9. Early Embryology 2

Question 1

At what points during development is a baby most at risk?

Answer 1

During pre embryonic most lethal effects likely.

Embryonic and fetal still very sensitive, CNS always remains at high risk.

Question 2

What are the key stages of foetal development?

Answer 2

Fertilisation and implantation
Gastrulation
Neurulation
Segmentation
Folding

Question 3

What is the primitive streak?

Answer 3

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

Question 4

What occurs during migration and invagination?

Answer 4

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.

Question 5

Where does migration and invagination occur?

Answer 5

Only two regions at top and tail end where left open containing little mesoderm, where future mouth and anus located.

Question 6

What are the derivatives of epiblast?

Answer 6

Ectoderm outside -
organs and structures, NS
Mesoderm -
supporting tissues, muscle, cartilage, bond, vascular
Endoderm - internal
epithelial linings

Question 7

What is situs inversus?

Answer 7

Complete mirror image viscera, commonly results from immotile cilia, heart on right side

Question 8

How does the body ensure there is left-right asymmetry?

Answer 8

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.

Question 9

What is gastrulation?

Answer 9

Third week of development
Marks start of embryonic period
Bilaminar disc now trilaminar
Primitive streak forms caudal epiblast, correct placement of tissues and structures

Question 10

What begins to occurs during week 4?

Answer 10

Neurulation and notochord development continues

Question 11

What is the notochord?

Answer 11

Solid rod of cells running in the midline, signalling roles

Directs conversion of overlying ectoderm to neuroectoderm

Question 12

What occurs during neurulation?

Answer 12

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

Question 13

Describe the different layers of mesoderm that surround the neural tube

Answer 13

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

Question 14

What are somites?

What happens when these break down?

Answer 14

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.

Question 15

How many somites do we have during development?

Answer 15

31 for 31 pairs of spinal nerves

Question 16

What does a somite innervate?

Answer 16

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.

Question 17

What is the buccopharyngeal membrane?

Answer 17

The site of future mouth

Question 18

How does folding occur to correctly position structures?

Answer 18

Cephalocaudal folding folding head and tail over.
Lateral folding follows.
Ensures ectoderm layer now completely enclosed by endoderm.

Question 19

What does folding look like through a sagittal section?

Viewing body as if cut half down middle at midline separating each arm and leg

Answer 19

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.

Question 20

What does folding look through a transverse section?

Sliced through middle horizontally

Answer 20

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.

Question 21

What does folding achieve?

Answer 21

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.

Question 22

What has occurred by the end of the fourth week?

Answer 22

Nervous system begun to form
Segments appeared assigning specific tasks to specific cells e.g. myotome skeletal etc
Everything in correct place