Third Week of Development Flashcards
1
Q
The third week of development coincides with which week off the menstrual period
A
the 1st missed menstrual period (5th week)
2
Q
What events occur in the third week of development
A
- Primitive streak is formed creating the 3 germ layers
- The notochord, neural tube and somites are formed
3
Q
What is gastrulation
A
- This is when the bilaminar disc converts to a tri-laminar disc when the primitive streak is formed
4
Q
Where is the primitive streak developed?
A
- On the dorsal aspect (in communication with the amniotic sac), caudally along the midline
- This includes the primitive streak → groove, node → pit
5
Q
Explain the process of gastrulation
A
- Epiblast cells thicken to form the primitive streak on the caudal end with a node on the cranial end
- The primitive streak cells die, making this thickening hollow
- Primitive streak = primitive groove and primitive node = primitive pit meaning there’s a space where thickening was
- The cells near the edge of primitive streak secrete FGF8 laterally to bind unto epiblast cells causing the cells to migrate towards the streak downwards replacing the hypoblast. Hypoblast = endoderm and epiblast = ectoderm
- More epiblast cells move through primitive groove in a forward motion, filling in between creating the mesoderm
6
Q
Explain the development of the notochord
A
- FGF8 is released and the ectodermal cells migrate through the primitive pit cranially towards the prechordal plate forming a tube which is called the notochord
7
Q
What is the purpose of the notochord?
A
- Helps develop the axial skeleton
- Induces the formation of vertebral bodies
- later becomes the nucleus pulposus of the intervertebral discs
- Induces the ectoderm to form the neural plate
8
Q
Where are the three places that no mesodermal cells are found
A
- Where the notochord is
- Where the prechordal plate is
- Where the cloacal plate is
9
Q
What is neurulation
A
- The formulation of the neural tube by the thickening of the ectoderm which is complete by the end of the 4th week
10
Q
Explain the process of neurulation
A
- The notochord releases growth factors for ectodermal cells to proliferate to form the neural plate day 17
- The longitudinal part of plate is pulled downwards = neural groove and transverse part of the plate is folded into itself along the primitive pit to the prechordal plate = neural fold
- Ectodermal cells around the edges of the neural fold → neural crest cells on day 18 cells fuse with each other = neural tube
- Tube goes underneath ectoderm to sit on top of notochord and neural crest cells hovers on top of tube. Day 21
- Tube has cranial and caudal ends still open (neuropores) that are closed off on day 25 (anterior) and day 27 (posterior)
11
Q
What are Neural Tube Defects (NTD)
A
- This is when the anterior and/or posterior neuropores fail to close
- Cells use folate to synth. DNA, lack of folate = no DNA synth. = cells won’t close
- Anterior neuropore failure = anencephaly (cephal for head!)
- Posterior neuropore failure = Spina Bifida
- Pregnant women should take 400µg of folic acid per day to prevent NTDs
12
Q
Describe the differentiation of the intra-embryonic mesoderm
A
- Mesoderm differentiates to the paraxial, intermediate and the lateral mesoderm
- Top part of lateral mesoderm = intra-embryonic somatic mesoderm
- Bottom part of lateral mesoderm = intra-embryonic splanchnic mesoderm
- The space in between merges on the far end to create ‘holes’ called the intraembryonic coelom
- Paraxial mesoderm = Somites (sclera, myo, and dermatomes)
13
Q
Describe the development of somites
A
- Paraxial mesoderm develops and breaks into cubed shaped cells = somites that have a cavity in the center = somatocoeles
- Somites differentiates to the sclerotomes (vertebrae), myotomes (muscles of the neck, body wall and limbs), and dermatomes (dermis of the scalp, neck and trunk)
14
Q
Purpose of the intraembryonic mesodermal layer
A
- Intraembryonic coelom: gives ride to pericardial, peritoneal and pleural layers
- Intraembryonic somatic mesoderm + ectoderm = body wall (somatopleure)
- Intraembryonic splanchnic mesoderm + endoderm = gut wall (splanchnopleure)
15
Q
What happens in the early development of the cardiovascular system
A
- Consists of vasculogenesis and angiogenesis
- 1º chorionic villi are developed in the 2nd week and are converted into the 2º villi when the extra-embryonic mesodermal cells grow into the 1º villi.
- The extra-embryonic mesoderm differentiated to form blood vessels converting the villi to tertiary
