13 Embryology embryonic period

Question 1

Embryonic period overview

Answer 1

weeks 3-8
Period of the greatest change so therefore greater risk of major congenital malformation (teratogenesis) due to environmental exposure or drug therapy
gastrulation–>neurolation–>segmentation–>folding

Question 2

first 4 weeks

Answer 2

foundations are laid for development. need sufficient cells of the right type in the right place.

Question 3

Gastrulation: what is a primitive streak?

Answer 3

at the end of the second week the epiblast is a uniform disk. in the third week the primitive streak starts to form on the dorsal surface. at the end of the streak like in the middle theres the primitive node.
the formation of the streak is the beginning of gastrulation

Question 4

Gastrulation: bilaminar layer to trilaminar

Answer 4

bilaminar layer: epiblast (top) and hyperblast. Epiblast start to multiple and migrate to the primitive layerand invaginate throught he primitive streak. they undergo differentiation so three layers are formed–> the hyperblast are no longer needed.

three new layers (trilaminar): ectoderm, mesoderm, endoderm. GET ALL THE TISSUES OF THE BODY FROM THESE LAYERS.

So there are three layers formed apart from two patches where the mesoderm is not there and the endoderm and ectoderm touch. these are the openings.

Question 5

Ectoderm derivative:

Answer 5

organs and structures that maintain contact with the outside world eg nervous system and epidermis

Question 6

Mesoderm derivatives:

Answer 6

supporting tissues eg muscle, cartilage, bone, vascular system

Question 7

Endoderm derivatives:

Answer 7

internal structures eg epithelial lining of GI tract, resp tract and paremchyma of glands

Question 8

Situs inversus

Answer 8

complete mirror image viscera commonly results from immotile cilia. no associated morbidity normally unless theres both normal and mirror-image disposition

Question 9

So how is left-right assymetry achieved?

Answer 9

during embryonic development. there are cells that are ciliated at the node that beat in a characteristic way and are able to push left sided signals to the left. so side specific signalling cascade.

Question 10

Week 4 neurulation: notochord?

Answer 10

Notochord is part of the mesoderm layer that run down the midline of the disk. it is a solid rod of cells important in signallin. it directs the conversion of overlying ectoderm to differentiate into neurectoderm. the cells will be in the very local vascinity.
Notochord signal causes pverlying ectoderm to thicken and form the neural plate. The edges elevate out of the plane of the disk and curl towards each other to creat e the neural tube

Question 11

Whats happening in the mesoderm while this is happening?

Answer 11

The mesoderm is differentiating. there is paraxial mesoderm around the neural tube and then intermediate mesoderm between that and at the ends somatic mesoderm and splanchnic mesoderm kind of levers at each end. space between these is called the intraembryonic coelom (gap).

Question 12

Somites?

Answer 12

they are blocks of mesoderm cells (paraxial) arranged around a small cavity. they are formed in pairs: around 31 made (more made but degraded). this is then followed by organised degeneration where the ventral wall of the somite breaks down, leading to the formation of the sclerotome. these cells will form skeletal structures.

Further organisation of the dorsal portion forms the combined dermomyotome. the myotome proliferates and migrates and dermatome disperses.

Question 13

Somite derivatives

Answer 13

Dermatome: skin section
Myotome: muscle section
Sclerotome: hard tissue section

Question 14

Implication of segmentation

Answer 14

Organisation of mesderm into somites gives rise to repeating structures: vertebrae, ribs, intercostal muscles, spinal cord segments.

Question 15

31 somites make:

Answer 15

31 segments ie 31 pairs of spinal nerves

Question 16

What happens next? folding

Answer 16

head and tail and sides folding called cephalocaudal folding. so ectoderm on outside.

Question 17

What does folding achieves

Answer 17

Draws together the margins of the disk: creating a ventral body wall, pulling amniotic membrane around the disk. embryo becomes suspended within the amniotic sac puling connecting stalk ventrally.