Embryology Flashcards
What happens on days 1-2?
Fertilisation: repeated mitotic divisions (cleavage)
What happens on day 3?
Morula is formed which is a ball of cells that are identical post-fertilization
What happens on day 5?
Blastocyst is formed which has a cavity called the blastocele and 2 cell layers:
- Trophoblast (contributes to placenta)
- Embryoblast (inner cell mass that forms baby)
What happens on day 6?
- Blastocyst loses its membrane (zona paloca) which allows it to stick to the uterine epithelium
- Cells of blastocyst differentiate + become something else once stuck to epithelium (esp. in trophoblastic layer)
- Implantation: invasion of uterine lining + wall
- This must happen fast because it needs the foeto-maternal exchange membrane (deep in uterine epithelium) for blood supply + nutrients
What happens on day 8?
Blastocyst becomes a synctiotrophoblast (multi-nucleated cell mass) with an amniotic cavity + embryoblast has become a bilaminar disc including epiblast (top layer) + hypoblast (bottom layer)
This invades the endometrium using digestive enzymes that break down proteins + fats
What happens on day 9?
The synctiotrophoblast has gone further into endometrium + started to invade blood vessels
Primary yolk sac is formed when a layer of cells peels off bottom layer which can bring some nutrition in
Where can ectopic pregnancies occur?
Uterine tubes but also peritoneal cavity because ovulation takes place into there
Can carry on developing if its gets a blood supply breaking down any tissue it wants via digestive enzymes
What happens at day 12?
Implantation has occurred fully + embryo is now called a cytotrophoblast
Finger-like projections called maternal sinusoids formed which will develop into placenta
2 layered disc now has amniotic cavity above + yolk sac below surrounded by extraembryonic mesoderm consisting of cells + ECM for padding (cavities forming within this)
What happens at day 13?
Chorionic cavity formed by cavitation of extra-embryonic mesoderm
Connecting stalk formed of extraembryonic mesoderm (forms future umbilicus which will bring in nutrients)
Primary -> secondary yolk sac
Bilaminar embryonic disc formed (epiblast + hypoblast)
What day is the first day to notice menstrual periods are late?
15
What happens at day 15?1
Prochordal plate (mouth) + cloacal membrane (anus)
Primitive streak + node form from raised areas of epiblast undergoing rapid cell division; primitive streak extends from cloaca to primitive node
Primitive groove + pit are indentations of ectoderm in centre of streak + node where cells are migrating down
What are the other terms for epiblast and hypoblast?
Epiblast = ectoderm
Hypoplast = endoderm
What is gastrulation?
When epiblast cells from the primitive streak migrate towards the hypoblast -> forms 3 germ cell layers because migrating epiblast cells form the intraembryonic mesoderm + replace the hypoblast
Happens at days 18-24
What membranes remain mesoderm free?
Buccopharyngeal (was prochordal plate)
Cloacal
What forms the notochord? What will it go onto form?
Primitive node/pit cells
Structures in the vertebral column
What will ectoderm, endoderm and mesoderm go onto form in the adult?
Ectoderm/epiblast: skin/neural tissue
Mesoderm: paraxial (somite), intermediate (genito-urinary) + lateral plate (serous membranes)
Endoderm/hypoblast: lining of gut tube, respiratory system + urinary system
What range of muscles form from the mesoderm?
Skeletal muscle Bone Connective tissue The heart Urogenital system
What do somites form?
Axial skeleton (skull, vertebrae + ribs), associated muscle + dermis of skins
Each somite is formed by a single spinal nerve + migrate to form parts of the body (explains dermatomal sensory mapping of skin)
What occurs at days 18-24 as well?
Folding: flat disc needs to be folded into prawn shaped -> folding puts skin (ectoderm) on outside + gut tube lining (endoderm) on inside
What types of folding occurs at weeks 3-4?
Longitudinal: folds head + tail ends towards eachother
Lateral: folds lateral sides of disc towards each other meeting in the anterior midline
Describe the fluid filled sacs present in the embryonic disc at day 13.
