basis for all organs and body tissues
What happens to the epiblast around day 15?
Along the midline near the caudal end a primitive streak is formed
This defines the body axis e.g. cranial vs caudal + right vs left
How does invagination occur
The primitive streak deepens into a primitive node with a primitive pit which is a depression in the streak
This depression goes across to the caudal end and the epiblast cells then detach and slip down this depression.
These cells replace the hypoblast and becomes a new layer = endoderm
What are the remaining cells of the epiblast post invagination referred to?
The ectoderm forming the most exterior distal layer
Where are the mesoderm cells?
The cells left in between the new endoderm and the epiblast turned ectoderm
Which organs are made form endoderm?
GI Liver pancreas lung thyroid
Which organs are made frome ecotderm?
CNS
neural cest
SKin epithelium
tooth enamel
Which organs are made from mesoderm
Blood muscle Gonads kidneys adrenal cortex bone cartilage
what event occurs on day 13 after grastulation?
Notochord formation
What is a notochord?
rod like tube structure of cartilage like cells
forms along embryo midline, under ectoderm
What does the notochord act as?
key organizing centre for neurulation and mesoderm development, it does this by controlling the neural plate
Describe the formation of the neural tube and CNS?
the notochord signals direct the neural plate ectoderm to invaginate forming neural groove
creates two ridges called neural folds running along the cranio -caudal axis
neural crest cells specified in neural folds
Once the neural folds develop what happens?
The folds move together over neural groove
folds fuse forming a hollow tube = neural tube
the neural tube overlays with epidermis
migration of the neural crest cells from folds
The neural tube starts as a hollow open tube, what must happen for CNS development to proceed?
Closure at head end around day 23
Closure at tail end at day 27
- closure at head end precedes formation of brain structures
What does the failure of neural tube closure cause?
Developmental defects:
- anencephaly - absense of most skull and brain due to failure of closure at HEAD END of neural tube
- Spina bifida - lower spine gap due to failure to close TAIL END of neural tube
What do neural crest cells make in the cranial area?
- cranial neurones
- glia
- lower jaw
- middle ear bones
- facial cartilage
What do neural crest cells make in the cardiac area?
- aortic arch
- pulmonary artery septum
- large arteries wall musculoconnective tissue
What do neural crest cells make in the trunk area
- dorsal root ganglia
- sympathetic ganglia
- adrenal medulla
- aortic nerve cluster
- melanocytes
What do neural crest cells make in the vagral and sacral area?
- parasympathetic ganglia
- enteric nervous system
What defect causes pigmentation disorders?
Neural crest defect
What defect causes deafness?
neural crest - ossicle bones
What defect can cause cardiac and facial defects?
neural crest defect
What defect can cause failure to innervate the gut?
Neural crest defect
What are the properties of neural crest cells?
endoderm derives
plastic
migrate extensively during development to give rise to other cell types
What is somitogenesis?
Formation of somites
What are somites?
arise from paired blocks of paraxial mesoderm lflanking the neural tube and notochord
How are the pairs of praxial mesoderm crates around the neural tube?
Parts of the paraxial mesoderm CONDENSE and BUD OFF creating somite pairs either side
How are 44 pairs of somites created?
Somitogenesis commences at the head end and progresses down the long axis of the embryo
rate of ‘budding’ or appearance of somite pairs if species-specific, as is the number of pairs
- check slide for images
What two tissues do somites initially form?
- Sclerotome = vertebrae and rib cartilage
- dermomyotome = which subdivides to form
1. dermatome (skin, connective and fat tissues of neck and truck
2.myotome
What two foldings in the embryo leads to the primitive gut?
ventral folding - where head and tail ends curl together
lateral folding - where the two sides of the
embryo roll
As these folds are folding it pinches off part of the yolk sac to form the primitive gut
What 3 parts does the primitive gut form?
Foregut
Midgut
Hindgut
What organs does the foregut give rise do?
esophagus stomach upper duodenum liver gall bladder pancreas
What organs does the midgut give rise do?
lower duodenum
remainder of s.intestine
acsending colon
2/3 of transverse colon
What organs does the hindgut give rise do?
1/3 transverse colon
descending colon
rectum
upper anal canal
When is fetal heartbeat detectable?
6 weeks gestational age
How does the heart develop?
tube of mesoderm
beating and pumping blood around day 22
How do the lungs develop?
Arise from lung bud and endodermal structure adjacent to foregut
lung bud splits into two at the end and progressively branches through development
How do the gonads develop?
forms the mesoderm as bipotential structures known as gonadal/genital ridges
What does it mean by bipotential structures?
not committed to testis or ovaries
How does a XY embryo develop?
presence of SRY gene on Y chromosome directs gonadal cells to become Sertoli cells
triggers testis development, Leydig cell formation and testosterone development
How does an XX embryo develop?
absence of SRY leads to gonadal cells adopting Granulosa cell fate and ovary development
requires reinforcement by FOXL2
