Embryonic Phase - overview Flashcards
Define embryonic phase:
Period 3-8 weeks after fertilisation, when all structures and systems are formed.
Name the outermost primary germ layer:
Ectoderm
Name the innermost primary germ layer:
Endoderm
Define primitive streak:
Narrow groove that develops on dorsal surface of epiblast layer of bilaminar disc.
It determines the orientation of the embryo.
Epiblast cells migrate through the streak to form the mesoderm and endoderm.
Define gastrulation:
Phase during week 3, where the bilaminar disc is reorganised into a trilaminar disc.
List the most important derivatives of the ectoderm:
- Skin + appendages (hair/nails)
- Brain + spinal cord
- Peripheral nerves
- Retina, Iris, lens + cornea of eye
List the most important derivatives of the mesoderm:
- Connective tissue (bone/cartilage/blood/lymph etc)
- Muscle (+ heart)
- Kidneys + ureters
List the most important derivatives of the endoderm:
- GI tract + glands/organs
- Respiratory tract
- Bladder + urethra
Skin and brain are both derivatives of which germ layer?
Ectoderm
Heart and kidneys are both derivatives of which germ layer?
Mesoderm
Liver, lungs and bladder are all derivatives of which germ layer?
Endoderm
Which layer of the bilaminar disc gives rise to all 3 layers of the trilaminar disc?
Epiblast layer
Name the 2 areas of the trilaminar disc in which mesoderm is absent:
1) Buccopharyngeal membrane
2) Cloacal membrane
Why does the absence of mesoderm (ie at the cloacal membrane) mean this will be a future site of rupture?
Mesoderm gives rise to the CVS, so an area without mesoderm will necrose due to lack of nutrients/o2 = rupture.
During which week after fertilisation is the trilaminar disc formed?
Week 3
Describe the formation of the notochord:
Epiblast cells migrate through the cranial part of the primitive pit differentiate into prenotochordal cells, and form a solid rod of cells running in the midline
Cells from which layer of the bilaminar disc form the notochord?
Epiblast
Name the solid rod of cells (created in week 3) which define the midline of the embryo:
Notochord
The notochord eventually regresses, forming what?
Nucleus pulposus of the intervertebral disc
What is the name given to the individual segments of the paraxial mesoderm?
Somites
When do the 1st pair of somites appear?
Day 20
Do the somites appear cranio-caudally, or caudo-cranially?
Cranio-caudally
How many somites are formed in total?
44 paired somites, then some regress to form 31 pairs (spinal nerves)
How many spinal nerves do we have?
31 pairs
Somites are derived from what germ layer?
Mesoderm (paraxial)
Name the 4 types of mesoderm:
1) Paraxial
2) Intermediate
3) Somatic
4) Splanchnic
Why are the buccopharyngeal and cloacal membranes destined to rupture?
Lack of mesoderm = no blood supply = necrosis
Describe the shape of the neural plate:
Key-hole shaped
What stimulates the formation of the neural cord?
Notochord
Describe the formation of the neural cord:
Notochord stimulates overlying ectoderm to thicken (forming neural plate)
Edges elevate out of plane of disc, and curl towards each other = tube
Which germ layer proliferates the fastest, leading to lateral and cephalo-caudal folding?
Ectoderm
List some main outcomes of lateral and cephalo-caudal folding:
- Formation of the primitive heart tube
- Moves the primitive heart tube + diaphragm into thoracic region
- Forms ventral body wall
- Surrounds embryo in amniotic fluid
- Pulls connecting stalk (future umbilical cord) ventrally
- Forms primitive gut
Name the ventral section of a somite:
Dermamyotome
What does the ventral part of a somite give rise to (dermamyotome)?
- Ventral body wall
- Limb muscles + dermis
- Back muscles + dermis
What does the dorsal part of a somite give rise to (sclerotome)?
- Vertebrae
- Ribs
Define dermatome:
Strip of skin supplied by single spinal nerve
Define myotome:
Muscle/group of muscles supplied by a a single spinal nerve