Amniotic cavity on top of embryonic disc and a yolk sac below
Chorionic cavity surrounds + contains all of this
What happens to the embryonic disc during lateral folding?
Ectoderm (skin) folds around disc
Endoderm folds to form a tube
Amniotic cavity drawn around entire disc so amniotic membrane is reflected on future umbilicus
What does longitudinal folding do to the embryo?
Moves heart, brain, mouth + anus into adult position in a process called reversal where tissue of folding disc is folded toward + reflected at future umbilicus
How is the gut tube formed?
During embryonic disc folding from the endoderm running from the prochordal plate (mouth) to cloacal membrane (anus)
It herniates through the umbilicus due to rapid growth, it rotates 270o anticlockwise around SMA + goes back in to the abdomen to its final disposition where it is least likely to become blocked
What does the bladder form from?
Dilated terminal portion of gut tube
What is the embryological name for the umbilicus?
Vitello-intestinal duct
What is omphalocele?
When the gut tube growth, herniation + rotation goes wrong so a loop of gut is still in the umbilicus surrounded by amniotic sac
What is gastroschisis?
When the gut tube growth, herniation + rotation goes wrong so a loop of gut is still in the umbilicus but this is NOT in the amniotic sac i.e. gut went back into abdomen after herniation but came back out through a defect in the abdominal wall
What happens if the gut tube rotation goes wrong?
Clockwise rotation or malrotation increases risk of gut tube volvulus/obstruction
What is Meckel’s Diverticulum?
Remnant of the vitellointestinal duct from the SI (stomach contents may get stuck here causing inflammation + pain like appendicitis)
What is am imperforate anus?
Abnormal urorectal septum formation where anus does not open so faecal matter cannot get out
How does the cloaca develop?
Starts as common waste outflow for urine + faeces -> urorectal septum divides cloaca into bladder + rectum meeting the cloacal membrane to form the perineal body
What problems can occur if the urorectal septum does not form properly?
Faecal matter in urinary system or urine in the rectum -> infections
Where does the notochord form? What does it become?
In the mesoderm layer
Forms part of vertebral column (e.g. nucleus pulposus of intervertebral discs) but NOT the spinal cord (overlying ectoderm becomes neural in nature)
What is sacrococcygeal teratoma?
Remnants of primitive streak tissue remains at the end of the coccyx predisposing this area to tumours (can go unnoticed in men until adulthood where they get pelvic tumours)
What occurs to the neural plate on day 19-25?
It forms + then folds to form the neural groove + subsequently, the neural tube
What are neural crest cells (NCCS)? What do they do?
Neural crest is where neural plates meeting, this becomes NCCs:
Involved in development of multiple body tissues + regions -> can become melanocytes, craniofacial cartilage, bone, SM, peripheral + enteric system & glia
What conditions are associated with defective neural crest cell (NCC)migration?
Craniofacial skeleton defect
Disordered gut motility if vagal-derived NCCs fail to populate the ENS
What significant events happen at day 25 and day 27?
25: Cranial neuropore closes
27: causal neuropore closes
I.E. neural tube zips up from heard toward tail
What is spina bifida?
Defective neural tube closure together with anomalies of covering tissues
Non-fusion of the vertebral arch
What are the different types of spina bifida (order from most mild to severe)?
- Occulta: unfused vertebral arch - may present with hairy red bit of skin on back
- Meningocele: subarachnoid space + dura mater prolapsed out of vertebral disc - bump protruding out of back
- Meningomyelocoele: spinal cord prolapsed as well - meroanencephaly
- Rachischisis/myeloschisis: no spinal cord tissue developed at all - not compatible with life
What to pharyngeal arches contribute towards?
Formation of neck, lower face + ear
How many pharyngeal arches form? How many develop?
6 initially form
5 develop (I, II, III, IV + VI)
What is the innervation and blood supply to the pharyngeal arches?
Each arch innervated by a cranial nerve
Each arch receives blood from an aortic arch artery
How old is the embryo when most of the major organ systems have formed? How big is it at this point?
8 weeks
30mm
