Embryology

Question 1

What happens on days 1-2?

Answer 1

Fertilisation: repeated mitotic divisions (cleavage)

Question 2

What happens on day 3?

Answer 2

Morula is formed which is a ball of cells that are identical post-fertilization

Question 3

What happens on day 5?

Answer 3

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)

Question 4

What happens on day 6?

Answer 4

1. Blastocyst loses its membrane (zona paloca) which allows it to stick to the uterine epithelium
2. Cells of blastocyst differentiate + become something else once stuck to epithelium (esp. in trophoblastic layer)
3. Implantation: invasion of uterine lining + wall
4. This must happen fast because it needs the foeto-maternal exchange membrane (deep in uterine epithelium) for blood supply + nutrients

Question 5

What happens on day 8?

Answer 5

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

Question 6

What happens on day 9?

Answer 6

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

Question 7

Where can ectopic pregnancies occur?

Answer 7

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

Question 8

What happens at day 12?

Answer 8

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)

Question 9

What happens at day 13?

Answer 9

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)

Question 10

What day is the first day to notice menstrual periods are late?

Answer 10

15

Question 11

What happens at day 15?1

Answer 11

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

Question 12

What are the other terms for epiblast and hypoblast?

Answer 12

Epiblast = ectoderm

Hypoplast = endoderm

Question 13

What is gastrulation?

Answer 13

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

Question 14

What membranes remain mesoderm free?

Answer 14

Buccopharyngeal (was prochordal plate)

Cloacal

Question 15

What forms the notochord? What will it go onto form?

Answer 15

Primitive node/pit cells

Structures in the vertebral column

Question 16

What will ectoderm, endoderm and mesoderm go onto form in the adult?

Answer 16

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

Question 17

What range of muscles form from the mesoderm?

Answer 17

Skeletal muscle
Bone
Connective tissue
The heart
Urogenital system

Question 18

What do somites form?

Answer 18

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)

Question 19

What occurs at days 18-24 as well?

Answer 19

Folding: flat disc needs to be folded into prawn shaped -> folding puts skin (ectoderm) on outside + gut tube lining (endoderm) on inside

Question 20

What types of folding occurs at weeks 3-4?

Answer 20

Longitudinal: folds head + tail ends towards eachother

Lateral: folds lateral sides of disc towards each other meeting in the anterior midline

Question 21

Describe the fluid filled sacs present in the embryonic disc at day 13.

Answer 21

Amniotic cavity on top of embryonic disc and a yolk sac below

Chorionic cavity surrounds + contains all of this

Question 22

What happens to the embryonic disc during lateral folding?

Answer 22

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

Question 23

What does longitudinal folding do to the embryo?

Answer 23

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

Question 24

How is the gut tube formed?

Answer 24

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

Question 25

What does the bladder form from?

Answer 25

Dilated terminal portion of gut tube

Question 26

What is the embryological name for the umbilicus?

Answer 26

Vitello-intestinal duct

Question 27

What is omphalocele?

Answer 27

When the gut tube growth, herniation + rotation goes wrong so a loop of gut is still in the umbilicus surrounded by amniotic sac

Question 28

What is gastroschisis?

Answer 28

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

Question 29

What happens if the gut tube rotation goes wrong?

Answer 29

Clockwise rotation or malrotation increases risk of gut tube volvulus/obstruction

Question 30

What is Meckel's Diverticulum?

Answer 30

Remnant of the vitellointestinal duct from the SI (stomach contents may get stuck here causing inflammation + pain like appendicitis)

Question 31

What is am imperforate anus?

Answer 31

Abnormal urorectal septum formation where anus does not open so faecal matter cannot get out

Question 32

How does the cloaca develop?

Answer 32

Starts as common waste outflow for urine + faeces -> urorectal septum divides cloaca into bladder + rectum meeting the cloacal membrane to form the perineal body

Question 33

What problems can occur if the urorectal septum does not form properly?

Answer 33

Faecal matter in urinary system or urine in the rectum -> infections

Question 34

Where does the notochord form? What does it become?

Answer 34

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)

Question 35

What is sacrococcygeal teratoma?

Answer 35

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)

Question 36

What occurs to the neural plate on day 19-25?

Answer 36

It forms + then folds to form the neural groove + subsequently, the neural tube

Question 37

What are neural crest cells (NCCS)? What do they do?

Answer 37

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

Question 38

What conditions are associated with defective neural crest cell (NCC)migration?

Answer 38

Craniofacial skeleton defect Disordered gut motility if vagal-derived NCCs fail to populate the ENS

Question 39

What significant events happen at day 25 and day 27?

Answer 39

25: Cranial neuropore closes 27: causal neuropore closes I.E. neural tube zips up from heard toward tail

Question 40

What is spina bifida?

Answer 40

Defective neural tube closure together with anomalies of covering tissues Non-fusion of the vertebral arch

Question 41

What are the different types of spina bifida (order from most mild to severe)?

Answer 41

1. Occulta: unfused vertebral arch - may present with hairy red bit of skin on back 2. Meningocele: subarachnoid space + dura mater prolapsed out of vertebral disc - bump protruding out of back 3. Meningomyelocoele: spinal cord prolapsed as well - meroanencephaly 4. Rachischisis/myeloschisis: no spinal cord tissue developed at all - not compatible with life

Question 42

What to pharyngeal arches contribute towards?

Answer 42

Formation of neck, lower face + ear

Question 43

How many pharyngeal arches form? How many develop?

Answer 43

6 initially form 5 develop (I, II, III, IV + VI)

Question 44

What is the innervation and blood supply to the pharyngeal arches?

Answer 44

Each arch innervated by a cranial nerve Each arch receives blood from an aortic arch artery

Question 45

How old is the embryo when most of the major organ systems have formed? How big is it at this point?

Answer 45

8 weeks 30mm