9. Early Embryology 2 Flashcards
At what points during development is a baby most at risk?
During pre embryonic most lethal effects likely.
Embryonic and fetal still very sensitive, CNS always remains at high risk.
What are the key stages of foetal development?
Fertilisation and implantation Gastrulation Neurulation Segmentation Folding
What is the primitive streak?
At the end of the second . week, epiblast formed uniform disc
By third week, primitive streak on dorsal surface of epiblast, end is the primitive node
Defines beginning of an individual
Bioethics? No more testing
Conjoined twins where does not separate
What occurs during migration and invagination?
Epiblast layer divides and differentiates, moves down from area of streak.
Pushes through columnar epithelial layer, displaces hypoblast to create three layers.
Now creates trilaminar, epiblast and hypoblast now ectoderm, mesoderm, endoderm - replaced hypoblast layer.
Where does migration and invagination occur?
Only two regions at top and tail end where left open containing little mesoderm, where future mouth and anus located.
What are the derivatives of epiblast?
Ectoderm outside - organs and structures, NS Mesoderm - supporting tissues, muscle, cartilage, bond, vascular Endoderm - internal epithelial linings
What is situs inversus?
Complete mirror image viscera, commonly results from immotile cilia, heart on right side
How does the body ensure there is left-right asymmetry?
Ciliated cells present at primitive streak and node.
Create left ward flow of signalling molecules.
Whilst cells of epiblast undergo differentiation.
Moves away from notochord.
What is gastrulation?
Third week of development
Marks start of embryonic period
Bilaminar disc now trilaminar
Primitive streak forms caudal epiblast, correct placement of tissues and structures
What begins to occurs during week 4?
Neurulation and notochord development continues
What is the notochord?
Solid rod of cells running in the midline, signalling roles
Directs conversion of overlying ectoderm to neuroectoderm
What occurs during neurulation?
Has different path to rest of ectoderm, thickens so much it lifts away from ectoderm
Creation of neural plate, curls to create neural tube
Cranial and caudal ends prominent, curling of embryonic disc
Describe the different layers of mesoderm that surround the neural tube
Chordamesoderm below forming notochord.
Paraxial mesoderm either side forming head and somites.
Intermediate mesoderm forming kidneys and gonads.
Lateral plate forms splanchnic - body, ectoderm skin and somatic - viscera, GI tract
Intraembryonic coelum in between two lateral plates first space inside embryo to build viscera
What are somites?
What happens when these break down?
Regular blocks of mesoderm cells arranged around a small cavity.
Then ventral wall of somite breaks down forming sclerotome - path to form skeletal elements.
Then formation of dermamyotome to the side of sclerotome, then further proliferation creates dermatome - deep layer of skin, myotome - muscle precursor.
How many somites do we have during development?
31 for 31 pairs of spinal nerves
What does a somite innervate?
Derivatives innervated by the nerve that corresponds to that segment
Joint to spinal ganglion and spinal segment, joint to dermamyotome - a pair either side of each segment of spinal cord.
What is the buccopharyngeal membrane?
The site of future mouth
How does folding occur to correctly position structures?
Cephalocaudal folding folding head and tail over.
Lateral folding follows.
Ensures ectoderm layer now completely enclosed by endoderm.
What does folding look like through a sagittal section?
Viewing body as if cut half down middle at midline separating each arm and leg
Three layered disc begins to curl, heart begins to enter thoracic region, neural tube exerting downward force.
Folding can occur as curling continues, amniotic sac entirely enclosing embryo, part of yolk sac drawn into embryonic body.
What does folding look through a transverse section?
Sliced through middle horizontally
Somites shown forming at mesoderm, space opening up in lateral forming splanchnic and somatic.
Folding continues, mesoderm curling over with ectoderm down over endoderm. Folding of yolk sac, ectoderm folding helps creation of mid gut in yolk sac. Yolk sac pinched off.
What does folding achieve?
Draws together margins of the disc, creates ventral body wall pulling amniotic membrane around disk.
Embryo suspended in amniotic sac, pulls connecting stalk ventrally.
Creates primordium of cut, creates new cavities, heart in correct place.
What has occurred by the end of the fourth week?
Nervous system begun to form
Segments appeared assigning specific tasks to specific cells e.g. myotome skeletal etc
Everything in correct place
