11 - Embryology II Flashcards
Which period of foetal development has the greatest changes?
The embryonic period
- All major structures and systems are formed
At what stage is there the greatest risk of congenital malformation?
The embryonic period as it is highly sensitive.
Pre-embryonic malformations are lethal, so pregnancy is lost.
The germ layers (trilaminar disc) are all formed from the epiblast. What happens to the hypoblast?
No longer needed so degenerates
Following gastrulation, holes develop towards the caudal and cranial ends of the mesoderm (the ectoderm and endoderm are still intact), what do these develop into?
Cranial end - future mouth
Caudal end - future anus
What is situs inversus? When does this cause a problem?
- Complete mirror image of viscera
- Common result of immotile cilia in embryonic development
- No morbidity but problems arise if there is normal and mirror-image disposition
How is left-right asymmetry developed in gastrulation?
- Ciliated cells at the primitive node results in left-sided flow of signalling molecules (absence of signals on right)
- Side-specific signalling cascades are activated
What is the next stage of embryonic development after gastrulation?
Neurulation
What is the first stage of neurulation?
- Formation of the notochord
- Notochord is a solid rod of cells that forms along the midline of the mesoderm
What does the notochord do?
- Thickens and converts the overlying cells in the ectoderm into neurectoderm
- Endoderm below is not affected as it doesn’t contain the receptors required
What happens to the neurectoderm formed by the notochord?
- Neurectoderm forms the neural plate
- Edges elevate out of the plane of the disc, curl towards each other and fuse, creating the neural tube
After neurulation, what different parts of the mesoderm are formed and where are they?
- Paraxial mesoderm - surrounds the neural tube
- Intermediate mesoderm - lateral to the paraxial mesoderm
- Somatic mesoderm - at the edge of the mesoderm layer on the ectoderm (upper) border
- Splanchnic mesoderm - at the edge of the mesoderm layer on the endoderm (lower) border
What is the space between the somatic and splanchnic mesoderm known as?
Intraembryonic coelom
The paraxial mesoderm splits into 31 pairs of blocks of mesoderm along its length, these are called ………….
Somites
Somites degenerate at their ………. wall. This portion forms the ………. which is the basis of the skeletal system.
Ventral
Sclerotome
The dorsal portion of the somite forms the ………………….. which splits into the ………….. and ……………….
Dermomyotome
Dermatome and myotome
The dermatome forms the ……………..
The myotome forms the ……………….
The sclerotome forms the ……………..
The dermatome forms the dermis of the skin
The myotome forms the muscles
The sclerotome forms the bones
The organisation of the mesoderm into somites gives rise to repeating structures such as:
- Vertebrae
- Ribs
- Intercostal muscles
- Spinal cord segments
How do the germ layers fold?
- Cephalocaudal folding - cephalic and caudal ends fold over toward the midline (ectoderm on the outside)
- Lateral folding - lateral sides fold over toward the midline (ectoderm on the outside)
- Small opening left in the middle to the yolk sac
- Like a drawstring purse - only ectoderm left on the outside
Describe how the embryo folds and how the position of key structures such as the amniotic and yolk sacs change.
- Amniotic sac folds down over the embryo
- Embryo curls over, keeping ectoderm on the outside
- Yolk sac brought up into the space created by the folding embryo, part is pinched off and degenerates
- Heart is brought into the centre
The yolk sac that is brought up into the embryonic cavity forms the ….. ………
GI tract
Folding results in the embryo’s suspension within the ……. …….. surrounded by the ……….. …………..
Amniotic sac
Amniotic membrane
